Micro Flashcards

Question

25. Which class of antibiotics is used for the treatment of serious gram-negative infections as well as infections with Mycobacterium tuberculosis? A. Cephalosporins B. Penicillins C. Tetracyclines D. Aminoglycosides Microbiology/Apply knowledge of fundamental biological characteristics/Antibiotics/1

Answer 1

D The aminoglycoside antibiotics are bactericidal agents that act by inhibiting protein synthesis. They show a low incidence of bacterial resistance but must be monitored carefully because at high doses they can cause ototoxicity and nephrotoxicity. The group includes amikacin, gentamicin, tobramycin, kanamycin, streptomycin, and spectinomycin. These drugs are usually administered intravenously or intramuscularly because they are poorly absorbed from the gastrointestinal tract

Answer 2

B CIN agar inhibits the growth of many organisms from the family Enterobacteriaceae. Yersinia spp. are also recovered from MacConkey and Salmonella-Shigella agars. Y. enterocolitica are seen as “bull’s eye” colonies on CIN agar after 48 hours at room temperature.

Answer 3

D Y. pestis is on the list of agents of bioterrorism. Isolation and identification should be performed in a facility with a Level II or higher biosafety rating. If there is a high risk of aerosolizing the specimen during processing, procedures should be performed under Level III biosafety conditions. Recovery of Y. pestis is highest if the specimen is cultured within 2 hours of collection

Answer 4

A The oxidase positive test result rules out the members of the Enterobacteriaceae family. Colonies of A. hydrophilia and Plesiomonas spp. (both oxidase positive) might be mistaken for Vibrio spp. because all three grow as clear colonies on MacConkey agar, are β-hemolytic on blood agar, and are oxidase positive. Aeromonas spp. will also grow on CNN agar and mimic Y. enterocolitica (a member of the Enterobacteriaceae family and oxidase negative).

Answer 5

A All three organisms are positive for oxidase production and are motile. Plesiomonas spp. do not grow on TCBS agar. Clear colonies on MacConkey agar and yellow colonies on TCBS agar indicate Vibrio or Aeromonas spp. However, only Vibrio spp. require Na+ (1% NaCl) in the medium for growth. Vibrio Aeromonas Plesiomonas Oxidase + + + Na+ requirement + Neg Neg Mannitol fermentation + + Neg Growth onTCBS + + Neg

Answer 6

C E. coli 0157:H7 ferments lactose and, therefore, appears as dark pink colonies on MacConkey agar. To differentiate E. coli 0157:H7 from normal fecal flora, MacConkey agar with sorbitol is used. E. coli 0157:H7 does not ferment sorbitol, and usually are colorless colonies.

Answer 7

A The family Enterobacteriaceae consists of more than 100 species and represents the most commonly encountered isolates in clinical specimens. All Enterobacteriaceae species ferment glucose, are nonsporulating, and are oxidase negative (except for Plesiomonas shigelloides, recently added to the family but which is oxidase positive). Most Enterobacteriaceae species are motile, but the genera Shigella and Klebsiella are not.

Answer 8

B The ONPG test detects β-galactosidase activity and is most useful in distinguishing late lactose fermenters from lactose nonfermenters. Some strains of E. coli are slow lactose fermenters and may be confused with Shigella spp., which do not ferment lactose. E. coli are ONPG positive, whereas Shigella spp. are ONPG negative.

Answer 9

A Acetoin or carbinol, an end product of glucose fermentation, is converted to diacetyl after the addition of the VP reagents (β-naphthol and 40% potassium hydroxide [KOH]). Diacetyl is seen as a red- to-pink–colored complex.

Answer 10

D Both MR and VP tests detect acid production from the fermentation of glucose. However, a positive MR test result denotes a more complete catabolism of glucose to highly acidic end products, such as formate and acetate, than occurs with organisms that are VP positive only (e.g., Klebsiella pneumoniae).

Answer 11

A The Simmons citrate test determines if an organism can utilize citrate as the sole source of carbon. The medium turns blue, indicating the presence of alkaline products, such as carbonate. Tubes are incubated a minimum of 24 hours at 35°C with a loose cap before reading

Answer 12

B The test for urease production is based on the ability of the colonies to hydrolyze urea in Stuart broth or Christensen agar to form CO2 and ammonia. These products form ammonium carbonate, resulting in alkalinization. This turns the pH indicator (phenol red) pink at pH 8.0

Answer 13

A The indole test detects the conversion of tryptophan (present in the media) to indole by the enzyme tryptophanase. Indole is detected by the reaction with the aldehyde group of p-dimethylaminobenzaldehyde (the active reagent in Kovac and Ehrlich reagents) in acid, forming a red complex

Answer 14

D Specific decarboxylases split dibasic amino acids (lysine, arginine, and ornithine), forming alkaline amines. These products turn the pH indicators in the medium (cresol red and bromcresol purple) from yellow to purple.

Answer 15

C LIA is used as an aid for the identification of Salmonella species. It contains phenylalanine, lysine, glucose, thiosulfate, ferric ammonium citrate, and bromcresol purple. Salmonella spp. produce H2S from thiosulfate. This reduces ferric ammonium citrate, forming ferrous sulfate and causing the butt to blacken. Salmonella also decarboxylate lysine to produce alkaline amines, giving the slant its purple color and differentiating it from Citrobacter spp., which are lysine decarboxylase negative

Answer 16

C Putrescine is the amine product of the decarboxylation of ornithine.

Answer 17

B Phenylalanine deaminase oxidatively deaminates phenylalanine, forming phenylpyruvic acid. When a solution of ferric chloride is added, the iron reacts with phenylpyruvic acid, forming a green-colored complex. Phenylalanine deaminase is found in the genera Morganella, Providencia, and Proteus and is an excellent test to determine if an organism belongs to this group. Rarely, isolates of Enterobacter may be phenylalanine deaminase positive as well.

Answer 18

C Both KIA and TSI contain 10-fold more lactose than glucose, peptone, and phenol red to detect acid production (turns yellow) and sodium thiosulfate and ferrous ammonium sulfate to detect H2S production. However, TSI contains sucrose, and KIA does not. Organisms fermenting either sucrose or lactose will turn the slant of the agar tube yellow. Therefore, some organisms (e.g., many species of Cedecea, Citrobacter, Edwardsiella, and Serratia) will produce a yellow slant on TSI but a red slant on KIA.

Answer 19

C The malonate test determines whether an organism can utilize sodium malonate as the sole source of carbon. Malonate is broken down, forming alkaline metabolites that raise the pH of the broth above 7.6. This causes bromthymol blue to turn from green to deep blue (Prussian blue). E. coli, Shigella, and most Salmonella are malonate negative, whereas Enterobacter and Salmonella (formerly Arizona) subgroups 2, 3a, and 3b are positive. Proteus, Providencia, Serratia, and Yersinia are also malonate negative.

Answer 20

B Escherichia, Salmonella, Shigella, and Yersinia are responsible for the majority of enteric diarrhea cases attributable to the Enterobacteriaceae family.

Answer 21

A An isolate of E. coli (shiga-like producing toxin of E. coli [STEC]) recovered from a stool culture in hemorrhagic colitis can be definitely identified only by serotyping. The isolate is identified as E. coli by the usual biochemical reactions. The strain of E. coli responsible for hemorrhagic colitis is O157:H7 and is usually negative for sorbitol fermentation. Colonies of this strain of E. coli appear colorless on MacConkey agar with sorbitol added.

Answer 22

A Serological confirmation of Shigella isolates is based on O antigen typing. If a suspected Shigella spp. is serologically typed with polyvalent sera before it has been correctly identified biochemically, a false-positive confirmation may occur with an isolate that is E. coli (i.e., anaerogenic, non–gas-producing, lactose-negative or - delayed, and nonmotile strains). These strains were formerly known as the Alkalescens-Dispar serotype.

Answer 23

D The Shigella spp. are lactose nonfermenters that, for the most part, are biochemically inert and are classified into serogroups A, B, C, and D as a result of their biochemical similarity. S. sonnei is the species most often isolated from diarrhea cases in the United States. It is more active biochemically than the other species because of ornithine decarboxylase and β-galactosidase activity. These enzymes, found in most strains of S. sonnei, distinguish it from other Shigella species.

Answer 24

B E. coli, when positive for lactose, indole, and ONPG, are usually motile. Shigella species do not ferment lactose or produce indole, lack β-galactosidase, and are nonmotile.

Answer 25

A Shigella spp. and Klebsiella spp. are, for the most part, nonmotile. Yersinia can be motile at 22°C but is nonmotile at 36°C. Other members of Enterobacteriaceae that have been isolated from human specimens and are usually nonmotile include Leminorella, Rahnella, and Tatumella (considered unusual enteric pathogens).

Answer 26

D Cold enrichment is especially useful when specimens contain large numbers of normal flora that are sensitive to prolonged exposure to near-freezing temperature. In addition to Yersinia, the technique has been used to enhance recovery of Listeria monocytogenes from specimens containing other bacteria

Answer 27

A Shigella spp. and Campylobacter spp. are both causes of diarrhea, abdominal pain, fever, and sometimes vomiting. Blood is present in the stools of patients infected with Shigella as a result of invasion and penetration of the bowel by the organisms. Young children may also exhibit bloody stools when infected with Campylobacter.

Answer 28

A V. cholerae and C. difficile are usually not included in a routine stool culture. If Vibrio spp. are suspected, a special request should be included. Although MacConkey agar will support the growth of Vibrio spp., normal enteric flora overgrow and occlude these organisms. C. difficile culture requires special media (e.g., CCFA) that inhibit other anaerobic flora and facultative anaerobic flora, and culture should be requested specifically if symptoms warrant. MacConkey agar with sorbitol will allow the E. coli O157:H7 to be recovered. Yersinia spp. can be detected on a regular MacConkey agar plate.

Answer 29

D All Enterobacteriaceae species are oxidase negative (the exception being Plesiomonas spp., which cluster with the genus Proteus). Through the use of nucleic acid–based methods, oxidase positive Plesiomonas is now included in the family Enterobacteriaceae. Because E. coli and Proteus spp. comprise the majority of the organisms recovered from clinical specimens, they can be initially identified through rapid testing without additional overnight testing. E. coli display a positive indole test, and the colonial morphology on MacConkey agar is distinctive, showing flat, pink (lactose-positive) colonies with a ring of bile precipitation. Proteus spp. swarm on blood agar and are indole negative.

Answer 30

A Morganella and Proteus spp. are motile, produce urease, and deaminate phenylalanine

Answer 31

B Test E. aerogenes (% positive) E. tarda (% positive) H2S 0 100 Sucrose >90 0 Indole <20 100 VP 100 0 Citrate 95 0

Answer 32

A C. sakazakii was formally called a yellow-pigmented E. cloacae and is best differentiated from E. cloacae by sorbitol fermentation (95% positive for E. cloacae and 0% for [E.] C. sakazakii). In addition, E. cloacae is usually positive for urease and malonate (65% and 75%, respectively) and (E.) C. sakazakii is usually negative (1% and less than 20%, respectively). Both are usually motile and arginine dihydrolase positive.

Answer 33

27. D DNase is not produced by Cedecea spp. but is produced (along with proteinases) by Serratia spp. Other key differential tests include lipase (positive for Cedecea, negative for Serratia) and gelatin hydrolysis (negative for Cedecea, positive for Serratia).

Answer 34

B Biochemical differentiation is essential because Citrobacter isolates may give a falsepositive result on agglutination testing with Salmonella grouping sera. C. freundii strains, like Salmonella spp., are usually H2S producers and may be confused with Salmonella spp. unless the proper biochemical tests are utilized. C. freundii and Salmonella spp. are adonitol, indole, and malonate negative. However, C. freundii is KCN positive, whereas Salmonella spp. are KCN negative. (C. freundii are late lactose fermenters and appear colorless at 24 hours on Mac).

Answer 35

B Morganella spp. are biochemically similar to Proteus spp., both being lactose negative, motile, and positive for phenylalanine deaminase and urease. However, Morganella spp. can be differentiated from Proteus spp. based on H2S, indole, ornithine decarboxylase, and xylose fermentation. Ewingella spp. are usually positive (70%) for lactose fermentation, whereas the other three genera are lactose negative

Answer 36

D K. oxytoca and K. pneumoniae are almost identical biochemically except for the ability to produce indole. Both organisms are nonmotile and usually test positive for urease, sucrose, and citrate. However, K. oxytoca is indole positive, and K. pneumoniae is indole negative.

Answer 37

B Proteus spp. are urease positive as are approximately 70% of Y. enterocolitica isolates. Both organisms are lactose negative and motile. However, Yersinia is motile at 22°C and not at 35°C (demonstrated by using motility media).

Answer 38

D Possible pathogenic strains of E. coli should be picked from MacConkey agar and subcultured onto MacConkey agar with sorbitol. After subculture, these strains can be serotyped or sent to a reference laboratory. Most E. coli normal flora ferment Dsorbitol and appear pink to red on MacConkey–sorbitol agar. The E. coli strain O157:H7 causes the enteric disease hemorrhagic colitis. It ferments D-sorbitol slowly or not at all and appears as colorless colonies on MacConkey–sorbitol agar.

Answer 39

C CIN medium is the best agar for the isolation of Yersinia strains because it inhibits growth of other coliforms, but it is not used routinely in clinical laboratories. Yersinia spp. grow well on MacConkey agar incubated at 37°C, but the colonies are much smaller than the other Enterobacteriaceae species; therefore, 25°C is the temperature recommended for isolation. Some serotypes of Yersinia may be inhibited on more selective media, such as Salmonella–Shigella or Hektoen. CNA agar inhibits the growth of gram-negative bacteria.

Answer 40

A C. freundii and Salmonella spp. are H2S positive and indole, VP, and phenylalanine deaminase negative. Biochemical characteristics that help to differentiate C. freundii from Salmonella include lactose fermentation (50% of C. freundii are lactose positive, whereas 100% of Salmonella are lactose negative) and urease production (70% of Citrobacter are positive and greater than 99% of Salmonella are negative).

Answer 41

A Shigella is the most likely organism biochemically. E. coli are usually indole and motility positive, and Proteus are motility and urease positive. Most Salmonella are H2S positive. Shigella and Campylobacter cause bloody diarrhea because they invade the epithelial cells of the large bowel; however, Campylobacter spp. do not grow on MacConkey agar, and they are oxidase positive.

Answer 42

D Enterobacteriaceae species are grouped according to their ability to ferment lactose, a β-galactoside. Salmonella, Shigella, Proteus, Providencia, and Morganella are usually lactose nonfermenters. Others—including certain strains of E. coli, S. sonnei, H. alvei, Serratia marcescens, and some Yersinia—appear to be lactose nonfermenters because they lack the permease enzyme that actively transports lactose across the cell membrane. However, true lactose nonfermenters do not possess β-galactosidase. The test for β-galactosidase uses the substrate o-nitrophenyl-β-galactopyranoside. At an alkaline pH, β-galactosidase hydrolyses the substrate, forming o-nitrophenol, which turns the medium yellow.

Answer 43

D The Kaufmann-White schema groups Salmonellae on the basis of the somatic O (heat-stable) antigens and subdivides them into serotypes based on their flagellar H (heat-labile) antigens. The Vi (or K) antigen is a capsular polysaccharide that may be removed by heating. In the United States, commercially available slide agglutination tests utilize polyvalent antisera (A, B, C-1, C-2, D, E, and Vi) to group the Salmonella spp. because 95% of isolated organisms belong to groups A through E with the Vi antiserum identifying the capsular or “K” antigen of Salmonella serotype typhi. Antiserum is also used against the flagella (“H”) antigens. If the Vi antigen is positive and the O antigen is negative, the isolate must be heated in boiling water for 15 minutes to remove the capsule and retested with the antisera. There are over 2,200 serotypes of Salmonella.

Answer 44

A The drugs of choice for the Enterobacteriaceae species vary, and several genera display patterns of resistance that aid in their identification. K. pneumoniae and Citrobacter diversus are resistant to ampicillin and carbenicillin; most Enterobacter spp. and Hafnia are resistant to ampicillin and cephalothin. Proteus, Morganella, and Serratia are resistant to colistin. Providencia and Serratia are resistant to multiple drugs. Several genera are resistant to chloramphenicol and most are resistant to penicillin.

Answer 45

B Strains of E. coli that produce one or both of the Shiga-like toxins (SLT I and SLT II) can cause bloody diarrhea (hemorrhagic colitis). In the United States, E. coli strain O157:H7 is the serotype most often associated with hemorrhagic colitis.

Answer 46

A Approximately 15 cases of Y. pestis infection are confirmed in the United States annually. Most originate in the Southwest. It is necessary to be aware of this regional occurrence because untreated cases are associated with a mortality rate of approximately 60%. Y. pestis is not fastidious and grows well on blood agar. It is inactive biochemically, and this helps differentiate it from the other Enterobacteriaceae species. Y. pestis is the only member of the Enterobacteriaceae family that infects humans via an insect vector (animal transmission by a flea bite).

Answer 47

D K. pneumoniae and E. cloacae display similar IMViC (indole, MR, VP, and citrate) reactions (00++) and TSI results. However, approximately 65% of E. cloacae strains are urease positive compared with 98% of those of K. pneumoniae. Enterobacter spp. are motile, and Klebsiella are nonmotile. The antibiotic pattern of resistance to carbenicillin and ampicillin is characteristic of the nonmotile Klebsiella spp.

Answer 48

D Typically, the IMViC reactions for the organisms listed are: E. coli: (++00) (PD = Neg) S. typhi: (0+00) Y. enterocolitica: (V+00) P. vulgaris: (++00) (PD = +)

Answer 49

A The most likely organism is a species of Shigella. Typically, Salmonella spp. produce H2S-positive colonies that display black centers on the differential media (except on MacConkey agar). The biochemical tests listed are necessary to differentiate Shigella from E. coli because some E. coli strains cross-react with Shigella-typing sera. Shigella spp. are one of the most common causes of bacterial diarrhea; group D (S. sonnei) and group B (S. flexneri) are the species most often isolated.

Answer 50

B S. marcescens has been implicated in numerous nosocomial infections and is recognized as an important pathogen with invasive properties. Gelatin hydrolysis and DNase are positive for both the Proteus spp. and Serratia, but the negative urease and phenylalanine deaminase are differential. E. cloacae does not produce DNase, gelatinase, or lysine decarboxylase but will display red-pigmented colonies on MacConkey agar.

Answer 51

A Although P. mirabilis is more frequently recovered from patients with UTIs, P. vulgaris is commonly recovered from immunosuppressed patients. P. mirabilis is indole negative and ornithine decarboxylase positive but otherwise is very similar to P. vulgaris

Answer 52

C Most members of the Enterobacteriaceae family produce detectable growth on MacConkey agar within 24 hours. Y. enterocolita produces non–lactose-fermenting colonies on MacConkey agar, salmon-colored colonies on Hektoen agar, and yellow or colorless colonies on XLD agar. If Y. enterocolitica is suspected, specialized agar (CIN) is employed. The typical “bull’s eye” colonies, dark red with a translucent border, can be confused with Aeromonas spp. that appear similarly on CIN agar. To differentiate, an oxidase test must be performed because Yersinia spp. are oxidase negative and Aeromonas spp. are oxidase positive

Answer 53

C Inflammation with bleeding of the mucosa of the large intestine (hemorrhagic colitis) is a result of an enterohemorrhagic E. coli (EHEC) infection associated with certain serotypes, such as E. coli O157:H7. The source of the E. coli infection is from ingestion of undercooked ground beef contaminated with fecal matter or drinking raw milk.

Answer 54

A Y. pestis is the cause of bubonic and pneumonic plague. Bubonic plague causes swelling of the groin lymph nodes (bubos), whereas pneumonic plague involves the lungs. The infection caused by bubonic plague may result in fulminant bacteremia that is usually fatal. The transmission is from rodents (rats, ground squirrels, or prairie dogs) to humans by the bite of fleas (vectors) or by ingestion of contaminated animal tissues. Pneumonic plague is acquired via the airborne route when there is close contact with other pneumonic plague victims.

Answer 55

B Preliminary serological grouping of the Salmonella spp. and Shigella spp. should be performed because reliable commercial polyvalent antisera are available. Sorbitolnegative (MacConkey agar with sorbitol) colonies of E. coli should be tested by using commercially available antisera for somatic “O” antigen 157 and flagellar “H” antigen 7. However, Y. pestis isolates should be sent to a public health laboratory for testing because clinical laboratories generally do not have the typing sera available.

Answer 56

D Bubonic plague involves an inflammatory swelling of the lymph nodes of the axilla and groin, whereas pneumonic plague is associated with an airborne route involving the lungs. Both infections are caused by the same member of the Enterobacteriaceae family—Y. pestis.

Answer 57

A Point mutations occur in most members of the Enterobacteriaceae family that result in production of a β-lactamase that hydrolyzes broad-spectrum antibiotics, such as the cephalosporins, as well as penicillin and monobactam antibiotics. These are known as ESBL producers. The most common ESBL organisms are K. pneumoniae and E. coli. ESBL strains are detected by demonstrating their resistance to β-lactam antibiotics.

Answer 58

D Serotyping for grouping of Salmonella spp., Shigella spp., and E.coli 0157:H7 using commercially available slide agglutination test kits is done by most clinical laboratories. Nucleic acid–based studies are usually performed in reference laboratories if the demand is minimal in a clinical laboratory. MALDI-TOF MS is used for the identification of Enterobacteriaceae species. Microbial genomic research showed that many mass spectral peaks represented ribosomal proteins when compared with 16S rRNA gene sequencing and could be used to identify clinically important bacteria (Enterobacteriaceae) with the incorporation of mass spectral libraries and software. One limitation has been identified in that MALDI-TOF MS cannot differentiate between E. coli and Shigella spp. Species-level identification compared with the reference method showed optimal performance in the case of many of the Enterobacteriaceae.

Answer 59

D S. sonnei (Serotype D) is the most isolated species of Shigella in the United States. Humans are the only reservoirs, and of the estimated 500,000 cases of shigellosis in the United States per year, S. sonnei is responsible for 85% of the infections. It is primarily a pediatric infection, with 60% seen in children under 10 years of age.

Answer 60

A E. coli (STEC) is the organism referred to as enterohemorrhagic E. coli, which produces a toxin similar to the Shiga toxin produced by S. dysenteriae. Inflammation and bleeding of the mucosa of the large intestine can lead to kidney failure and HUS. HUS is characterized by low platelet counts and hemolytic anemia, leading to transfusions and dialysis and death. The two most common serotypes are 0157:H7 and 0157:NM (nonmotile). Unlike in dysentery, the wet preps of the stools usually lack white blood cells (WBCs).

Answer 61

D P. shigelloides, now considered a new member of the Enterobacteriaceae family, causes gastrointestinal infections. Research shows that it may cross-react with Shigella grouping antisera (Group D) causing misidentification. If an oxidase test is performed first, this error can be avoided (Shigella = oxidase negative, whereas Plesiomonas = oxidase positive).

Answer 62

C NFB will grow on the slant of TSI or KIA, but they do not acidify the butt (glucose fermentation), as do the Enterobacteriaceae. NFB can be cytochrome oxidase positive or negative, but all the Enterobacteriaceae species, except Plesiomonas shigelloides, are oxidase negative. Enterobacteriaceae species grow well on MacConkey agar and reduce nitrate to nitrite, but NFB grow poorly or not at all on MacConkey and most do not reduce nitrate. Nearly 70% of NFB recovered from clinical specimens are strains of P. aeruginosa, Acinetobacter spp., or Stenotrophomonas maltophilia

Answer 63

A The use of OF tubes helps to determine the presumption of a nonfermentative bacillus (glucose oxidation positive and glucose fermentation negative). The positive cytochrome oxidase test and pigment production indicate a possible Pseudomonas species. Several NFB produce pigments that aid in species identification: P. aeruginosa produces yellow pyoverdins (fluorescein) and/or pyocyanin (blue aqua pigment). The characteristic grapelike odor of aminoacetophenone and growth at 42°C are characteristic of P. aeruginosa.

Answer 64

D Both organisms are oxidase positive, motile, and produce pyoverdin. Both are negative for ONPG and DNase. The differentiating tests are: Test P. aeruginosa P. putida Mannitol + Neg Reduction of + Neg nitrate to nitrite 42°C growth + Neg

Answer 65

A Acinetobacter spp. are nonmotile rods, which appear as coccobacillary forms in clinical specimens. All are oxidase negative and catalase positive. P. aeruginosa reduces nitrate to nitrite, whereas Acinetobacter spp. do not. The genus Acinetobacter is divided into two groups: saccharolytic or glucose-oxidizing species and asaccharolytic or nonglucose species.

Answer 66

B The two genera, Acinetobacter and Alcaligenes, are very similar. Both use oxidation for the metabolism of carbohydrate, with some strains being nonsaccharolytic. Both grow well on MacConkey agar. However, Acinetobacter is nonmotile and oxidase negative. Alcaligenes is motile by peritrichous flagella and oxidase positive.

Answer 67

C The two genera, Pseudomonas and Stenotrophomonas, are motile and grow well on MacConkey agar. However, P. aeruginosa is oxidase positive and grows at 42°C but is motile only by polar monotrichous flagella. S. maltophilia is oxidase negative, does not grow at 42°C, and is motile by a polar tuft of flagella

Answer 68

B P. aeruginosa is often recovered from the respiratory secretions of patients with CF. If the patient is chronically infected with the mucoid strain of P. aeruginosa, the biochemical identification is very difficult. The mucoid strain results from production of large amounts of alginate, a polysaccharide that surrounds the cell.

Answer 69

A P. stutzeri produces dry, wrinkled colonies that are tough and adhere to the media as well as smooth colonies. B. pseudomallei produces similar colony types but is distinguished by biochemical tests and susceptibility to the polymyxins. The colonies of P. stutzeri are buff to light brown because of the relatively high concentration of cytochromes.

Answer 70

D B. pseudomallei produces wrinkled colonies resembling P. stutzeri. Infections are usually asymptomatic and can be diagnosed only by serological methods. The organism exists in soil and water in an area of latitude 20° north and south of the equator (mainly in Thailand and Vietnam). Thousands of U.S. military personnel were infected with these bacteria during the 1960s and 1970s. The disease may reactivate many years after exposure and has been called the “Vietnamese time bomb.”

Answer 71

A Both organisms produce yellowish pigment and have polar tuft flagella, but the oxidase and DNase tests are differential. Test B. cepacia S. maltophilia Pigment on BAP Green-yellow Lavender-green Oxidase + Neg DNase Neg + Motility + + Glucose OF (open) + + Maltose OF (open) + + Lysine decarboxylase + +

Answer 72

D The oxidase test and red pigment (pyorubin), as well as growth at 42°C, distinguish P. aeruginosa from the other nonfermenters listed, particularly B. cepacia, which is also associated with CF.

Answer 73

A Alcaligenes spp. and Bordetella spp. (nonpertussis) are two organisms that are very similar biochemically, but B. bronchiseptica is urease positive. Both organisms are oxidase positive, grow on MacConkey agar, and are motile by peritrichous flagella. B. bronchiseptica grows well on MacConkey agar, but other species of Bordetella are fastidious gram-negative rods. B. bronchiseptica is the cause of mild respiratory infections, whereas B. pertussis is the cause of whooping cough

Answer 74

A Chryseobacterium spp. and Acinetobacter spp. often produce a yellow pigment on blood or chocolate agar and are nonmotile. Acinetobacter spp. are oxidase negative, grow on MacConkey agar, and are coccobacillary on the Gram-stained smear. In contrast, Chryseobacterium spp. are oxidase positive, do not grow on MacConkey agar, and are typically rod shaped. Elizabethkingia (formerly Chryseobacterium) meningosepticum is highly pathogenic for premature infants. The organism is transmitted to the neonate via the birth canal and is seen in outbreaks of neonatal units as well as long-term care facilities

Answer 75

D Moraxella spp. are oxidase positive and nonmotile, and this distinguishes them from Acinetobacter spp. and most Pseudomonas spp. Moraxella spp. are highly sensitive to penicillin, but Acinetobacter spp. and Pseudomonas spp. are penicillin resistant. M. lacunata is implicated in infections involving immunosuppressed patients. M. lacunata is usually associated with eye infections but can also be the cause of bacteremia and respiratory infections in the immunosuppressed patient.

Answer 76

B Growth on Cetrimide (acetyl trimethyl ammonium bromide) agar is used for the isolation and presumptive identification of P. aeruginosa. With the exception of P. fluorescens, the other pseudomonads are inhibited along with related nonfermentative bacteria. Colonies of P. aeruginosa appear as yellow-green to blue-green colonies on Cetrimide agar.

Answer 77

C P. aeruginosa is a cause of a significant number of burn wound infections; these organisms can exist in distilled water and underchlorinated water. Acinetobacter spp. are oxidase negative and Moraxella spp. are highly susceptible to penicillin, and this helps rule them out as possible causes

Answer 78

D All species of Acinetobacter are oxidase negative, are nonmotile, and grow on MacConkey agar. Elizabethkingia (formerly Chryseobacterium) spp. produce yellow pigment (like some Acinetobacter) but are oxidase positive and do not grow well on MacConkey agar. B. cepacia also produces a yellow pigment but is motile. P. aeruginosa is motile and grows on Mac agar

Answer 79

A In the nitrate test, nitrites formed by bacterial reduction of nitrates will diazotize sulfanilic acid. The diazonium compound complexes with α-naphthylamine, forming a red product. Media containing nitrates are used for the identification of nonfermenters. When testing nonfermenters, it is wise to confirm a negative reaction using zinc dust. The diazonium compound detects nitrite only, and the organism may have reduced nitrates to nitrogen, ammonia, nitrous oxide, or hydroxylamine. Zinc ions reduce residual nitrates in the media to nitrites. A red color produced after addition of zinc indicates the presence of residual nitrates, confirming a true negative reaction. If a red or pink color does not occur after adding zinc, then the organism reduced the nitrate to a product other than nitrite, and the result is considered positive

Answer 80

A S. putrefaciens produces abundant H2S on KIA or TSI. Shewanellae are the only nonfermenters that produce H2S on these media.

Answer 81

A B. cepacia (93%) are weakly oxidase positive and motile. Chryseobacterium spp. are oxidase positive but are nonmotile.

Answer 82

B P. aeruginosa is an opportunistic organism that is not part of the human normal flora. Contact lens solution contamination, eye injury, or contact lens eye trauma are factors that contribute to P. aeruginosa eye infections. The characteristic blue-green pigment on Mueller-Hinton agar (pyocyanin pigment) produced by P. aeruginosa and the high resistance to antibiotics aid in its identification.

Answer 83

D P. aeruginosa is highly resistant to many antimicrobial drugs as well as being one of the most often cultured opportunistic organisms. This virulence factor allows for many nosocomial infections, such as UTI, wounds (in burn patients), bacteremia, respiratory tract infection, and CNS infection

Answer 84

A B. mallei is rarely transmitted to humans. It is the causative agent of glanders in mules, donkeys, and horses. It is not part of the human skin flora, and the most likely transmission to humans is through broken skin

Answer 85

B B. pseudomallei infections often produce abscesses in organs (liver, spleen, lungs) as well as on the skin, in soft tissue, and in joints and bones. Vietnam War veterans especially may harbor these organisms, which are limited to tropical and subtropical environments (Southeast Asia and Australia). The organism may surface years later after surviving in a latent state within phagocytes. There is a high prevalence of this organism on the surfaces of rice paddies in northern Thailand.

Answer 86

D Patients with CF usually do not escape P. aeruginosa infections completely. P. aeruginosa produces alginate that accounts for the “wet, mucoidy” appearance of colonies. This overproduction of alginate is thought to cause the inhibition of phagocytosis. The light blue color results from lower production of polyamine, and the wet or mucoidy appearance is caused by overproduction of alginate. The result is chronic infections in patients with CF with the “wet” form of P. aeruginosa.

Answer 87

A Acinetobacter spp. often colonize immunocompromised patients through intravenous or urinary catheters. Acinetobacter spp. are nonmotile, oxidase negative, coccobacilli, which grow well on MacConkey agar.

Answer 88

B S. maltophilia are oxidase negative, whereas all the other choices are oxidase positive (B. gladioli are weakly oxidase positive).

Answer 89

C The growth of yellow or green colonies on the selective TCBS agar is dependent on whether the organism ferments sucrose (producing yellow colonies). Vibrio spp. also grow well on 5% sheep blood, chocolate, and MacConkey agars. Enrichment with alkaline peptone broth, pH 8.4, helps recover Vibrio spp. from stool specimens.

Answer 90

B V. parahaemolyticus appear as green colonies on TCBS agar, whereas V. cholerae appear as yellow colonies on TCBS. V. cholerae is the only Vibrio species that causes a positive string test result. In the test, a loopful of bacterial colonies is suspended in sodium deoxycholate, 0.5%, on a glass slide. After 60 seconds, the inoculating loop is lifted out of the suspension. V. cholerae forms a long string resembling a string of pearls. Salmonella spp. and Shigella spp. are oxidase negative

Answer 91

B The only Campylobacter spp. that hydrolyze hippurate are C. jejuni and subsp. doylei. However, some strains of P. aeruginosa grow on agar selective for Campylobacter at 42°C. C. fetus usually will not grow at 42°C but will grow at 25°C and 37°C.

Answer 92

A Campylobacter spp. are best recovered in a microaerophilic atmosphere (reduced O2). The use of a CO2 incubator or candle jar is not recommended because the amount of O2 and CO2 do not permit any but the most aerotolerant Campylobacter to survive. Cultures for Campylobacter should be incubated for 48 to 72 hours before reporting no growth.

Answer 93

D H. pylori is found in specimens from gastric secretions and biopsies and has been implicated as a cause of gastric ulcers. It is found only in the mucous-secreting epithelial cells of the stomach. Both H. pylori and C. jejuni are catalase and oxidase positive. However, Helicobacter spp. are urease positive, which differentiates them from Campylobacter spp. Test H. pylori C. jejuni Nitrate reduction Neg + Hippurate hydrolysis Neg + Urease + Neg Cephalothin sensitivity Sensitive Resistant Nalidixic acid sensitivity Resistant Sensitive

Answer 94

C Aeromonas hydrophilia and other Aeromonas spp. have been implicated in acute diarrheal disease as well as cellulitis and wound infections. Infections usually follow exposure to contaminated soil, water, or food. Aeromonas growing on enteric media are differentiated from the Enterobacteriaceae species by demonstrating that colonies are oxidase positive. The Aeromonas are sometimes overlooked as pathogens because most strains grow on selective enteric agar as lactose fermenters.

Answer 95

A P. shigelloides is an NLF that will resemble Shigella spp. on MacConkey agar. Both are TSI Alk/Acid and urease negative. Plesiomonas produces indole and Shigella usually causes delayed production of indole. However, Plesiomonas is oxidase positive, whereas Shigella spp. are oxidase negative. P. shigelloides has been added to the Enterobacteriaceae family through the use of nucleic acid–based methods and is the only member that is oxidase positive

Answer 96

B Both these bacteria cause diarrhea, grow well on enteric agar, and may be confused with other pathogenic gram-negative rods. Both organisms are positive for oxidase, motility, indole, and lysine decarboxylase. The following reactions are differential: Test A. hydrophilia P. shigelloides β-Hemolysis + Neg on sheep blood agar DNase + Neg Voges-Proskauer + Neg

Answer 97

A Aggregatibacter spp. (formerly Actinobacillus spp.) and formerly Centers for Disease Control and Prevention (CDC) groups HB-3 and HB-4 share many biochemical characteristics of the Haemophilus spp. Infections most often associated with this gram-negative coccobacillus are subacute bacterial endocarditis and periodontal disease (its main habitat is the mouth). The most common human isolate is Aggregatibacter (formerly Actinobacillus) actinomycetemcomitans, which grows slowly on chocolate agar. It is positive for catalase, nitrate reduction, and glucose fermentation. It does not grow on MacConkey agar and is negative for oxidase, urease, indole, X, and V requirements.

Answer 98

B C. hominis (indole positive) is a gram-negative coccobacillus biochemically similar to Aggregatibacter (Actinobacillus) spp. (indole negative). Like Aggregatibacter (formerly Actinobacillus) spp., it is a cause of endocarditis. However, Cardiobacterium spp. are positive for cytochrome oxidase and negative for nitrate reduction and catalase, whereas most Aggregatibacter (formerly Actinobacillus) spp. are negative for oxidase and positive for nitrate reduction and catalase. C. hominis will grow on blood agar after 48 to 72 hours in 5% CO2 at 35°C, but Aggregatibacter (formerly Actinobacillus) requires chocolate agar.

Answer 99

A E. corrodens is a part of the normal flora of the upper respiratory tract and the mouth. It is often seen after trauma to the head and neck, dental infections, and human bite wounds. It requires blood for growth. The organism causes pits in the agar, where colonies are located. The smell of bleach may be apparent when the plates are uncovered for examination. Aggregatibacter (formerly Actinobacillus) spp. and C. hominis both utilize several carbohydrates, and Proteus spp. are oxidase negative

Answer 100

A Both K. kingae and E. corrodens are gram-negative rods that are oxidase positive and catalase negative. Both grow well on blood and chocolate agars and cause pitting of the media, and neither grows on MacConkey or XLD agar. However, K. kingae strains produce a narrow zone of β-hemolysis on sheep blood agar similar to that of group B streptococci.

Answer 101

B Kingella spp. are gram-negative coccobacilli or plump-looking rods. They are part of the normal flora of the upper respiratory and urogenital tracts of humans. Infection is seen primarily in patients having underlying heart disease, poor oral hygiene, or iatrogenic mucosal ulcerations (e.g., radiation therapy), in whom the organism is recovered from blood cultures.

Answer 102

C The Capnocytophaga species C. gingivalis, C. sputigena, and C. ochracea are part of the normal oropharyngeal flora of humans; however, C. canimorsus and C. cynodegmi (formerly CDC groups DF-2 and DF-2-like bacteria) are associated with infections resulting from dog bite wounds and cat bites as well as scratch wounds.

Answer 103

B Pasteurella multocida (P. canis) is part of the normal mouth flora of cats and dogs and is frequently recovered from wounds inflicted by them. It produces large amounts of indole and therefore an odor resembling that of colonies of E. coli. Pseudomonas spp. are also catalase and oxidase positive but can be ruled out because it grows on MacConkey agar and does not produce indole.

Answer 104

D B. pertussis is an oxidase-positive, nonmotile, gram-negative coccobacillus and appears as small, round colonies resembling droplets of mercury on Bordet-Gengou media with sheep blood agar. It is fastidious and does not grow on chocolate or MacConkey agar. However, B. pertussis adapts to blood agar, growing within 3 to 6 days. This organism is the cause of whooping cough, which can be prevented by immunization with the diphtheria, tetanus, pertussis (DPT) vaccine. The DPT vaccine contains diphtheria and tetanus toxoids and killed whole-cell B. pertussis.

Answer 105

A Although blood agar will support the growth of Brucella spp., Castenada bottles are the medium of choice. Castenada bottles contain a slant of enriched agar medium that is partially submerged and surrounded by an enriched broth medium. As the specimen is injected into the bottles and mixed, the agar slant is simultaneously coated with the blood (or bone marrow). Brucella is the cause of undulant fever (acquired by ingesting infected, unpasteurized milk products) and is responsible for many cases of fever of unknown origin. Brucella spp. are facultative intracellular (multiply in phagocytic cells) organisms and grow very slowly, usually requiring 4 to 6 weeks for recovery. Brucella melitensis is the most frequently recovered species.

Answer 106

A B. abortus can be differentiated from B. melitensis by the following reactions: H2S Urease CO2 Requirement Basic Fuchsin Thionin (20 mg) B. melitensis Neg V Neg + growth + growth B. abortus + + +/V + growth (Neg) No growth

Answer 107

C F. tularensis (the causative agent of tularemia) is a fastidious gram-negative rod that is best recovered from lymph node aspirates and tissue biopsies. It is oxidase negative, nonmotile, and inert biochemically. Cysteine blood agar is the medium of choice, but F. tularensis will grow on commercially prepared chocolate agar because it contains X factor and is supplemented with a growth enrichment (IsoVitaleX) that contains cysteine. F. tularensis may not grow well on MacConkey agar.

Answer 108

D L. pneumophila should be recovered on buffered CYE agar. This agar is nonselective but can be made more selective for Legionella spp. by addition of the antibiotics cefamandole, polymixin B, and anisomycin. Any small, glistening, convex colonies on buffered CYE agar after 2 to 3 days of incubation that do not grow on L-cysteine– deficient buffered CYE agar or routine nonselective media should be further tested by the direct fluorescent antibody (DFA) test for confirmation of L. pneumophila. Rapid detection by using enzyme immunoassay (EIA) and immunochromotographic methods in tissue and urine has proven successful, as well as polymerase chain reaction (PCR) for respiratory specimens.

Answer 109

A Both H. influenzae and H. aegyptius are glucose, urease, oxidase, and catalase positive. H. influenzae (biotype II) is positive for both indole and xylose, whereas H. aegyptius is negative for both tests. Biotype II encompasses 40% to 70% of H. influenzae strains recovered from clinical specimens. H. aegyptius is responsible for epidemics of conjunctivitis in children.

Answer 110

C The V factor, NAD, must first be released from RBCs before it can be assimilated by Haemophilus spp. Chocolate agar is made by heating blood agar to lyse RBCs. The released NAD is directly available to those Haemophilus species requiring it. Chocolate agar also contains the X factor (hemin). All Haemophilus except H. ducreyi and H. aphrophilus (now called Aggregatibacter aphrophilus) require V factor, whereas X factor is required by H. influenzae, H. haemolyticus, and H. ducreyi.

Answer 111

A Staphylococcus growing on sheep blood agar produce NAD (V factor) as a metabolic by-product and β-hemolysins which lyse sheep RBCs, releasing hemin (X factor). This satellite feature allows pinpoint-sized colonies of Haemophilus spp. to grow around Staphylococcus colonies. Sheep blood agar alone does not support the growth of Haemophilus spp., which require V factor because of the presence of V factor– inactivating enzymes that are present in the agar.

Answer 112

B Production of β-hemolysis is used to distinguish these two species from other Haemophilus with the same X and V requirements. Horse blood agar furnishes X factor and, when supplemented with yeast extract, supports the growth of Haemophilus spp. Sheep blood agar is not used because it contains growth inhibitors for some Haemophilus spp. The chart at the top of the page summarizes the characteristics of the Haemophilus spp.

Answer 113

B The majority of H. influenzae infections occur in children under 5 years of age and are caused by capsular serotype b (Hib), one of six serotypes designated a through f. This capsular strain appears to contain a virulence factor that makes it resistant to phagocytosis and intracellular killing by neutrophils. The nontypable strains do not have a capsule and are part of the normal respiratory flora. Serotyping of Haemophilus is performed by mixing colonies with agglutinating antibodies available as commercial agglutination kits. Identification is also done by using PCR.

Answer 114

C A. (H.) aphrophilus does not require either X or V factor for growth and is differentiated from the other Haemophilus species by its ability to produce acid from lactose and a positive ALA test. H. influenzae and H. haemolyticus are incapable of synthesizing protoporphyrin from Δ-ALA and are negative for this test.

Answer 115

B H. ducreyi requires exogenous X factor and causes genital lesions referred to as “soft chancres.” It is difficult to grow and requires commercial chocolate agar or gonococcus base medium containing 1% to 2% hemoglobin, 5% fetal calf serum, and 1% IsoVitaleX enrichment. The plates must be incubated at 33oC in a 3% to 5% CO2 moist environment for 2 to 3 days. Recovery of H. ducreyi from specimens constitutes a reportable sexually transmitted disease (chancroid).

Answer 116

A Roughly 20% of H. influenzae strains produce β-lactamase, which hydrolyses and inactivates the β-lactam ring of ampicillin (and penicillin).

Answer 117

B H. influenza is the main cause of meningitis in young, unvaccinated children. Although several biotypes of H. parainfluenzae produce indole and urease, H. parainfluenzae does not require X factor for growth. H. ducreyi requires X factor, but not V factor. A. (H.) aphrophilus does not require either X factor or V factor for growth.

Answer 118

A The X factor requirement for growth is the cause of many inaccuracies when identifying Haemophilus spp. requiring this factor. False-negative results have been attributed to the presence of small amounts of hemin in the basal media, or X factor carryover from colonies transferred from primary media containing blood. The ALA test determines the ability of an organism to synthesize protoporphyrin intermediates in the biosynthetic pathway to hemin from the precursor compound Δ-ALA. Haemophilus species that need exogenous X factor to grow are unable to synthesize protoporphyrin from Δ-ALA and are negative for the ALA test. These include H. influenzae, H. haemolyticus, H. aegyptius, and H. ducreyi.

Answer 119

A Bartonella spp., frequently the cause of zoonoses infections, are difficult to grow on primary culture media because they are facultative intracellular organisms. Bartonella henselae is the organism found in domestic cats, the main reservoir. When CSD is suspected from the patient’s history, blood cultures should be smeared and Gram staining performed. Bartonella spp. are biochemically inert—that is, they are negative for oxidase, catalase, indole, and urease tests. Therefore, commercial identification systems, DNA amplification for various genes, and indirect immunofluorescence assays (IFAs) are used to identify these organisms

Answer 120

B B. pertussis, the cause of whooping cough, is highly contagious during the 5- to 10- day period after acquisition. The incidence of whooping cough is greater in nonimmunized individuals, and therefore, is higher in children ages less than 1 year. B. bronchiseptica is only rarely found in humans, but may cause respiratory disease in animals. Unlike B. pertussis it is positive for nitrite, urease, and motility. A direct identification by DFA using polyclonal antibody to detect B. pertussis in smears from nasopharyngeal material is the quickest method and less costly compared with PCR.

Answer 121

D Persons handling samples suspected of containing F. tularensis must wear gloves and use a biological safety cabinet (follow biosafety Level [BSL]-2 controls). For cultures, BSL-3 controls must be followed. Tularemia (rabbit fever or deer fly fever) is one of the most common laboratory-acquired infections, and it is recommended that specimens be sent to a reference laboratory for identification and serological testing. F. tularensis requires cysteine and cystine to grow. It may grow on chocolate agar supplemented with IsoVitaleX and also on charcoal yeast extract agar used to isolate Legionellae

Answer 122

B Elizabethkingia (formerly Chryseobacterium) meningosepticum can cause septicemia and meningitis in neonates and immunocompromised adults. The ability to encapsulate, produce proteases, and survive in chlorinated tap water are factors that contribute to hospital-acquired infections with this bacterium.

Answer 123

A C. canimorsus are part of the oral flora of dogs. The organisms require at least 5% CO2 for growth and grow slowly on blood and chocolate agars. Colonies can grow in 48 hours if cultured in high CO2 on BHI agar with 5% sheep blood.

Answer 124

B Blood cultures growing small, gram-negative rods should alert the microbiologist to the possibility of infection with one of the five AACEK (formerly HACEK) organisms. Although responsible for less than 5% of bacterial endocarditis overall, greater than 50% of endocarditis cases caused by gram-negative rods result from one of them.

Answer 125

D Legionella spp. stain poorly, if at all, with Gram stain. Legionella pneumophilia is not acid fast, although L. micdadei, which accounts for a small percentage of Legionella pneumonia infections, is acid-fast positive. Specimens suspected of containing Legionella spp. should be handled in a Class II biological safety cabinet. Legionella spp. require buffered charcoal–yeast extract (BCYE) agar for growth and will not grow on MacConkey agar. Because culture can take up to 10 days, rapid diagnosis by direct immunofluorescence (DIF) and DNA amplification are preferred. Direct fluorescent antibody (DFA) tests are not as sensitive as culture or PCR but are specific and can be used to rapidly confirm a positive serological test, which may be positive in the absence of disease. MALDI-TOF MS can be used to identify isolates, but this method cannot distinguish serotypes. PCR methods using respiratory specimens directly, (without culture for the identification of Legionella spp.) are 99% specific and 85% sensitive.

Answer 126

A Tissue specimens are immersed in urea broth or placed in urea agar. A positive test indicates the presence of urease produced by H. pylori. Another method is the urea breath test, which is less invasive

Answer 127

B AACEK (formerly HACEK) denotes the following slow-growing gram-negative bacilli causing subacute bacterial endocarditis: Aggregatibacter aphrophilus; Aggregatibacter actinomycetemcomitans; Cardiobacterium hominis; Eikenella corrodens; and Kingella kingae

Answer 128

B H. ducreyi may take up to 7 days to grow on chocolate agar (requiring X factor for growth) and preferably at 33°C with high humidity. Testing for chancroid disease requires specialized media

Answer 129

C To separate C. jejuni subsp. jejuni from C. jejuni subsp. doylei (both are positive for hippurate hydrolysis and grow at 37°C and 42°C), it is best to use cephalothin (30-μg disk) and nalidixic acid (30-μg disk) antibiotic testing. C. jejuni subsp. jejuni are resistant to cephalothin and susceptible to nalidixic acid. C. jejuni subsp. doylei are susceptible to both disks

Answer 130

A B. pertussis produces a virulence factor, pertussis toxin (PT). The organism attaches to the respiratory ciliated epithelial cells of the upper respiratory tract, causing coughing and irritation. The recommended method for identification of the toxin in serum is by ELISA using paired samples of acute and convalescent sera. PCR on nasopharyngeal swabs as well as DFA tests are available. MALDI-TOF MS can also be used successfully for identification

Answer 131

B The catalase test (utilizing a 3% hydrogen peroxide [H2O2] solution stored in a brown bottle under refrigeration) is positive for the Staphylococcus and Micrococcus spp. Members of the Streptococcaceae family are negative. Staphylococcus spp. Micrococcus spp. Open tube (oxidation) + + Closed tube (fermentation) + Neg

Answer 132

A Both micrococci and staphylococci are catalase-positive and gram-positive cocci. On direct smears, they both appear as pairs, short chains (resembling Streptococcus spp.), or clusters. However, the micrococci fail to produce acid from glucose under anaerobic conditions. The OF tube reactions are provided in the table above.

Answer 133

C Lysostaphin is an endopeptidase that cleaves the glycine-rich pentapeptide cross bridges in the staphylococcal cell wall peptidoglycan. The susceptibility of the staphylococci to lysostaphin (less than 200 μg/mL) is used to differentiate them from the micrococci. Staphylococci are susceptible and show a 10- to 16-mm zone of inhibition, whereas micrococci are not inhibited and will grow up to the disk.

Answer 134

D The slide coagulase test using rabbit plasma with ethylenediaminetetraacetic acid (EDTA) detects bound coagulase or “clumping factor” on the surface of the cell wall, which reacts with the fibrinogen in the plasma. This test is not positive for all strains of S. aureus, and a negative result must be confirmed by the tube method for detecting “free coagulase” or extracellular coagulase. The tube test is usually positive within 4 hours at 35°C; however, a negative result must then be incubated at room temperature for the remainder of 18 to 24 hours. Some strains produce coagulase slowly or produce fibrinolysin, which dissolves the clot at 35°C. Latex agglutination procedures utilize fibrinogen and immunoglobulin G (IgG)-coated latex beads that detect protein A on the staphylococcal cell wall

Answer 135

B In addition to coagulase, the virulence of S. aureus is attributed to hyaluronidase, which damages the intercellular matrix (basement membrane) of tissues. β-Lactamase– producing strains are able to inactivate penicillin and ampicillin, making the organism resistant to these antibiotics. Lecithinase is not produced by S. aureus, and urease is not a virulence factor.

Answer 136

C S. aureus is the organism most often recovered from female patients. These strains produce toxic shock syndrome toxin 1 (TSST-1). Toxic shock syndrome is attributed to the use of certain highly absorbent tampons by menstruating females. The toxin is also recovered from sites other than the genital area and produces fever and lifethreatening systemic damage as well as shock.

Answer 137

A S. intermedius may be falsely identified as S. aureus in dog bite wounds because it infects mammals and certain birds, but not usually humans. Cases involving humans result from animal bites and are most often seen in persons who work closely with animals. S. intermedius may give a positive latex agglutination coagulase test

Answer 138

D S. epidermidis represents 50% to 80% of all coagulase-negative Staphylococcus spp. recovered from numerous clinical specimens. It is of special concern in nosocomial infections because of its high resistance to antibiotics.

Answer 139

B S. epidermidis produces an extracellular slime (exopolysaccharide or biofilm) that enhances the adhesion of these organisms to indwelling plastic catheters. The slime production is considered a virulence factor and is associated with infections from prostheses.

Answer 140

A Methicillin-resistant S. aureus (MRSA) and methicillin-resistant S. epidermidis (MRSE) are called heteroresistant. This refers to two subpopulations in a culture, one that is susceptible and the other that is resistant to antibiotic(s). The resistant population grows more slowly than the susceptible one and can be overlooked. Therefore, the more resistant subpopulation should be promoted growthwise by using a neutral pH (7.0–7.4), cooler incubation temperatures (30°C–35°C), the addition of 2% to 4% NaCl, and incubation up to 48 hours. For identification of MRSA, molecular assays (using amplified DNA) and MALDI-TOF MS (used directly from blood cultures) have proven successful.

Answer 141

B S. saprophyticus is coagulase negative and resistant to 5 μg of novobiocin. When using the standardized Kirby-Bauer sensitivity procedure, a 6- to 12-mm zone of growth inhibition is considered resistant. Susceptible strains measure 16 to 27 mm (inhibition) zones

Answer 142

C S. aureus can produce fibrinolysins that dissolve the clot formed by the coagulase enzyme. The tube method calls for an incubation of 4 hours at 35°C to 37°C and 18 to 24 hours at room temperature. Both must be negative to interpret the result as coagulase negative. This organism is coagulase positive and, therefore, identified as S. aureu

Answer 143

D Vancomycin, along with rifampin, is used for strains of S. aureus that are resistant to the β-lactams. MRSA strains pose problems when reading the zone sizes for these strains. Their heteroresistance results in a film of growth consisting of very small colonies formed within the defined inhibition zone surrounding the antibiotic disk. Initially, this appears as a mixed culture or contaminant

Answer 144

A The production of acetoin by S. aureus from glucose or pyruvate differentiates it from S. intermedius, which is also coagulase positive (positive for clumping factor and free coagulase or tube test). This test is the Voges-Proskauer (VP) test. Acetoin production is detected by addition of 40% KOH and 1% α-naphthol to the VP test broth after 48 hours of incubation. A distinct pink color within 10 minutes denotes a positive test result.

Answer 145

C Micrococcus spp. utilize glucose oxidatively but not under anaerobic conditions (sealed tube). Staphylococcus spp. utilize glucose oxidatively and anaerobically. The catalase differentiates the Micrococcaceae family (positive) from the Streptococcaceae family (negative).

Answer 146

D S. epidermidis and S. saprophyticus are the two possibilities because they are both catalase positive, coagulase negative, urease positive, and ferment lactose. Novobiocin susceptibility is the test of choice for differentiating these two species. S. epidermidis is sensitive, but S. saprophyticus is resistant to 5 μg of novobiocin. UTIs caused by S. saprophyticus are often seen in cultures from sexually active young females.

Answer 147

D S. aureus is novobiocin sensitive and cannot be ruled out by a negative clumping factor test. Most S. aureus produce β-hemolysis on sheep blood agar plates and are mannitol salt positive (produce acid and are not inhibited by the high salt concentration). The tube test (to detect free coagulase) should be performed because the slide test was negative.

Answer 148

A Staphylococci are susceptible to furazolidone, giving zones of inhibition that are 15 mm or greater. Micrococcus spp. are resistant to furazolidone, giving zones of 6 to 9 mm. The test is performed as a disk susceptibility procedure using a blood agar plate

Answer 149

A A bacitracin disk (0.04 unit) is used to identify group A β-hemolytic streptococci, but it will also differentiate catalase-positive organisms. A zone of 10 mm or greater is considered susceptible. The Staphylococcus species are resistant and grow up to the disk, whereas Micrococcus species are sensitive.

Answer 150

C The modified oxidase test (Microdase) is used to rapidly identify catalase-positive gram-positive cocci as Micrococcus spp. (positive) or Staphylococcus spp. (negative). Filter paper disks that are saturated with oxidase reagent (tetramethyl-pphenylenediamine in dimethylsulfoxide) are used. A colony of the isolate is rubbed onto the paper. Oxidase-positive organisms produce a purple color within 30 seconds or a blue color within 2 minutes if using the commercial product Microdase.

Answer 151

B Streptococcus spp. are facultative anaerobes that grow aerobically as well and are oxidase and catalase negative. To demonstrate streptolysin O on blood agar, it is best to stab the agar to create anaerobiosis because streptolysin O is oxygen labile.

Answer 152

A Group A β-hemolytic streptococci are the cause of scarlet fever, and some strains produce toxins (pyrogenic exotoxins A, B, and C) that cause a scarlatiniform rash. Streptococcus pyogenes is the cause of puerperal fever, streptococcal toxic shock, rheumatic fever, and a severe “flesh-eating” bacterial infection.

Answer 153

A The antistreptolysin O (ASO) titer is used to indicate a recent infection with group A β-hemolytic streptococci. Streptolysin O may also be produced by some strains of groups C and G streptococci

Answer 154

A The bacitracin A disk test is no longer used, but the PYR, hippurate hydrolysis, and CAMP tests are used for the presumptive identification of the β-hemolytic streptococci. In addition to group A, groups C, F, and G are also β-hemolytic and give a positive test result for bacitracin (a zone of inhibition of any size). Therefore, a positive bacitracin A disc test does not confirm the presence of group A β-hemolytic streptococci. Group A strep test positive for PYR but negative for hippurate and the CAMP test. Group B streptococci test negative in the PYR test but positive in the hippurate and CAMP tests. Groups C, G and F test negative in all three tests.

Answer 155

B SXT is inhibitory to most streptococci except S. pyogenes and Streptococcus agalactiae. For this reason, SXT is used in a commercially available streptococcal selective agar (SSA) as a primary plating agar for the detection of group A streptococci. Latex agglutination test kits are used for the rapid detection of β- hemolytic streptococci Lancefield groups A, B, C, F, and G.

Answer 156

D Streptococci that are not group A or B may be either resistant or susceptible to bacitracin but are usually susceptible to SXT. β-Hemolytic Streptococci Bacitracin Sulfamethoxazole-Trimethoprim Group A Susceptible Resistant Group B Resistant Resistant Non-A, non-B Susceptible or resistant Susceptible

Answer 157

A The CAMP test (described by Christie, Atkins, and Munch-Petersen in 1944) refers to a hemolytic interaction (flaming arrowhead) that is seen on a blood agar plate between the β-hemolysins produced by most strains of S. aureus and an extracellular protein produced by both hemolytic and nonhemolytic isolates of group B streptococci. In a CAMP test, the plate must be placed in an ambient air incubator at 35°C to 37°C. Group A streptococci may be CAMP positive if the plate is incubated in a candle jar, high CO2 atmosphere, or anaerobically.

Answer 158

D The bile esculin test differentiates those bacteria that can hydrolyze esculin and also grow in the presence of 4% bile salts or 40% bile. The bile esculin slant or plate is inoculated on the surface and incubated for 24 to 48 hours in a non-CO2 incubator. Group D streptococci (enterococci and nonenterococci) are positive, causing blackening of half or more of the slant within 48 hours. Viridans streptococci are negative (do not grow or hydrolyze esculin).

Answer 159

B The bile solubility test can be performed directly by dropping 2% sodium deoxycholate onto a few well-isolated colonies of S. pneumoniae. The bile salts speed up the autolysis observed in pneumococcal cultures. The colonies lyse and disappear when incubated at 35°C for 30 minutes, leaving a partially hemolyzed area on the plate. The same phenomenon can be seen using a broth culture; addition of 10% deoxycholate to broth containing S. pneumoniae results in visible clearing of the suspension after incubation at 35°C for 3 hours.

Answer 160

A Optochin (P disk) at a concentration of 5 μg/mL or less inhibits the growth of S. pneumoniae but not viridans streptococci. However, optochin at a concentration in excess of 5 μg/mL inhibits viridans streptococci as well if incubated under 5% CO2 at 35°C for 18 to 24 hours. A zone of inhibition of 14 mm or more around the 6-mm disk is considered a presumptive identification of S. pneumoniae. A questionable zone size should be confirmed by performing a bile solubility test

Answer 161

D E. faecalis will grow in 6.5% salt, and the nonenterococci (S. bovis and S. equinus) will not. This test distinguishes the enterococci group from S. bovis and S. equinus (nonenterococci group). Both groups grow on bile esculin agar. Viridans streptococci are negative for 6.5% salt growth and will not grow on bile esculin agar

Answer 162

B Approximately 80% of group B streptococci are capable of growing in 6.5% salt broth; however, they do not hydrolyze esculin or grow in media containing 4% bile salts.

Answer 163

D A precipitin reaction seen microscopically with methylene blue stain (microprecipitin reaction) occurs between the carbohydrate of the capsule of S. pneumoniae and anticapsular antibody. The antibody may be type specific or polyvalent. Binding of antibodies to the bacteria causes the capsule to swell, identifying the organisms as S. pneumoniae. Antigen detection kits are used to identify S. pneumoniae recovered from CSF and blood cultures. S. pneumoniae is the cause of 95% of the bacterial pneumonias in unvaccinated children and the leading cause of bacterial meningitis

Answer 164

A The PYR hydrolysis test is highly specific for group A streptococci and Enterococcus spp. The test detects the pyrrolidonylarylamidase enzyme, which hydrolyzes PYR

Answer 165

D The β-hemolytic streptococci—not of groups A or B—are sensitive to SXT and may be either sensitive or resistant to bacitracin. Groups A and B are both resistant to SXT. Group A and E. faecalis are PYR positive. E. faecalis is also positive for bile esculin and 6.5% salt broth. Latex agglutination tests identify β-hemolytic groups A, B, C, F, and G

Answer 166

C VRE (Vancomycin-Resistant Enterococcus) denotes E. faecalis and Enterococcus facium, which have shown resistance to most antimicrobials used to treat infections caused by gram-positive cocci (ampicillin, methicillin and other β-lactams) and therefore have mutated and now are resistant to vancomycin. Group A streptococci are sensitive to bacitracin and negative for bile esculin and 6.5% salt broth. Group B streptococci will grow in 6.5% salt broth but are negative for bile esculin and PYR. The non–group A, non–group B streptococci will not grow in 6.5% salt broth and are sensitive to SXT. E. faecalis is positive for bile esculin, 6.5% salt broth, and PYR.

Answer 167

D The staphylococcal streak, across the NVS inoculum, provides the nutrients needed. Very small colonies of NVS can be seen growing adjacent to the staphylococcal streak on the blood agar plate in a manner similar to the satellite phenomenon of Haemophilus spp. around S. aureus. NVS are normal flora of the oral cavity but may cause life-threatening infections when introduced into sterile areas (cause of endocarditis). Blood culture media will supply the necessary nutrients for their recovery.

Answer 168

A E. faecalis is resistant to penicillin and ampicillin as well as some of the aminoglycoside antibiotics. Pneumococci, group B streptococci, and S. bovis are PYR negative

Answer 169

B S. bovis and S. salivarius are physiologically and biochemically similar. They are both PYR and 6.5% salt broth negative and bile esculin positive, but only S. bovis is positive for mannitol and starch reactions. See the following chart: Bacitracin PYR BE 6.5% Salt Mannitol Starch S. bovis R Neg + Neg + + S. salivarius R Neg + Neg Neg Neg

Answer 170

B Group B streptococci (S. agalactiae) are resistant to both bacitracin and SXT. Unlike group A and E. faecalis, the group B streptococci are negative for PYR. With some exceptions, group B streptococci will grow in 6.5% salt broth. Group B strep infections in newborns are the result of premature rupture of the mother’s membranes. The infection in a newborn happens within the first 5 days after birth and results in fever, sepsis, meningitis, and respiratory distress

Answer 171

A Both N. gonorrhoeae and N. meningitidis grow selectively on MTM as a result of the addition of vancomycin and colistin, which inhibit gram-positive and gram-negative bacteria, respectively. Trimethoprim is added to inhibit swarming of Proteus spp. because a rectal swab may be used for culture. Nystatin and amphotericin B are used to prevent growth of yeasts and molds from vaginal specimens. N. lactamica will grow on MTM but is rarely involved in human infection.

Answer 172

D Upon subculture from a primary plate, various sizes and appearances of gonococci result from multiple nutritional requirements, such as arginine–hypoxanthine–uracil (AHU)–requiring strains. Colony size and coloration (or light reflection) are the basis of the Kellogg scheme (types T1 through T5). Types T1 and T2 have pili on the surface, whereas T3, T4, and T5 do not. Transparent media are not used routinely, but opaque and transparent colonial differences of the gonococci can be seen when using it.

Answer 173

D All of the listed Neisseria spp. grow on MTM and are oxidase positive. N. lactamica is a nonpathogenic component of normal throat flora resembling N. meningitidis, but it grows well on selective MTM agar. Presumptive identification of N. meningitidis or N. gonorrhoeae is stated only if the source of the specimen (i.e., urogenital or CSF) is given. The identification must be confirmed by further testing, such as carbohydrate utilization tests, DNA tests, or rapid latex slide agglutination tests.

Answer 174

A N. gonorrhoeae colonies recovered from selective MTM media give an immediate 4+ positive reaction (bubbling) when 30% H2O2 is added. The catalase test uses 3% H2O2. This is a presumptive test for N. gonorrhoeae. N. meningitidis and N. lactamica give a weak or delayed bubbling reaction. M. catarrhalis is catalase positive, whereas the Superoxol reactions are negative, weak, or delayed and it shows a variable growth pattern on MTM.

Answer 175

B Moraxella spp. are oxidase and catalase positive, as are the gonococci. Neisseria spp. and M. catarrhalis will keep their typical coccal morphology after overnight incubation on blood agar with a 10-unit penicillin disk (CO2 incubation). Other Moraxella spp. form long filaments or long spindle-shaped cells when grown near a 10-unit penicillin disk

Answer 176

A Gram staining of urethral discharge (in men only) that shows typical gonococcal cells in PMNs should be reported “presumptive N. gonorrhoeae, confirmation to follow.” With female patients, the normal vaginal flora contain gram-negative cocci and diplococci resembling gonococci, and therefore, no presumptive identification should be reported for N. gonorrhoeae from the vaginal Gram-stained smear.

Answer 177

A N. lactamica utilizes lactose by producing the enzyme β-galactosidase. All other Neisseria spp. that grow on MTM media are lactose negative.

Answer 178

C CTA agar with 1% carbohydrate and phenol red pH indicator added is used for the identification of Neisseria species. CTA carbohydrates must be placed in an ambient air incubator because a high CO2 concentration may reduce the pH, causing a falsepositive (acid) result. The utilization of carbohydrates by some fastidious gonococcal strains may take up to 72 hours to produce a color change in the pH indicator is rarely used because identifications are done by rapid biochemical, immunologic, or enzymatic methods as well as with nonamplified DNA probes.

Answer 179

D N. lactamica is part of the normal vaginal and throat flora and is the only Neisseria species that grows on MTM that utilizes lactose. Other saprophytic Neisseria spp. may utilize lactose but do not grow on MTM media.

Answer 180

A M. catarrhalis is part of the normal upper respiratory flora but is implicated in lower respiratory infections, including pneumonia. It produces stunted growth on MTM and is DNase positive, characteristics that differentiate it from the other saprophytic Neisseria spp.

Answer 181

A Oxacillin is the drug used to screen staphylococci for resistance to antibiotics having the β-lactam ring. Included in this group are penicillin, cephalosporin, monobactam, and carbapenem. MRSA defines strains of staphylococci that are resistant to all of these antibiotic groups. MRSA strains are treated with vancomycin or oxazolidinone.

Answer 182

D The standard Kirby-Bauer method used for disk diffusion susceptibility testing recommended by the Clinical and Laboratory Standards Institute (CLSI) is 20 mm. The test requires 20- to 24-hour incubation at 35°C in 5% to 7% CO2. If the zone size is 19 mm, an MIC should be performed on isolates from CSF or blood.

Answer 183

C A test for β-lactamase production should be performed on all isolates of Staphylococcus and Enterococcus as well as N. gonorrhoeae and H. influenzae. Some organisms may appear to be penicillin/ampicillin susceptible by the conventional Kirby-Bauer disk diffusion method for β-lactamase producers, but still produce small amounts of β-lactamase. An example is heteroresistant mecA-positive S. aureus (the mecA gene confers penicillin resistance and is present in all MRSA strains).

Answer 184

B By using a 15-μg erythromycin disk adjacent to a 2-μg clindamycin disk in a disk diffusion procedure, a flattening of the clindamycin zone occurs in inducible clindamycin resistance, causing the zone to look like the letter “D.” This is referred to as being D-zone positive. The CLSI recommends that inducible clindamycin resistant strains be reported as resistant, with a comment that clindamycin may still be effective in some patients; however, an alternative therapy is recommended for treatment of the infection

Answer 185

A S. aureus (VRSA) is determined if the MIC result is 16 μg/mL or greater. Strains showing high levels of resistance (MIC greater than 8 μg/mL) and intermediate resistance (MIC 4 to 8 μg/mL) are thought to have structural changes within the cell wall. Two new agents available to combat vancomycin-resistant strains are linezolid and daptomycin.

Answer 186

B Strains of S. aureus producing toxins A and B are the cause of “scalded skin syndrome,” which refers to skin infections that can be manifested as small blisters or covering 90% or greater of the body surface in newborns.

Answer 187

B The only spore former listed is the Bacillus spp., which grow as large, spreading colonies on blood agar plates. Pseudomonas spp. are gram-negative rods; Corynebacterium spp. appear as small, very dry colonies on BAP; Listeria spp. appear as very small β-hemolytic colonies on BAP, resembling Streptococcus spp.

Answer 188

A Both species of Bacillus are catalase and lecithinase positive and produce acid from glucose. B. cereus is β-hemolytic and motile, but B. anthracis is neither. See the chart below. β Hemolysis Motility Oxidase Catalase Lecithinase Glucose B. cereus + + Neg + + + B. anthracis Neg Neg Neg + + +

Answer 189

D The best specimen is the suspected food itself. Stool cultures are not useful because B. cereus is part of the normal fecal flora. The suspected food can be the source of food poisoning by B. cereus if 100,000 or greater organisms per gram of infected food are demonstrated.

Answer 190

A The best differentiating test to perform on a suspected B. anthracis culture is the 10- unit penicillin disk test. B. anthracis is susceptible, but other Bacillus spp. are not. Organisms suspected to be B. anthracis should be sent to a reference laboratory for final confirmation. All tests should be performed in a biological safety cabinet, and personnel should wear protective clothing to reduce risk from possible production of aerosols.

Answer 191

A The best differentiating test to perform on a suspected B. anthracis culture is the 10- unit penicillin disk test. B. anthracis is susceptible, but other Bacillus spp. are not. Organisms suspected to be B. anthracis should be sent to a reference laboratory for final confirmation. All tests should be performed in a biological safety cabinet, and personnel should wear protective clothing to reduce risk from possible production of aerosols. Catalase Motility CAMP H2S Bile L. monocytogenes + + (umbrella) Wet prep = tumbling + Neg + Group B streptococci Neg Neg + Neg Neg

Answer 192

C E. rhusiopathiae is catalase negative, whereas the other three organisms are catalase positive. E. rhusiopathiae is seen primarily as a skin infection on the fingers of workers handling fish, meat, and poultry, as well as in farmers. Colonies growing on blood agar are small and transparent, may be either smooth or rough, and are often surrounded by a green tinge. E. rhusiopathiae is characterized by H2S production in the butt of a TSI slant, which differentiates it from other catalase-negative, grampositive rods. Motility observation notes a “bottle brush” appearance in gel media at 22°C.

Answer 193

A Lactobacillus spp. produce both long, slender rods or short coccobacilli that form chains. Lactobacillus spp. are part of the normal flora of the vagina (are not considered pathogens) and are sometimes confused with the streptococci. Lactobacillus spp. are absent or few when bacterial vaginosis is present with G. vaginalis.

Answer 194

D Corynebacterium spp. recovered from a throat culture are usually considered part of the normal throat flora. C. diphtheriae is an exception and should be suspected when one of the conditions described occurs. In this event, direct inoculation on Loeffler serum medium or tellurite medium and the following biochemical tests should be performed to confirm the identification of C. diphtheriae. Gelatin hydrolysis = Neg Motility = + Acid from glucose = + Catalase = + Urease = + Carbohydrate fermentation = +

Answer 195

D A Gram-stained smear from a vaginal secretion showing many squamous epithelial cells loaded with pleomorphic gram-variable (positive and negative) bacilli is considered presumptive for G. vaginalis. Such cells are called clue cells. Other important findings are: β-Hemolysis on BAP = + Oxidase = Neg Catalase = Neg Hippurate hydrolysis = +

Answer 196

B Nocardia spp. should be suspected if colonies that are partially acid fast by the traditional method are positive with the modified acid-fast method using Kinyoun stain and 1% sulfuric acid as the decolorizing agent. The other organisms listed are negative with acid-fast staining

Answer 197

B Serological tests of the patient’s serum for evidence of syphilis are routinely performed, but culturing is not because research animals must be used for inoculation of the suspected spirochete. T. pallidum stains poorly or not at all by either the Gram or acid-fast technique. Darkfield microscopy for direct visualization, phase-contrast, or indirect immunofluorescence using fluorescein-conjugated antihuman globulin may be used for visualization of spirochetes. Tests for specific antibodies to T. pallidum are available in a wide range of immunoassay formats, including chemiluminescence and point-of-care immunochromatography. Therefore, a reverse algorithm that screens with a specific test for treponemal antibody is often used. If positive, this is followed by a nontreponemal test (rapid plasma regain [RPR] or Venereal Disease Research Laboratory [VDRL]) to assess active infection. If the treponemal test is positive but the nontreponemal test negative, then a second treponemal test, usually the Treponemal pallidum-particle agglutination (TP-PA), is used to confirm the initial positive screening test. Also, PCR can be used to identify T. pallidum DNA within lesions and exudates. Because the bacterium is fastidious, blood samples should be collected in EDTA, CSF should be frozen and sent on dry ice, and samples should be analyzed as soon as possible.

Answer 198

A Borrelia spp. are often seen on Wright-stained smears of peripheral blood as helical bacteria with three to 10 loose coils. They are gram negative but stain well with Giemsa stain. B. recurrentis, is the cause of relapsing fever, a louse-borne epidemic. The use of thick and thin smears is helpful in observing the organism.

Answer 199

D Lyme disease may result in acute arthritis and meningitis and is caused by B. burgdorferi. This spirochete is carried by the deer tick belonging to the Ixodes genus (I. dammini in the eastern and north-central United States and I. pacificus in the northwest United States). The life cycle of the tick involves small rodents, such as the white-footed mouse and the white-tailed deer.

Answer 200

A Serological analysis using immunofluorescence or EIA is the method of choice for diagnosis of Lyme disease. Titers of IgM remain high throughout the infection. B. burgdorferi can be cultured directly from lesions, and darkfield microscopy can be used for detection of spirochetes in blood cultures after 2 to 3 weeks of incubation at 34°C to 37°C. PCR on synovial fluid, CSF, and skin biopsies is used for detection of early- and late-stage Lyme disease when there is clinical suspicion but serology is negative

Answer 201

B A common cause of respiratory tract illness, M. pneumoniae, generally causes a selflimited infection (3–10 days) and usually does not require antibiotic therapy. M. pneumoniae can be cultured from the upper and lower respiratory tracts onto specially enriched (diphasic) media but is most frequently diagnosed on the basis of the change in antibody titer from acute to convalescent serum by using EIA or other serological methods. PCR amplification is used to detect relevant Mycoplasma and Ureaplasma spp.

Answer 202

A Genital mycoplasmas (M. hominis and U. urealyticum) are grown on specific agars. M. hominis is grown on “M” agar containing arginine and phenol red. Colonies of mycoplasma are 50 to 300 μm in diameter and display a “fried egg” appearance with red holes. U. urealyticum is isolated from genital specimens on “U” agar (containing urea and phenol red) and then subcultured to A7/A8 agar. Colonies of Ureaplasma are small and golden brown on A7/A8 agar

Answer 203

B U. urealyticum is the only human mycoplasma that hydrolyzes urea. The manganous chloride–urea test utilizes manganous chloride (MnCl2) in the presence of urea. Urease produced by the organism hydrolyzes the urea to ammonia. This reacts with MnCl2 forming manganese oxide, which is insoluble and forms a dark brown precipitate around the colonies. The reaction is observed under a dissecting microscope and is a rapid test for the identification of U. urealyticum.

Answer 204

D All of the listed organisms produce mycelium (aerial or substrate), causing them to appear branched with Gram staining, but only the Nocardia spp. are positive for modified acid-fast stain. Nocardia is an opportunistic pathogen, and cultures typically have a “musty basement” odor.

Answer 205

A Corynebacterium spp. are part of the normal upper respiratory tract flora. Organisms display typical pleomorphic shapes often resembling letters, such as “Y” or “L,” and metachromatic granules. Identification of C. diphtheriae, however, requires selective culture media and biochemical testing.

Answer 206

A The “whiff” test is used for a presumptive diagnosis of an infection with G. vaginalis. A fishy odor is noted after the addition of one drop of 10% KOH to the vaginal washings. This odor results from the high concentration of amines found in women with vaginosis caused by G. vaginalis. Presumptive identification includes “clue cells” seen on Gram-stained vaginal washings or wet-mount preps.

Answer 207

B EIA and MIF tests, which are performed on serum, detect antibodies to C. trachomatis but may also detect antibodies that cross react with other organisms. Cell culture is time consuming and dependent on the quality of the specimen. PCR testing is both sensitive and specific and may be performed on urine.

Answer 208

A The cold agglutinin test is nonspecific, time consuming, and, therefore, outdated for the diagnosis of atypical pneumonia. Although M. pneumonia can be grown on artificial media, many specimens fail to grow, and growth can take up to 3 weeks. CF tests are not as sensitive or specific as EIA tests. Direct antigen assay by immunofluorescence is specific but has a sensitivity at least two orders of magnitude below DNA amplification methods. Although PCR testing is the most sensitive method, it may detect the presence of M. pneumonia in the absence of infection. EIA methods are sensitive, but some persons fail to produce antibodies especially in extrarespiratory infections, and antibody tests alone may detect past, rather than current, infections. Adults may fail to produce an IgM response, especially in recurrent infections, so the best confirmatory approach is a combination of EIA and direct antigen testing

Answer 209

A Chlamydophila psittaci is the new taxonomic classification for Chlamydia psittaci. This bacterium is found naturally in psittacine birds and other avian species. Psittacosis, the human infection, is the result of contact with pet birds and imported nonquarantined birds or from occupational contact in poultry farming or processing. Inhalation of the organisms from aerosols, fecal material, or feather dust causes a respiratory infection, and the specimen of choice is a throat swab or sputum.

Answer 210

D Ehrlichia and Anaplasma spp. are intracellular bacteria and are the causative agents of human monocytotropic ehrlichiosis (HME) and human granulocytotropic anaplasmosis (HGA). The organisms infect humans, dogs, and cattle through the bite of an infected Ixodes tick (named the Lone-Star tick transmitting Ehrlichia spp.), which also transmits Borrelia spp., causing Lyme disease. Blood, CSF, and serum are used for serological or PCR testing. A diagnosis can be made by direct observation of the basophilic inclusions (morulae) in leukocytes of Giemsa- or Wright-stained blood or buffy coat smears. This occurs in about 20% of HME in which the organisms are present in the monocytes and more frequently in HGA in which they are found in the granulocytes

Answer 211

B In stages I and II of Lyme disease, EIA is performed on serum for antibodies. PCR testing and culture of Borrelia spp. may be performed on a skin biopsy but would be unnecessary in this situation given the characteristic rash associated with Lyme disease. Synovial fluid, skin biopsy, and CSF may be tested for Borrelia spp. using PCR when Lyme disease goes untreated and evolves to stage III.

Answer 212

A R. equi are facultative intracellular organisms that are found within macrophages. They are found in soil and animal feces. R. equi is usually opportunistic and infects immunocompromised individuals as well as farm workers through inhalation of the organisms

Answer 213

D Culture techniques are time consuming and difficult to use as a screening procedure. The fastidious M. genitalium are difficult to grow. Molecular NAAT testing using conventional PCR will take 2 to 3 days for results, whereas real-time PCR will yield results in 1 day. M. genitalium has been implicated in the sexual transmission of HIV

Answer 214

D In general, anaerobic organisms lack superoxide dismutase (SOD) and catalase enzymes. The anaerobic bacteria are subdivided on the basis of their O2 requirement. Obligate anaerobes are killed by exposure to atmospheric O2 for 10 minutes or longer. Facultative anaerobes grow under aerobic or anaerobic conditions. Microaerophilic organisms do not grow in an aerobic incubator on solid media and only minimally under anaerobic conditions. However, they will grow in minimal O2 (5% O2). SOD is produced by many anaerobes and catalyzes the conversion of superoxide radicals to less toxic H2O2 and molecular O2.

Answer 215

C Eh is most affected by pH and is expressed at pH 7.0. In cultivating anaerobic bacteria, reducing agents, such as thioglycollate and L-cysteine, are added to anaerobic transport and culture media to maintain a low Eh. Certain anaerobes do not grow in the media above a specific critical Eh level.

Answer 216

A KVA allows growth of Bacteroides spp., Prevotella spp., and Fusobacterium spp. and inhibits most facultative anaerobic gram-negative rods and gram-positive bacteria (both aerobic and anaerobic). PEA inhibits facultative gram-negative bacteria but will support gram-positive aerobes and anaerobes and gram-negative obligate anaerobes. CCFA is selective for C. difficile from stool, whereas THIO broth supports grampositive and gram-negative aerobes and anaerobes

Answer 217

D The incidence of anaerobic bacteria recovered from the urine is approximately 1% of isolates. The other three types of infection are associated with a 60% to 93% incidence of anaerobic recovery. Urine is not cultured routinely under anaerobic conditions unless the specimen has been obtained surgically (e.g., suprapubic aspiration).

Answer 218

A Peptone yeast and chopped meat with carbohydrates support the growth of anaerobic bacteria. The end products from the metabolism of the peptone and carbohydrates are volatile fatty acids that help identify the bacteria. After incubation, the broth is centrifuged, and the supernatant injected into a gas–liquid chromatograph. Peaks for acetic, butyric, or formic acid, for example, can be identified by comparison with the elution time of volatile organic acid standards

Answer 219

B Anaerobic bacteria can be identified by analysis of metabolic products using GLC. Results are evaluated, along with Gram staining characteristics, spore formation, and cellular morphology, to make the identification.

Answer 220

C C. perfringens produces a double zone of β-hemolysis on blood agar, which makes identification relatively easy. The inner zone of complete hemolysis is caused by a θ- toxin and the outer zone of incomplete hemolysis is caused by an α-toxin (lecithinase activity). The Bacteroides spp. are gram-negative bacilli, and C. difficile is lecithinase negative and does not produce a double zone of β-hemolysis

Answer 221

A Egg yolk agar (modified McClung or neomycin egg yolk agar) is used to determine the presence of lecithinase activity, which causes an insoluble, opaque, whitish precipitate within the agar. Lipase activity is indicated by an iridescent sheen or pearly layer on the surface of the agar

Answer 222

D Lipase is produced by some Clostridium spp. and is seen as an iridescent pearly layer on the surface of the colonies that extends onto the surface of the egg yolk agar medium surrounding them. C. perfringens, the most frequently isolated Clostridium species, is negative for lipase production.

Answer 223

B Spores are generally not demonstrated from clinical specimens containing C. perfringens, which is the only species producing a double zone of hemolysis. The reactions in the chart above distinguish the four species listed. PCR testing for C. perfringens identification is available

Answer 224

A Spore appearance and location, along with Gram stain morphology, aids in distinguishing the Clostridium spp. Round, terminal spores (drumstick spores) are demonstrated when C. tetani is grown in chopped meat with glucose broth. Recognition of spores is particularly important because C. tetani sometimes appears as gram negative.

Answer 225

A Infant botulism is the most frequent form occurring in the United States. Epidemiological studies have demonstrated that infant botulism results from the ingestion of spores via breastfeeding or exposure to honey. Preformed toxin has not been detected in food or liquids taken by the infants. C. botulinum multiplies in the gut of the infant and produces the neurotoxin in situ.

Answer 226

B Foodborne botulism in adults and children is caused by ingestion of the preformed toxin (botulinum toxins A, B, E, and F) in food. The neurotoxins of C. botulinum are protoplastic proteins made during the growing phase and released during lysis of the organisms. C. botulinum is listed as a potential bioterrorism agent (as a possible bioweapon) by the CDC. Confirmation of botulism is made by demonstration of the botulinum neurotoxin (BoNT) in food, serum, gastric, or stool specimens. The CDC can be reached 24 hours a day for emergency assistance when botulism is suspected. The mouse bioassay is the recommended method for identification of the toxin BoNT, which is extremely dangerous (Biosafety Level 3).

Answer 227

A C. botulinum and C. sporogenes have similar characteristics biochemically (see the following chart), and definitive identification of C. botulinum is made by the toxin neutralization test for its neurotoxins in serum or feces. Specimens should be kept at 4°C and sent to the CDC for culture and toxin assays. Spore Type Motility Lipase GLC Products C. botulinum Subterminal + + A, (P)*, B, (IB)†, IV‡ C. sporogenes Subterminal + + A, (P), B, (IB), IV

Answer 228

B C. difficile is also implicated in hospital- and nursing home–acquired diarrhea and colitis. Clinical testing for C. difficile includes culture and cytotoxin testing. Because culture takes 3 days and will detect nontoxigenic strains that do not cause diarrheal disease, immunoassays using antibodies against either the A toxin or both the A and B toxins are most frequently employed. Assays detecting both toxins are only slightly more sensitive because infections producing only B toxin are infrequent. The cytotoxin assay requires that specimens (only liquid or nonformed stools) be shipped to a reference laboratory on dry ice or kept at 4°C to 6°C if done in-house for up to 72 hours. Nucleic acid–based assays are also available.

Answer 229

D The culture and Gram staining of the puncture wound site usually does not produce any evidence of C. tetani.The diagnosis is usually based on clinical findings, which are characterized by spastic muscle contractions, lockjaw, and backward arching of the back caused by muscle contraction. The tetanus neurotoxin (TeNT) is an exotoxin that disrupts nerve impulses to the muscles.

Answer 230

B The Bacteroides group grows well in 20% bile (primary anaerobic plates include BBE agar or bile-esculin plates) and is resistant to penicillin two-unit disks with the exception of B. ureolyticus. Most Prevotella spp. are also resistant to penicillin twounit disks, but most Fusobacterium spp. and Porphyromonas spp. are sensitive

Answer 231

D B. ureolyticus spp. is the only species listed that is susceptible to penicillin and produces urease. The other organisms listed are resistant to penicillin.

Answer 232

A Pigmenting Porphyromonas spp. and Prevotella spp. also show hemolysis on anaerobic blood agar and do not grow on BBE agar

Answer 233

B Fusobacterium spp. are usually spindle-shaped, slim rods, whereas the other genera are small rods (variable length for Bacteroides spp. and tiny coccoid rods for Prevotella and Porphyromonas spp.). Fusobacterium spp. and Porphyromonas spp. are susceptible to penicillin two-unit disks, whereas most Bacteroides spp. and Prevotella spp. are resistant.

Answer 234

D Veillonella spp. are gram-negative cocci. All four genera are part of the normal human flora and are the anaerobic cocci most frequently isolated from blood cultures, abscesses, wounds, and body fluids. The Streptococcus spp. are facultative anaerobes

Answer 235

A P. acnes is a non–spore former and is described as a diphtheroid-shaped rod. It is part of the normal skin, nasopharynx, genitourinary tract, and gastrointestinal tract flora but is implicated as an occasional cause of endocarditis and is seen in the inflammatory process of acne. P. acnes is referred to as the “anaerobic diphtheroid.”

Answer 236

D Wounds infected with clostridia are characterized by invasion and liquefactive necrosis of muscle tissue with gas formation. The most frequent isolate is C. perfringens followed by C. novyi and C. septicum. C. perfringens is also seen in food poisoning with the release of the toxin when large numbers of the organism are ingested

Answer 237

A A. israelii is part of the normal flora of the mouth and tonsils but may cause upper or lower respiratory tract infections. The sulfur granules are granular microcolonies with a purulent exudate. Like Nocardia, Actinomyces produces unbranched mycelia and is sometimes (erroneously) considered a fungus. It has also been implicated in pelvic infection associated with intrauterine contraceptive devices (IUDs).

Answer 238

D The anaerobes are not suited for the broth disk elution or disk agar diffusion tests because of their slow rate of growth. The Kirby-Bauer method reference charts are not designed to be used as a reference of susceptibility for anaerobes. The three methods listed use Brucella blood agar or broth, at 35°C to 37°C. incubation (anaerobically) and take up to 48 hours for results

Answer 239

B B. fragilis infections usually occur below the diaphragm and it is the most commonly encountered anaerobe in anaerobic infections. The Gram-stained smear showing “safety pin” and pleomorphic bacilli is often seen as a preliminary clue in abdominal fluids

Answer 240

A Contamination by fungi and other bacteria contributes to lower yields of mycobacteria in a 24-hour sample. The first morning specimen collected by expectoration or nebulization produces the highest concentration of mycobacteria in sputum.

Answer 241

B A strong decontamination solution (6% NaOH or greater) may kill or severely damage the mycobacteria. A 4% NaOH solution is mixed with an equal volume of N-acetyl-Lcysteine (NALC), a digestant or mucolytic agent, to yield a final working concentration of 2% NaOH. The time of exposure of the specimen to the digestion/decontamination solution must be monitored because overtreatment may result in fewer positive results on culture

Answer 242

C All four media contain malachite green as an inhibitory agent of nonmycobacteria, but the Petragnani medium contains a higher concentration (0.052 g/dL) compared with the Löwenstein-Jensen (0.025 g/dL), Middlebrook 7H10 (0.0025 g/dL), or ATS (0.02 g/dL) medium. The last is used for normally sterile specimens, such as from CSF and bone marrow. A combination of solid (17 days for growth) and a liquid (10 days for growth) media is recommended. If there is no growth after 8 weeks, the cultures are reported as negative and discarded.

Answer 243

A Carbolfuchsin (fuchsin with phenol) stains mycobacteria red and does not decolorize after the acid–alcohol is added. The background and any other bacterial elements will decolorize and are counterstained blue by the methylene blue. A fluorescent staining procedure may be used as an alternative to acid-fast staining. Auramine fluorochrome produces bright yellow fluorescent mycobacteria and auramine–rhodamine causes an orange-red (gold) fluorescence against a dark background. A fluorescent microscope must be used, but with this method, the smear can be scanned with a 25× objective instead of the 100× objective, permitting more rapid identification of mycobacteria.

Answer 244

A M. tuberculosis is positive for niacin accumulation, whereas the other three species are niacin negative. M. ulcerans is associated with skin infections (in the tropics), does not grow at 37°C (optimal temperature is 33°C), and is not recovered from sputum. A serpentine pattern of growth indicates production of cording factor, a virulence factor for M. tuberculosis.

Answer 245

D M. bovis is also called the bovine tubercle bacillus. A nonvirulent strain, bacillus Calmette-Guérin (BCG), is used as a tuberculosis vaccine throughout the world. Infections with M. bovis resemble infections caused by M. tuberculosis and are seen in circumstances where there is close contact between humans and cattle.

Answer 246

A Growth rates of mycobacteria are used along with biochemical tests as an aid to identification. M. fortuitum grows within 3 to 5 days at 37°C and is used as the control for rapid growers. M. tuberculosis grows in 12 to 25 days at 37°C and is a control organism for slow growers. In addition to M. fortuitum, M. chelonei is a rapid grower (3–5 days at 28°C–35°C). In addition to M. tuberculosis, M. avium, and M. kansasii are slow growers (10–21 days at 37°C).

Answer 247

A M. tuberculosis does not produce pigmentation in the dark or after exposure to light (photochromogen). A common tapwater scotochromogen is M. gordonae. The pathogenic scotochromogens are M. szulgai, M. scrofulaceum, and M. xenopi. M. kansasii is a photochromogen producing a yellow pigment following exposure to light and red β-carotene crystals after long incubation periods

Answer 248

D M. kansasii is a photochromogen that causes chronic pulmonary disease (classic tuberculosis). The other three species cause cutaneous or subcutaneous disease. It is important to culture skin lesions at the correct temperature to facilitate growth. Optimal Temperature Growth at 37°C M. marinum 30°C–32°C Poor M. haemophilum 28°C–32°C Poor or no M. ulcerans 33°C No

Answer 249

B Niacin production is common to all mycobacteria. However, the niacin accumulates as a water-soluble metabolite in the culture medium when the organism cannot form niacin ribonucleotide. M. tuberculosis, M. simiae, and some strains of M. marinum, M. chelonae, and M. bovis lack the enzyme and therefore are called niacin positive because of the accumulation of niacin detected in the test medium.

Answer 250

D One milliliter of an equal mixture of 30% H2O2 (Superoxol) and Tween 80 (a strong detergent) is added to a 2-week-old subculture on the Löwenstein-Jensen medium and placed upright for 5 minutes. Catalase activity is determined semiquantitatively by measuring the height of the column of bubbles produced above the culture surface.

Answer 251

A M. bovis and M. tuberculosis are very similar biochemically, and some strains of M. bovis also accumulate niacin. The T2H test differentiates M. tuberculosis from M. bovis. M. tuberculosis is not inhibited by T2H.

Answer 252

C The hydrolysis of Tween 80 is usually positive when testing the clinically insignificant mycobacteria. M. fortuitum, M. chelonae, and M. gordonae are saprophytic (and opportunistic) species, but M. kansasii is a pathogen. M. kansasii hydrolyses Tween 80 more rapidly than the other species (within 3–6 hours). A positive reaction is indicated by a change in the color of neutral red from yellow to pink.

Answer 253

A M. xenopi causes a pulmonary infection resembling M. tuberculosis and is frequently isolated from patients with an underlying disease, such as alcoholism, AIDS, diabetes, or malignancy. It is often recovered from hot water taps and contaminated water systems and is a possible source of nosocomial infection. The other three species cause skin infections and grow on artificial media at a much lower temperature than M. xenopi (below 32°C).

Answer 254

C With the exception of M. tuberculosis, M. avium complex is the Mycobacterium species most often isolated from patients with AIDS. It is biochemically inert, which is a distinguishing factor for identification. M. avium complex is highly resistant to the antibiotics used to treat tuberculosis, including those used in multidrug therapy. Treatment with streptomycin, rifampin, ethionamide, ethambutol with cycloserine, or kanamycin has shown little success

Answer 255

A Both pathogenic and saprophytic mycobacteria may produce urease, and urease production is used to differentiate several mycobacteria species. Biochemically, M. scrofulaceum is identical to M. gordonae, except for the urease reaction for which M. scrofulaceum is positive and M. gordonae is negative. Urease reactions for the other pathogenic mycobacteria are: M. tuberculosis = + M. kansasii = + M. bovis = + M. avium complex = Neg

Answer 256

A The ATS recognizes three levels of laboratory services for mycobacteria testing. Level I laboratories are those that grow mycobacteria and perform acid-fast stains but do not identify M. tuberculosis (they may or may not perform drug susceptibility tests on M. tuberculosis). Level II laboratories perform all of the functions of Level I laboratories and also identify M. tuberculosis. Level III laboratories identify all mycobacteria species from clinical specimens and perform drug susceptibility tests on all species.

Answer 257

B The CAP lists four options for laboratories to follow in order to correlate the services provided with guidelines for inspection and accreditation. A laboratory’s performance on CAP proficiency tests is evaluated by interlaboratory comparison with laboratories within these levels of performance.

Answer 258

A M. marinum is typically recovered from cutaneous wounds resulting from infection when the skin is traumatized and comes into contact with inadequately chlorinated fresh water or salt water, such as in swimming pools or fish aquariums. The other three species are slow growers at 37°C. M. tuberculosis and M. avium complex are nonphotochromogens. M. avium complex is urease negative, M. tuberculosis is positive for niacin and nitrate, and M. kansasii is positive for nitrate and catalase.

Answer 259

D M. gordonae is a nonpathogen, scotochromogen, and rapid grower (7 days at 37°C). Rarely, it is implicated in opportunistic infections in patients with shunts, prosthetic heart values, or hepatoperitoneal disease. The other three species are pathogenic mycobacteria.

Answer 260

D Second-line antibiotics used to treat first-line drug-resistant tuberculosis include cycloserine, ethionamide, kanamycin, rifabutin, ciprofloxacin, doxycycline, ofloxacin, SXT and capreomycin

Answer 261

D Standard therapy using isoniazid and rifampin for classic, uncomplicated pulmonary tuberculosis is 9 months. The patient may not respond to therapy, even when the organism is susceptible to the antibiotics in vitro; therefore, cultures must be kept for up to 1 year to facilitate testing of additional antibiotics should the infection become refractory to therapy.

Answer 262

A Acid-fast smears are standardized by the ATS for reporting the number of AFB seen. The following criteria should be used to uniformly report results: 1 to 2 AFB per smear = Report number seen and request another sample 3 to 9 AFB per smear = Report as rare (1+) 10 or more per smear = Report as few (2+) 1 to 9 per oil immersion field = Report as numerous (3+) Greater than 9 per oil immersion field = Report as (4+)

Answer 263

D Mycobacteria spp. growing on MacConkey agar and other routine media are usually nonpathogens. However, M. chelonae, M. abscessus, and M. fortuitum are rapid growers that will grow on MacConkey agar (with no crystal violet) within 5 days and are considered pathogens causing disseminated diseases in immunocompromised individuals.

Answer 264

D M. tuberculosis is a slow grower with a prolonged culture time of 12 to 25 days and requires 3 to 6 weeks for definitive identification and antibiotic susceptibility testing. The acid-fast and fluorochrome smears remain the number one rapid test for the initial detection of mycobacterial infection. A positive smear has a predictive value of 96% when all laboratory and clinical findings are considered. Mass spectrometry (MALDITOF) is used for early culture identification when there is moderate culture growth. Sequencing uses minimal or very little culture growth for identification. DNA probes are available for rapid identification of M. tuberculosis, M. bovis, and M. avium. Amplification of a species-specific target region of DNA is done by PCR with a labeled (biotinylated) oligonucleotide primer. The PCR product is detected by denaturation and hybridization to a capture probe. After washing to remove unbound DNA, strepavidin conjugated to an enzyme is added. After washing to remove unbound conjugate, substrate is added. The presence of product indicates a positive result.

Answer 265

C A positive PPD test result indicates a person who is latently infected with M. tuberculosis. Such persons are asymptomatic and not infectious but have a 10% risk of developing tuberculosis during their lifetime.

Answer 266

A M. leprae and M. microti are different from all other mycobacteria because they cannot be cultured in vitro. Biopsies from nodules and plaques of skin that show numerous AFB are presumptively diagnosed as positive for M. leprae. Confirmatory identification is made by nucleic acid testing (NAT) using PCR.

Answer 267

A Greater than 90% of M. avium complex organisms that are isolated from patients with AIDS are M. avium. These are differentiated from M. intracellulare and M. chimaera, which comprise the other two members of the complex by performing NAT.

Answer 268

B M. microti is found in guinea pigs, rabbits, cats, and other warm-blooded animals. It is a cause of tuberculosis in immunocompromised humans but is known to cause tuberculosis in immunocompetent persons as well. M. microti cannot be cultured in vitro but can be distinguished from M. leprae by its distinctive “croissant-like” morphology. Diagnosis is confirmed by performing NAT.

Answer 269

A M. haemophilum, M. ulcerans, and M. marinum grow optimally at 27°C to 30°C. M. xenopi grows optimally at 42°C to 45°C, and is frequently isolated from hot water systems and storage tanks. All other mycobacteria are cultured at 35°C to 37°C.

Answer 270

B M. avium subsp. paratuberculosis is known to cause an ulcerative intestinal disease with chronic diarrhea in cattle known as Johne disease. Although M. paratuberculosis has been isolated from the intestines of humans with Crohn disease with significantly greater frequency than controls, the organism has not yet been proven to cause ileitis in humans

Answer 271

B M. marinum is usually recovered from skin lesions that have been in contact with underchlorinated freshwater, saltwater, or tropical fish aquariums

Answer 272

A M. tuberculosis tests positive in the niacin (nicotinic acid) accumulation test, which is done after sufficient cultural growth is observed. Even though all other species produce niacin, M. tuberculosis produces the most which is tested directly on the L-J slant that is at least 3 weeks old, provided sufficient numbers of the typical buff-colored, slowgrowing, rough colonies (serpentine) are observed.

Answer 273

D This smear cannot be reported as positive or negative because only two AFB were observed per 300 fields. The two AFB observed could be contaminants in the oil immersion or on the lens of the microscope from a previous positive AFB smear. A new specimen should be requested. The fluorochrome staining method saves time when scanning smears for AFB. With the use of a UV microscope, a lower magnification (450×) allows for more fields to be scanned in less time. Positive results using a fluorochrome staining method, must be confirmed using an acid-fast staining method.

Answer 274

B Presently, BCG (an attenuated strain of M. bovis) has been used in many parts of the world to vaccinate susceptible individuals against tuberculosis.

Answer 275

D Specimens for fungal culture must be kept in a moist, sterile environment. Swabs that are dried out or submitted with insufficient material on them should be rejected. Generally, swabs are inadequate for the recovery of fungi because they are easily contaminated by surrounding skin flora.

Answer 276

A A solution of 10% KOH is used for contaminated specimens, such as skin, nail scrapings, hair, and sputum, to clear away background debris that may resemble fungal elements. Normally sterile specimens (CSF, blood, and bone marrow) do not require KOH for clearing

Answer 277

B Meningitis caused by C. neoformans is diagnosed through culture, biochemical reactions, and rapid agglutination tests for cryptococcal antigen. The India ink test is not diagnostic for cryptococcal meningitis because positive staining results are demonstrated in less than 50% of confirmed cases. A positive India ink test shows yeast cells in CSF with a surrounding clear area (the capsule) because the capsule of C. neoformans is not penetrated by ink particles

Answer 278

B Superficial dermatophytes rarely invade the deeper tissues and are the cause of most cutaneous fungal infections. Fungal infections of the skin are most often caused by Microsporum spp., Trichophyton spp., and Epidermophyton spp., although Candida spp. are sometimes implicated as the cause of nail infections.

Answer 279

D The true germ tube (filamentous extension from a yeast cell) is approximately one-half the width and three to four times the length of the cell with no true hyphae constriction at the point of origin. C. albicans produce germ tubes (95%), and a positive test is considered a presumptive identification

Answer 280

A C. tropicalis forms pseudohyphae that resemble true germ tubes by producing a constriction at the point of origin of the yeast cell. Germ tubes represent true hyphae without constriction, and therefore the test should have been repeated along with carbohydrate tests before making a presumptive identification. The other three species of yeast listed do not form hyphae

Answer 281

D Cornmeal agar with Tween 80 (polysorbate) reduces the surface tension and allows for enhanced formation of hyphae, blastospores, and chlamydoconidia

Answer 282

C Pseudohyphae are the result of a pinching-off process, blastoconidiation, with the growth of filaments with constrictions. Germ tubes are the beginning of true hyphae (no constrictions). Arthrospores are the result of a breaking-off process of true septate hyphae resulting in square conidia.

Answer 283

A A yeast that is isolated from CSF and is producing blastospores is most likely to be C. neoformans, which is positive for urease and produces brown colonies on birdseed agar.

Answer 284

A Most isolates of C. neoformans produce phenol oxidase when grown on Guizotia abyssinica medium (birdseed medium), producing brown to black pigmented colonies. C. neoformans is the only Cryptococcus species that oxidizes o-diphenol to melanin, which is responsible for the color.

Answer 285

D C. albicans and Candida dubliniensis, a variant of C. albicans, are the only yeasts that produce germ tubes within 1 to 3 hours of incubation at 37°C. C. tropicalis produces pseudohyphae after incubation for 3 hours, which may be mistaken for germ tubes. A careful evaluation of the tube origin for constriction is required to avoid a false-positive interpretation

Answer 286

B T. cutaneum and C. neoformans are both urease positive, but T. pullulans produces arthroconidia, and C. neoformans does not. In addition to Trichosporon spp., arthroconidia are produced by Geotrichum spp.

Answer 287

B Germ tube–negative isolates producing dark brown to black colonies on niger seed agar and a positive urease test are presumptive of C. neoformans. A positive germ tube test result is a presumptive identification for C. albicans as well as for C. dubliniensis. See the chart above.

Answer 288

D Cryptococcus spp. do not form either pseudohyphae or arthroconidia. Candida spp. produce blastoconidia or pseudohyphae. Trichosporon spp. produce pseudohyphae, blastoconidia, and arthroconidia. See the chart below

Answer 289

A Sexual spore production is a characteristic of the Ascomycotina, which produce an ascus (saclike structure) after the union of two nuclei. The resulting spore is termed ascospore. S. cerevisiae produces ascospores when grown on ascospore agar for 10 days at 25°C.

Answer 290

B Rhodotorula spp. produce pink- to coral-colored colonies on Sabouraud agar and cornmeal agar. It is usually considered a contaminant but is an opportunistic pathogen, and must be identified when found in specimens from immunosuppressed patients.

Answer 291

C Cornmeal Tween 80 agar supports the growth of C. albicans and the formation of its distinctive thick-walled, usually single, terminal chlamydoconidia. C. dubliniensis also produces chlamydoconidia (in pairs, triplets, and clusters) and germ tubes but is rarely implicated in disease

Answer 292

A The yeast isolate is inoculated directly into the molten agar base free of carbohydrates or is poured as a suspension onto a yeast nitrogen agar base plate. Carbohydrate disks are then added to the surface of the agar, and the plates are incubated for 24 to 48 hours at 30°C. Growth around the disk indicates the ability of the yeast to utilize the carbohydrate(s) as a sole source of carbon.

Answer 293

D This isolate is C. albicans, which also produces some true hyphae along with pseudohyphae. A positive germ tube is a presumptive identification along with the production of blastoconidia, terminal chlamydoconidia, and pseudohyphae

Answer 294

A C. tropicalis and C. kefyr (pseudotropicalis) differ in their arrangement of blastoconidia along the pseudohyphae. C. kefyr (pseudotropicalis) forms elongated blastoconidia arranged in parallel clusters that simulate logs in a stream. Trichosporon spp. and Geotrichum spp. form arthroconidia

Answer 295

B Dimorphic molds are in the yeast form in infected tissues because they are in the yeast form at 37°C. Specimens are cultured and incubated at both room temperature and 35°C to 37°C. To prove that a mold growing at room temperature (or 30°C) is a dimorphic fungus, conversion to the yeast form must be demonstrated via subculture and incubation at 37°C.

Answer 296

A The mold form of C. immitis shows barrel-shaped arthroconidia separated by empty cells (ghost cells) that cause an uneven staining effect when they are examined under a microscope. S. schenckii, H. capsulatum, and B. dermatitidis produce conidia that are round or oval in shape in the mold phase.

Answer 297

B S. schenckii is usually acquired by humans through thorns or splinters because it is commonly found on living or dead vegetation. It is called “rose gardener’s disease” because gardeners, florists, and farmers are most often infected. S. schenckii is often recovered from exudates of unopened subcutaneous nodules or open draining lesions.

Answer 298

A Sepedonium spp. are saprophytic molds that do not have a yeast phase and produce large spherical tuberculate macroconidia, similar to H. capsulatum. Histoplasmosis is a chronic granulomatous infection primarily found in the lungs that invades the reticuloendothelial system. Infection occurs via spores released from decaying bird or chicken droppings that are inhaled when disturbed.

Answer 299

A P. brasiliensis yeast forms are sometimes seen as a “mariner’s wheel” because multiple budding cells completely surround the periphery of the parent cell.

Answer 300

B Mucorales (formerly Zygomycetes) commonly recovered from clinical specimens are Rhizopus spp. and Mucor spp. Both display aseptate hyphae, whereas the other groups above display septate hyphae. Mucorales (Zygomycetes) usually not encountered in clinical specimens are also aseptate and include Lichtheimia spp. (formerly Absidia spp.), Rhizomucor spp., Cunninghamella spp., and Syncephalastrum spp.

Answer 301

A M. furfur has a worldwide distribution and causes a superficial, brownish, dry, scaly patch on the skin of light-skinned persons and lighter patches on persons with dark skin. M. furfur is not cultured because diagnosis can be made from microscopic examination of the skin scales. Skin scrapings prepared in KOH show oval- or bottleshaped cells that exhibit monopolar budding in the presence of a cell wall and also produce small hyphae

Answer 302

D Trichophyton spp., Microsporum spp., and Epidermophyton spp. are the organisms causing human dermatomycoses or cutaneous infections. Trichophyton spp. infect hair and nails, as well as skin. Infections with members of the genus Microsporum are confined to the hair and skin, whereas infections caused by the genus Epidermophyton are seen only on skin and nail

Answer 303

A Members of the genus Microsporum produce club-shaped microconidia and are usually pigmented white, buff, yellow, or brown. Epidermophyton does not display microconidia and produces yellow-green or yellow-tan colonies. T. rubrum can be differentiated from the other members of the genus by its distinctive cherry-red color. Trichophyton mentagrophytes may also produce a red pigment, but it is usually rose colored or orange, or deep red. T. tonsurans produces white-tan to yellow, suedelike colonies. T. schoenleinii produces white to cream-colored colonies, and T. violaceum produces port wine to deep violet colonies.

Answer 304

B M. audouinii and T. tonsurans may both cause epidemic tinea capitis in children. M. audouinii causes a chronic infection transmitted directly via infected hairs on caps, hats, combs, upholstery, and hair clippers. Infected hair shafts fluoresce yellow-green under a Wood lamp. M. audouinii does not usually sporulate in culture and forms atypical vegetative forms, such as antler and racquet hyphae and terminal chlamydoconidia. In contrast, M. canis produces spindle-shaped, thick-walled multicelled macroconidia, and M. gypseum produces ellipsoidal, multicellular macroconidia.

Answer 305

C Epidermophyton spp. do not produce microconidia; this differentiates them from Trichophyton spp. and Microsporum spp. Alternaria is not a dermatophyte. Epidermophyton floccosum is the most frequently isolated member of the genus and infects skin, but not hair

Answer 306

D T. schoenleinii is identified microscopically by its characteristic antler-shaped hyphae and chlamydoconidia in the absence of conidia.

Answer 307

A T. mentagrophytes may produce a deep red pigment seen through the reverse side of the agar plate that resembles the cherry-red pigment produced by T. rubrum. However, T. mentagrophytes can be differentiated by its ability to invade the hair shaft. T. rubrum grows on the surface of hair but does not penetrate the shaft.

Answer 308

A The dematiaceous molds are easily recognized and confirmed by observing darkyellow or brown septate hyphae upon microscopic examination

Answer 309

B A. niger is the only species listed producing black conidia, which causes a “pepper” effect as the colony grows. The reverse side of the agar plate remains buff or cream colored, which differentiates it from the dematiaceous (dark) molds.

Answer 310

A Phialophora, Exophiala, and Wangiella all produce phialides, but the last two genera form elongated, tubelike phialides without a collarette, as opposed to the flask-shaped phialides of Phialophora, which contain clusters of conidia at the tips.

Answer 311

C A. fumigatus is most often associated with compost piles and is found in the soil of potted plants. A. niger is the cause of cavitary fungus ball lesions of the lungs and nasal passages.

Answer 312

A Fusarium spp. are usually a contaminant but are sometimes seen as a cause of mycotic eye, nail, or skin infection in debilitated patients. Fusarium is a hyaline (light) mold and grows on Sabouraud agar plates at 30°C within 4 days. The other three organisms are members of the Dematiaceae family (dark molds).

Answer 313

B A. fumigatus is the most common cause of aspergillosis. It is characterized by sporulation only from the upper half or two-thirds of the vesicle. Colonies of A. niger are white with black pepper growth and produce phialides over the entire vesicle, forming the classic “radiate” head. A. flavus colonies are yellow to yellow-green and produce phialides that cover the entire vesicle and point out in all directions. A. terreus produces brown colonies and phialides that also cover the entire vesicle.

Answer 314

A Lichtheimia (Absidia) spp. are similar to Rhizopus spp. except for the location of rhizoids (rootlike hyphae). The rhizoids of Rhizopus spp. are located at the point where the stolons and sporangiophores meet, whereas those of Lichtheimia (Absidia) spp. arise at a point on the stolon between the rhizoids. Penicillium spp. and Aspergillus spp. do not form rhizoids.

Answer 315

B Immunocompromised patients are at risk for invasion of C. neoformans. The polysaccharide capsule of C. neoformans is not recognized by phagocytes, which cause patients with impaired cell-mediated immunity to become readily infected with C. neoformans

Answer 316

A Thermally dimorphic H. capsulatum produce microconidia and hyphal fragments at 37°C (yeast phase), whereas at 25°C (mold phase), the organism displays large, thickwalled, round macroconidia with knobby or knoblike projections. The yeast form is able to survive within circulating monocytes or tissue macrophages that can be demonstrated with Giemsa or Wright stain.

Answer 317

D C. immitis endospores are often confused with yeast cells, but they do not bud. C. immitis is endemic in the southwestern United States. Because arthroconidia are highly infectious, an open plate should not be used, and a slide culture test should not be performed. Rather, tubed media is used for testing, and all work should be performed in a biological safety cabinet.

Answer 318

A P. jirovecii (P. carinii) is classified as a fungus but formerly was classified as a parasite. It is best recovered by bronchoalveolar lavage or induced sputum in immunocompromised patients. Open lung biopsy sample was the specimen of choice before the AIDS epidemic. GMS stain is used to identify the organism which stains the cyst form in lung tissue, but not the trophic form. By using hematoxylin and eosin (H&E) staining on a smear, an eosinophilic, foamy material can be detected which is formed from the successful replication of the organism in alveolar lining fluid.

Answer 319

B Other Penicillium spp. are differentiated from Talaromyces (Penicillium) marneffei (thermally dimorphic) through conversion from the mold to yeast phase. Talaromyces (P.) marnerffei are seen as yeast at 35°C to 37°C on 5% sheep blood agar or in BHI broth. Other Penicillium spp. do not display a yeast phase. T. marneffei are recovered from the blood, skin, lymph nodes, bone marrow, and internal organs of immunocompromised patients.

Answer 320

A Open lung biopsy is not recommended for persons with AIDS because of the increased risk of death, infection, and complications associated with this surgical procedure. Persons with AIDS and P. jirovecii (carinii) pneumonia (PCP) can be diagnosed with chest radiography, special stains, and PCR methods from specimens obtained by bronchoalveolar lavage (BAL). Patients without AIDS have a much lower yield, and therefore, besides BAL, tracheal aspiration, endoscopic lung biopsy, cellular material from bronchial brushings, and open lung biopsy specimens are more often indicated.

Answer 321

C To determine an early diagnosis, soluble antigens (the galactomannan assay targets antigens of Aspergillus spp.) are detected by using such methods as ELISA, latex agglutination, and immunoblotting using serum or body fluids. Blood and sputum cultures are usually negative, and tissue biopsy is associated with a significantly higher risk of complications. Nucleic acid amplification assays, multiplex amplification and reverse transcription PCR (RT-PCR) have been developed which detect systemic aspergillosis in respiratory specimens. MALDI-TOF MS methodology has proven successful on old and young cultures, with 98.6% correct identification.

Answer 322

D Vascular invasion, followed by thrombosis, tissue infarction, and necrosis are the most common clinical manifestations of cutaneous infections caused by Rhizopus spp. Infections occur mainly in patients who are already being treated for other conditions, including lymphoma, leukemia, neutropenia, renal failure, and diabetic ketoacidosis.

Answer 323

A The yeast phase of H. capsulatum develops as small oval budding cells seen in macrophages. The yeast phase of P. brasiliensis develops large cells that bud creating a structure resembling a mariner’s wheel, and C. immitis produces very large spherules. At 25°C, P. brasiliensis produces small single conidia, and C. immitis produces arthroconidia. Candida spp. are not dimorphic. H. capsulatum is found in soil containing excrement of birds and bats. It is endemic in the Ohio and Mississippi River valleys, and the most common systemic mycosis in North America.

Answer 324

A C. immitis is a soil fungus and is endemic to the southwestern United States. Infection results from inhaling the arthroconidia, which form endospore-forming spherules and leads to infection within 1 to 3 weeks. Nonimmunocompromised people usually do not require treatment and are immune to reinfection. However, immunodeficient persons may develop symptomatic pulmonary and multiorgan infections

Answer 325

A M. furfur, the cause of tinea versicolor, is often diagnosed from a direct skin scraping specimen. The classic “spaghetti and meatball” formations of the yeast and hyphal cells are distinct. In fungemia, the blood culture specimen will show small yeast cells without pseudohyphae

Answer 326

B Microscopically, Scopulariopsis spp. resemble Penicillium spp. except for the following: Scopulariopsis spp. produce annellophores with flask-shaped annelides bearing large lemon-shaped, rough-walled conidia in chains; Penicillium spp. display brush-like conidiophores producing metulae with flask-shaped phialides, resulting in chains of small, smooth-walled conidia

Answer 327

C A “foot cell” is characteristic of Aspergillus fumigatus. The presence of a foot cell, which is a T- or L-shaped cell at the base of the conidiophore (it is not a separate cell), is characteristic and distinct for A. fumigatus which is thermotolerant, growing at temperatures up to 45°C.

Answer 328

C True viruses have nucleic acid that is either RNA or DNA, and this serves as the basis for initial classification. Members of these classes are further divided into groups that cause human disease on the basis of mode of transmission, tissues invaded, diseases produced, and antigenic characteristics

Answer 329

A RSV is the cause of croup, bronchitis, bronchiolitis, and interstitial pneumonia. Hospitalized children age less than 1 year are the most susceptible group.

Answer 330

D Coxsackie A virus, Coxsackie B virus, and the echoviruses are most commonly implicated in myocarditis and other syndromes, including acute cerebellar ataxia and hepatitis. Like poliovirus, infections are more common in the summer and fall seasons and gain entry through the gastrointestinal tract

Answer 331

A CMV infection in a previously healthy individual causes a self-limiting mononucleosis syndrome. CMV is an opportunistic pathogen that may produce lifelong infections and causes a variety of diseases, including congenital and neonatal infection, hepatitis, pneumonia, and disseminated infection in immunocompromised patients

Answer 332

A Influenza and adenoviruses are the main causes of respiratory infections, including the common cold, tracheobronchitis, and pneumonia. Adenoviruses also cause conjunctivitis, keratitis, cystitis, and gastroenteritis and are transmitted by aerosolized droplet, contaminated respiratory secretions, stool or fomites

Answer 333

B Rotaviruses have been implicated in both nosocomial infections and epidemic gastroenteritis. Children ages 3 to 24 months are most commonly affected. Diarrhea begins after an incubation period of 3 days, lasts for 2 to 10 days and is associated with vomiting and dehydration. In immunosuppressed children, rotavirus causes a chronic infection.

Answer 334

D Parvovirus causes a fever and characteristic “slapped cheek” rash in young children. Adults are usually immune, but immunocompromised persons may exhibit arthritis or anemia (the virus infects immature RBCs in bone marrow).

Answer 335

D Although the most common mode of transmission of hepatitis B is via needle puncture, it may also be transmitted by other parenteral means, including through sexual transmission and contact with contaminated blood via broken skin or mucous membranes

Answer 336

A Noroviruses are small RNA viruses that have been implicated in epidemics of community gastroenteritis as well as sporadic infections. Unlike rotaviruses, which cause gastroenteritis in infants and young children, noroviruses produce infections in all age groups

Answer 337

D Human papillomaviruses (HPVs) cause genital warts, the most prevalent sexually transmitted viral disease in the United States. Several strains, including HPV-6, HPV- 11, HPV-16, and HPV-18, are associated with penile, cervical, and vaginal neoplasia, causing more than 60% of the cancers. Because the virus cannot be cultured in vitro, diagnosis is usually made by using DNA probes, histopathology, or cytological examination of cutaneous biopsy of cells. A diagnostic characteristic of infected cells is koilocytosis, a perinuclear clearing in the squamous epithelium accompanied by nuclear atypia. Currently, the FDA has licensed three vaccines for HPV: Cervarix, Gardasil, and Gardasil-9.

Answer 338

D In addition to serological tests for antibodies against the virus and DNA probes that identify viral DNA or RNA, cytological and histological examination aid in the rapid diagnosis of several viruses. The rubella virus is detected serologically by testing for specific IgG and IgM antibodies. Testing is routinely done on pregnant women, as a prenatal precaution.

Answer 339

C The Western blot assay is no longer used to confirm a positive serological test of antibodies to HIV. Western blot detects antibodies but it may miss people in the window phase of infection, is not confirmatory for neonatal infections because of the presence of maternal antibodies, and has a long turnaround time resulting in loss of contact with the patient. For these reasons, the CDC recommends a new approach using a simultaneous test for antibodies to both HIV-1 and HIV-2 and p24 antigen to screen for infection by immunoassay. Positive specimens would be tested by separate immunoassays to identify whether the infection is HIV-1 or HIV-2. Samples testing negative in suspected cases should be tested by a nucleic acid amplification method, such as PCR for viral RNA for definite identification.

Answer 340

C Using direct immunofluorescence microscopy (DIF), rabies antigen can be detected in the cutaneous nerves surrounding the hair follicles of the posterior region of the neck (nuchal biopsy) and in the epithelial cells obtained through corneal impression. Antibodies to rabies can be detected in the serum and CSF of infected persons within 8 to 10 days of illness; however, infection usually occurs several months before the onset of symptoms. Isolation of virus from the saliva of the patient may be accomplished by mouse inoculation or by inoculation of susceptible cell culture lines with subsequent detection by immunofluorescent antibodies. Postmortem diagnosis of rabies is accomplished by using direct immunofluorescent assay of brain tissue. Brain tissue testing from the dead animal results in prompt treatment/prophylaxis of the human victim after an animal bite or injury

Answer 341

D Influenza virus types A, B, and C may be grown and isolated in cell cultures by using primary monkey kidney (PMK) cells permitting identification within 1 to 3 days in specialized laboratories performing viral cultures. Direct fluorescent and enzyme immunoassays by using monoclonal antibodies to nucleoprotein antigens in infected nasal epithelium are used for rapid diagnosis of both influenza A and influenza B infections. Influenza C testing is not routinely requested, and this type is referred to as “mild” influenza. RT-PCR is replacing the cell culture method because of better turnaround time, from collection of specimen to identification. It is also becoming the gold-standard for identifying respiratory viruses.

Answer 342

A DIF testing of vesicle (lesion) fluid for virus using fluorescein-conjugated antibodies is the most rapid method for diagnosis of genital herpes infection. Immunofluorescence and immunoperoxidase methods are also used to distinguish HSV-1 and HSV-2. PCR and real-time PCR are more sensitive rapid methods for detection and identification. Viral cell culture is also very sensitive and may yield a positive result within 24 hours when fluid contains a high concentration of virus. Vero cells or primary human embryonic cells are inoculated with vesicle fluid and examined for cytopathic effects (CPEs), the most common of which are large “balloon” cells and multinucleated giant cells.

Answer 343

C EBV serological testing for IgM-VCA (viral capsid antigen) during the illness may or may not be positive. Patients who present with an infectious mononucleosis–like syndrome should be tested for both EBV and CMV. Both viruses cause the same symptoms during the acute phase of the illness

Answer 344

A West Nile virus causes neurological diseases with meningitis and encephalitis at the top of the list. The animal reservoirs are birds, with humans being accidental hosts. Transmission of West Nile virus is from mosquito to bird.

Answer 345

A Hantavirus is transmitted by a rodent host, the deer mouse, was originally endemic in the southwest United States, but currently has been discovered throughout the country. The name of the hantavirus responsible for outbreaks in the southwest is the Sin Nombre virus. Breathing in excrement of mice is the most common mode of infection, and the lung is the site of initial infection, causing labored breathing or hantavirus pulmonary syndrome (HPS). Diagnosis is usually made by using an IgM/IgG ELISA assay and RT-PCR. Isolation of the virus by culture has not been accomplished to date. The primary site of infection for norovirus and rotavirus is the gastrointestinal area and for hantavirus the pulmonary sector.

Answer 346

D Rotavirus is one of the most common causes of gastroenteritis in infants and young children (ages 6 months to 2 years). Vomiting and diarrhea are also common symptoms of norovirus infections, but the prevalence of rotavirus during the winter months and the absence of illness in other family members point to rotavirus being a more likely cause. Commercial availability of immunoassays for rotavirus makes its diagnosis easier to establish and rule out than infection with norovirus.

Answer 347

D The decision to initiate antiviral therapy is based on the presence or absence of symptoms, CD4 lymphocyte count, and the viral load. For example, treatment is usually withheld from patients with CD4 counts greater than 350/μL and viral load less than 55,000/mL and is instituted in asymptomatic patients if the CD4 count is less than 200/μL, regardless of viral load. Treatment failure within the first year with three-drug regimens is 35% to 45%, and drug resistance testing (genotype and/or phenotype testing) is recommended to identify drug-resistant strains prior to initiating treatment. For successful antiviral therapy, the HIV plasma viral load should be reduced to undetectable levels.

Answer 348

A RSV is spread by large particles, such as dust, and is one of the most common causes of hospitalization for respiratory illness in infants age less than 1 year. RSV causes bronchiolitis, pneumonia, and croup in infants and upper respiratory illness in children. It has also been found to cause nosocomial infection in nursing homes. Diagnosis is made by using EIA, fluorescent antibody (FA) staining, and cell culture (HEp-2).

Answer 349

B Rabies virus can be detected by FA staining and PCR testing. The virus replicates at the site of the bite and penetrates the surrounding tissue, finding its way to the central nervous system. Because the source cannot be tested, the best immediate course of action is to initiate postexposure prophylaxis with antirabies globulin and to immunize the patient with the rabies vaccine.

Answer 350

A SARS virus was first identified in China in November 2002 when it caused a worldwide outbreak that was finally contained by July 2003. The virus belongs to the group of coronaviruses causing the common cold and is easily transmitted to health care workers who have close contact with infected individuals. This virus is the cause of a severe lower respiratory infection that can be fatal. Laboratory confirmation may be done by using the recommended method, that is, RT-PCR testing, which is available commercially. Typically, PCR is used on two different specimen types or the same specimen type submitted at least 2 days apart. If both tests are positive, the infection is confirmed. Cell culture (Vero-E6 cell line) can be performed if done in a BSL-3 or higher-level laboratory.

Answer 351

D HSV-2 infections produce genital lesions. Infants born prematurely with disseminated infection of HSV-2 from HSV-positive mothers have a mortality rate of 50% to 60%. Testing of pregnant women for antibody and cesarean section delivery can prevent most neonatal HSV infections because the virus enters the fetus during the delivery process

Answer 352

B Varicella-zoster virus is the cause of an infection with chickenpox. As an adult, the father is experiencing shingles, a reactivation of the virus. The virus lies dormant in the sensory (dorsal root) ganglia of the spinal nerves, and its reactivation produces a nonweeping, blisterlike rash on an inflamed skin base that follows the path of the underlying nerves.

Answer 353

B Transmission of rabies (rhabdovirus) occurs primarily from the bite of a rabid animal, but in 20% of human rabies cases, there is no known exposure to rabid animals. Donor tissues are not routinely tested for rabies. Diagnosis is achieved by performing fluorescent antibody staining of the infected tissues and electron microscopy, which shows the presence of bullet-shaped virus particles.

Answer 354

D Adult gastroenteritis caused by norovirus follows a 1 to 2 day incubation. Diagnosis is usually made on the basis of clinical criteria and confirmed by EIA, RT-PCR, or immune electron microscopy. Hepatitis E virus, also of the Caliciviridae family, also causes gastroenteritis 1 to 2 days after exposure but is not endemic in the United States and other developed countries

Answer 355

B Western blot is a test for detecting antibodies to HIV, and sufficient antibodies are not usually present in the first 3 to 4 weeks of infection to cause a positive test result. In some persons, antibodies do not appear until 3 months after the initial HIV infection. RT-PCR detects viral RNA and reduces the window phase of infection to approximately 1 week. RT-PCR tests for HIV-1 have 100% specificity at a cutoff of 20 to 40 copies RNA/mL. PCR is the preferred test for neonates because it avoids detection of maternal antibody and has replaced the Western blot as the confirmatory test of choice.

Answer 356

B Tests for toxoplasmosis, German measles, CMV, and HSV comprise a panel of infectious agents known as the TORCH panel. The TORCH panel consists of serological tests for antibodies to these organisms and has largely been replaced by more sensitive NAT when infection is suspected. The “O” in TORCH stands for “other,” and other viruses that cause transplacental infection, such as HBV and HIV are usually tested prenatally

Answer 357

A BK virus, a polyoma virus, is transmitted by direct contact with infected respiratory secretions. It has tropism for the urinary system and often causes a latent, asymptomatic infection in the kidneys. However, in immunocompromised patients, the virus is often implicated in renal and bladder infections. Kidney failure caused by BK virus is a significant concern in bone marrow and renal transplant recipients.

Answer 358

A The Ebola RNA virus is endemic in the Democratic Republic of the Congo with a human case fatality rate of 80% to 90%. The virus is transmitted from person to person or to caregivers through body fluids and respiratory droplets. Transmission of the virus to humans is from monkeys or other wild animals. It is speculated that bats harbor the virus as carriers or reservoirs. Using electron microscopy, the shape described is similar to that of the Marburg virus (“shepherd’s hook” shape), which causes a less severe viral hemorrhagic fever with a fatality rate of 23% to 25%. For diagnosis, electron microscopy (research laboratories), RT-PCR, and antibody detection with ELISA (IgG and IgM) are used along with cell culture in monkey cells in a BSL-4 laboratory. Both infections have no cure or established drug treatment. Several Ebola vaccines are being tested in clinical trials, as reported by the World Health Organization (WHO).

Answer 359

C The Zika virus is transmitted by the bite of the Aegypti mosquito, and the virus disappears from blood after 2 weeks; however, the virus can linger in sperm for 2 months. Zika virus outbreaks were seen in Brazil and southern Mexico. According to the Pan-American Health Organization (PAHO), reports of local transmission of Zika virus occurred throughout Mexico, with a peak in number of infections occurring in October 2016. To curb the transmission of the virus, genetically modified mosquitoes were used; after mating, the offspring would die before reaching adulthood. Spraying with insecticides kills the adults and larvae, and this was one way used to control outbreaks that occurred in Miami in 2016.

Answer 360

A SARS virus is detected with RT-PCR and can be cultured by using the Viro-E6 cell line. Low levels of the virus early in the infection poses a problem in diagnosing SARS.

Answer 361

B Hepatitis E virus (HEV) infection is the result of an infection with a new genus, Hepevirus. It is transmitted by the fecal–oral route and mimics hepatitis A, except that HEV has a high rate of fatality (10%–20%) among pregnant women. It is not endemic in the United States or in other developed areas of the world. Pregnant women should be cautioned about drinking contaminated water and traveling to parts of the world where HEV is endemic (India and Pakistan). Fulminant hepatitis, with a fatality rate of 30%, results if infection occurs during the third trimester. Diagnosis is through serology (IgG and IgM antibodies) as well as liver function tests (aspartate aminotransferase [AST], alanine aminotransferase [ALT], and bilirubin) during the early phase of the infection

Answer 362

C The appropriate procedure for the diagnosis of E. vermicularis would involve the microscopic examination of a series of consecutive cellulose tapes. Typical eggs and/or a rare adult pinworm could be identified under the microscope.

Answer 363

B As PMNs in stool begin to fragment and appear to have four nuclei, they will resemble E. histolytica/E. dispar cysts. However, E. histolytica/E. dispar cysts are rarely seen in cases of diarrhea. The species name E. histolytica is reserved for the true pathogen, whereas E. dispar is used for the nonpathogenic species. Unfortunately, they look identical morphologically. The only time E. histolytica could be identified morphologically would be from trophozoites containing ingested RBCs. Nonpathogenic E. dispar would not contain ingested RBCs. The correct way to report these organisms is E. histolytica/E. dispar (no trophozoites containing ingested RBCs) or E. histolytica (trophozoites seen that contain ingested RBCs). Physicians may base the treatment on patient symptoms.

Answer 364

B When eosinophils disintegrate, the granules reform into Charcot-Leyden crystals.

Answer 365

D Of the organisms listed, Diphyllobothrium latum cannot be transmitted sexually; this infection is acquired from the ingestion of infected raw or poorly cooked freshwater fish.

Answer 366

A East and West African trypanosomiasis (T. B. rhodesiense and T. B. gambiense) are acquired when infective forms are introduced into the human body through the bite of the tse-tse fly, not sand flea

Answer 367

C C. belli oocysts in various stages of maturity would be seen in the concentrate sediment or possibly the direct, wet preparation; these oocysts would stain positive with modified acid-fast stains

Answer 368

D T. vaginalis is site-specific. The organisms are found in the urogenital tract; thus, the intestinal tract is not the normal site for these organisms.

Answer 369

B The best technique to recover and identify D. fragilis trophozoites and rare cysts would be the trichrome-stained smear

Answer 370

C For a number of years, G. lamblia (G. intestinalis, G. duodenalis) has been implicated in both waterborne and foodborne outbreaks from the ingestion of infective cysts within contaminated water and food.

Answer 371

C E. gingivalis is known to be an inhabitant of the mouth and is characterized by morphology that resembles E. histolytica/E. dispar. However, E. gingivalis tends to ingest PMNs, whereas E. histolytica/E. dispar do not.

Answer 372

A The trophozoite of E. histolytica has evenly arranged chromatin on the nuclear membrane; a central, compact karyosome in the nucleus; clear pseudopodia; and ingested RBCs in the cytoplasm

Answer 373

D N. fowleri are free-living soil and water amoebae that cause primary amebic meningoencephalitis (PAM). The number of cases reported is few; however, the infection is very acute and almost always fatal.

Answer 374

A The rhabditiform larvae of S. stercoralis are characterized by the short buccal capsule (mouth) and large genital primordium, whereas hookworm larvae have a long buccal capsule and very small genital primordium.

Answer 375

A Toxocara spp. are the cause of visceral larva migrans and occur when humans accidentally ingest the infective eggs of the dog or cat ascarid. The larvae migrate through the deep tissues, including the eye. The test of choice is the serology.

Answer 376

D B. coli is a ciliate that can cause watery diarrhea in humans; however, it is not commonly found within the United States. It is the largest of the intestinal protozoa and can be found in proficiency testing specimens. Therefore, although it is not common, laboratories must still be able to identify these organisms.

Answer 377

B S. haematobium blood flukes reside in the veins over the bladder. When the eggs are passed from the body, they are often found in urine; egg viability can also be determined in unpreserved urine.

Answer 378

A P. westermani adult worms are found in the lung, and eggs may be coughed up in the sputum. Consequently, both sputum and stool (if the sputum containing the eggs is swallowed) are the recommended specimens for examination for the eggs.

Answer 379

A Both D. latum and P. westermani eggs are operculated and approximately the same size. The morphology is similar, although D. latum has a knob at the abopercular end and P. westermani has a thickened abopercular end and shoulders into which the operculum fits

Answer 380

A Both T. solium (pork tapeworm) and T. spiralis can be acquired from the ingestion of raw or poorly cooked infected pork.

Answer 381

B D. latum is the only operculated cestode egg that is found in humans; the infection is acquired from the ingestion of infected raw or poorly cooked freshwater fish.

Answer 382

A Although hydatid cysts are found in sheep or in humans (accidental intermediate host), the adult tapeworms of E. granulosus are found in the intestine of the dog.

Answer 383

C If humans ingest T. solium cysticerci in uncooked or rare pork, the adult tapeworm will mature within the intestine (human will serve as definitive host); if eggs from the adult tapeworm are ingested, then the cysticerci will develop in human tissues (accidental intermediate host), causing cysticercosis.

Answer 384

D The ingestion of raw freshwater fish containing the encysted larvae of D. latum will result in the development of an adult tapeworm within the human intestine.

Answer 385

B In infections with H. nana, humans serve as both intermediate and definitive hosts. When ingested, the oncosphere penetrates the intestinal mucosa, develops into the mature cysticercoid (human is intermediate host), and returns to the gut, where the adult tapeworm matures (human is definitive host). Thus, the life cycle can go from egg to adult tapeworm within the human host

Answer 386

A Babesia is an intracellular parasite that closely resembles the ring forms (early trophozoites) of P. falciparum. Often in babesiosis there are more rings per cell and the ring form is the only stage seen

Answer 387

C A. lumbricoides eggs require a period of development in the soil before they are infective for humans. The other organisms listed can be transmitted within the laboratory or in the hospital setting.

Answer 388

D Infection with T. gondii is acquired through the ingestion of infected rare or raw meats, infective oocysts from cat feces, or as a congenital transmission. The organism has a cosmopolitan distribution and although serological testing is generally the test of choice, the results may be very difficult to interpret in certain situations (e.g., congenital infection and immunocompromised patients).

Answer 389

A Oocysts of Cryptosporidium spp. can be found and identified using microscopic examination of fecal smears stained with modified acid-fast stains. They appear as purple-red-pink round objects, measuring approximately 4 to 6 μm. Often the four sporozoites and residual body can be seen within the oocyst wall

Answer 390

C O. volvulus microfilariae are normally found in the fluid right under the outer layer of skin. Therefore, the skin snip is the proper specimen to examine.

Answer 391

B The pathogenic sequelae of malarial infections with P. falciparum are the most severe of the four species listed. They can include massive hemolysis, blackwater fever, and multiple organ involvement, including the central nervous system (cerebral malaria).

Answer 392

B G. lamblia (G. intestinalis, G. duodenalis), D. fragilis, and Cryptosporidium have been implicated in nursery school outbreaks. Among the many protozoa and coccidia found in the human, these three organisms have become the most likely parasites in this type of setting

Answer 393

D Infections with E. vermicularis (the pinworm) may cause anal itching, sleeplessness, and possibly some vaginal irritation or discharge; however, dysentery (bloody diarrhea) has not been associated with this infection.

Answer 394

B The ova and parasite examination contains three components: the direct wet film (demonstrates protozoan trophozoite motility), the formalin-ethyl acetate concentration (demonstrates protozoan cysts, coccidian oocysts, and helminth eggs), and the trichrome or iron hematoxylin–stained smear (confirms protozoan cysts and trophozoites

Answer 395

C The cyst of I. bütschlii is characterized by a large glycogen vacuole that is seen on the wet smear (stains brown with iodine) and on the permanent stained smear (vacuole will appear clear). Occasionally the vacuole will be so large that the organism will collapse on itsel

Answer 396

B The determination of egg viability is important in schistosomiasis; therefore, the miracidial hatching test is helpful in demonstrating the egg viability of Schistosoma species. Once the eggs are hatched, the living miracidium larvae will be visible in the water

Answer 397

A Both G. lamblia (G. intestinalis, G. duodenalis) and Cryptosporidium have been implicated in waterborne outbreaks or diarrheal disease. These infections would result from the ingestion of Giardia cysts and/or Cryptosporidium oocysts

Answer 398

B Fecal immunoassays have become more commonly used for the diagnosis of infections with Giardia and Cryptosporidium. These tests are available in EIA, FA, or rapid cartridge formats

Answer 399

C With the possible exception of direct inoculation infection in the eye, the microsporidia are thought to initially infect the gastrointestinal (GI) tract through ingestion of the infective spores; infections in other body sites are thought to disseminate from the GI tract.

Answer 400

C The majority of eye infections with Acanthamoeba spp. have resulted from the use of contaminated eye care solutions, primarily the use of homemade saline. It is recommended that all solutions be discarded at the expiration date. Continued use may increase the risk of environmental contamination of the fluids

Answer 401

C Currently, the most sensitive method of those listed for the recovery of Acanthamoeba spp. from clinical specimens is the non-nutrient agar culture plate seeded with E. coli. The amoebae feed on the bacteria; both trophozoites and cysts can be recovered from the agar surface

Answer 402

A Although the microsporidia have been known as pathogens in many groups of animals, their involvement in humans has primarily been in immunocompromised patients, especially those with AIDS. Microsporidia can be found in different tissues, and currently, there are a number of genera implicated in human disease.

Answer 403

B The decolorizer in modified acid-fast stains (Kinyoun cold method, modified hot method) is usually 1% to 3% sulfuric acid rather than the much stronger acid alcohol used in the routine AFB stains

Answer 404

D T. trichiura (whipworm) may cause diarrhea and occasionally dysentery in very heavy infections; however, the worms are confined to the intestine, and myalgias are not seen in this helminth infection.

Answer 405

C H. nana has a thin eggshell containing a six-hooked embryo (oncosphere) and polar filaments that lie between the eggshell and the embryo. The striated eggshell is generally associated with the eggs of Taenia spp.

Answer 406

B The Knott concentration is designed to allow the recovery of microfilariae from a blood specimen. Dilute formalin (2%) is used; blood is introduced into the formalin, the red cells lyse, and the sediment can be examined as a wet preparation or permanent stained smear (Giemsa or hematoxylin-based stain) for the presence of microfilariae.

Answer 407

D The egg of S. mansoni is characterized by a large lateral spine; S. haematobium has the characteristic terminal spine.

Answer 408

C The request for blood films for malaria should always be considered a STAT request. Any laboratory providing these services should have coverage 24 hours a day, 7 days a week. In cases of P. falciparum malaria, any delay in diagnosing the infection could be fatal for the patient.

Answer 409

B A latent infection with S. stercoralis acquired years before may cause severe symptoms in the immunosuppressed patient (“autoinfective” capability of life cycle and migratory route through the body leading to the hyperinfection syndrome and disseminated disease).

Answer 410

D The collection of a series of consecutive Scotch/cellophane tape preparations is recommended for the diagnosis of infection with the pinworm E. vermicularis.

Answer 411

B Eosinophilic meningoencephalitis is a form of larva migrans and is caused by A. cantonensis, the rat lungworm. This infection, common in the Pacific region, is associated with CSF symptoms and sometimes eye involvement

Answer 412

D Duplicate cultures should be set up, and specific American Type Culture Collection (ATCC) strains should be cultured along with the patient specimens to confirm that the culture system is operating properly. This approach is somewhat different from that used in diagnostic bacteriology and mycology.

Answer 413

C As E. coli cysts mature, the cyst wall becomes more impenetrable to fixatives. Consequently, the cysts may be visible in the concentrate sediment but appear very distorted or pale on the permanent stained smear.

Answer 414

A The cause of hydatid disease is E. granulosus, and the human is classified as the accidental intermediate host. Infection occurs when humans accidentally ingest the eggs of E. granulosus and the hydatid cysts develop in the liver, lung, and other organs of the human instead of sheep (normal cycle).

Answer 415

D Bear meat is another excellent source of T. spiralis. In this case, the patient had evidently consumed poorly cooked infected bear meat, thus ingesting the encysted larvae of T. spiralis.

Answer 416

B The accidental ingestion of T. solium eggs can result in the disease called cysticercosis. The cysticerci will develop in a number of different tissues, including the brain, and humans are accidental intermediate hosts.

Answer 417

B The fact that the patient has received a transplant, is on immunosuppressive drugs, and has increasing diarrhea, suggests infection with microsporidia; the most appropriate method would be the modified trichrome stain

Answer 418

D On the basis of the history, the most relevant procedure to perform is the preparation and examination of thick blood films for the recovery and identification of microfilariae. The symptoms suggest early filariasis

Answer 419

C Cryptosporidium oocysts (unlike those of C. belli) are immediately infective when passed in stool, and nosocomial infections have been well documented with this coccidian

Answer 420

D T. cruzi, the cause of Chagas disease, exists in two forms within humans, the trypomastigote in the blood and the amastigote in the striated muscle (usually cardiac muscle and intestinal tract muscle

Answer 421

C Travel history (areas of drug resistance), the date of return to the United States (primary versus relapse case), and history of antimalarial medication and illness (severe illness, few organisms on smear) are very important questions to ask. Without this information, a malaria diagnosis can be missed or delayed with severe patient consequence

Answer 422

A Cryptosporidium can be transmitted through contaminated municipal water sources. Such outbreaks have been well documented

Answer 423

B The coccidian C. cayetanensis has been linked to mini-outbreaks of diarrheal disease. Epidemiological evidence strongly implicates various berries, basil, mesclun, and snow peas as likely causes. These outbreaks are very sporadic and tend to occur primarily in March through May, primarily from imported foods.

Answer 424

C The adult O. volvulus reside in subcutaneous nodules, and the microfilariae are found in the fluids right under the outer layers of skin; thus the appropriate diagnostic test is the microscopic examination of skin snips for the presence of microfilariae

Answer 425

A The pinworm, E. vermicularis, is the most common parasitic infection throughout the world, and the eggs are infective within just a few hours. It has been said, “You either had the infection as a child, have it now, or will have it again when you have children.”

Answer 426

B The eggs of T. trichiura (the whipworm) have been described as being barrel shaped, with a thick shell and two polar plugs, one at each end of the egg.

Answer 427

C The eggs of T. canis are infectious for humans and cause visceral and ocular larva migrans. These ascarid eggs of the dog can infect humans; the eggs hatch and the larvae wander through the deep tissues, occasionally the eye. In this case, the human becomes the accidental intermediate host.

Answer 428

A Both P. vivax and P. ovale infect young RBCs, have true stippling (Schüffner dots), contain malarial pigment, and have a true relapse stage in the life cycle

Answer 429

C Both Cryptosporidium and Strongyloides have the capability for internal autoinfection in their life cycles. This means that the cycle and infection can continue even after the patient has left the endemic area. In the case of Cryptosporidium, the cycle continues in patients who are immunocompromised and incapable of self cure.

Answer 430

D Infectious routes for microsporidial infections have been confirmed as ingestion and inhalation of the spores; direct transfer of infectious spores from environmental surfaces to the eyes has also been reported

Answer 431

D Routine O&P examinations usually do not allow the detection of Cryptosporidium oocysts and microsporidial spores; special stains are required. Modified acid-fast stains for Cryptosporidium and modified trichrome stains for the microsporidia are recommended

Answer 432

C Evidence of the cross/diagonal line polar tubules provides confirmation for microsporidial spores in the clinical specimen. Not all spores will contain visible polar tubules; the stain of choice is the modified trichrome stain (Ryan/blue or Weber/green

Answer 433

D Early ring forms of the fifth human malaria, P. knowlesi, resemble most closely ring forms of P. falciparum. However, in the early ring form stage, all human malaria species tend to look very similar

Answer 434

D On passage in stool from the gastrointestinal tract, the oocysts of Cryptosporidium are immediately infective; nosocomial infections have been documented with this organism. The oocysts of other coccidia have to undergo further development before they are infective

Answer 435

C Older developing stages (developing trophozoites, schizonts) of the fifth human malaria, P. knowlesi, tend to closely resemble those of P. malariae (band forms, “daisy head” mature schizont

Answer 436

D Autofluorescence is often used in preliminary testing for the presence of C. cayetanensis oocysts (8–10 μm). If positive, then the specimen can be stained using modified acid-fast stains for confirmation

Answer 437

A Characteristics of the fifth human malaria, P. knowlesi, include a rapid erythrocytic cycle of 24 hours, possible infection of all ages of RBCs (as with P. falciparum), and the potential for serious disease (as with P. falciparum).

Answer 438

A The use of a modified trichrome stain (Ryan/blue, Weber/green; both with a very high dye content) and light microscopy (oil immersion using the 100× objective) can be used to confirm a microsporidial infection

Answer 439

B Although the pathogenicity of Blastocystis spp. continues to be debated (some patients are symptomatic, and some are asymptomatic), it is now known that the organisms we see microscopically comprise a number of subtypes/strains/subspecies. Approximately half these subtypes are pathogenic for humans, and half are nonpathogenic; however, morphologically they all look the same. Correct reporting of this infection should be Blastocystis spp.; numbers should also be reported (rare, few, moderate, many, packed

Answer 440

D All of the changes mentioned are seen in a tube of EDTA blood that has been held too long prior to blood film preparation. Many morphological changes of the parasites, as well as lost organisms, occur under these conditions. These blood specimens should always be handled STAT (ordering, collection, processing, examination, and reporting).

Answer 441

D With thick blood films, the RBCs must be ruptured/laked either prior to or during staining. Without laking the RBCs, the stained smears would be too thick or too dark to be able to visualize any morphology. In contrast, the thin blood film is fixed prior to/during staining to preserve the RBC morphology; the parasite morphology within the infected RBC can be reviewed for possible diagnosis to the species level. The thin film has a much smaller amount of blood than the thick blood film.

Answer 442

A CNA inhibits most strains of S.saprophyticus. Therefore, blood agar should be used when culturing catheterized urine samples from young female patients. Most S. saprophyticus isolates are obtained from female patients ages 20 to 30 years.

Answer 443

C The high concentration of NaCl (7.5%) in mannitol salt agar allows for the recovery of S. aureus from heavily contaminated specimens while inhibiting other organisms. Furthermore, S. aureus ferments mannitol, thus allowing for easy detection of yellowhaloed colonies of S. aureus on red mannitol salt agar

Answer 444

B Generally, tube 2 or 3 is submitted to the microbiology laboratory for culture and Gram staining. To ensure recovery of any pathogens and correct diagnosis without contamination by other bacteria, immediate centrifugation and inoculation to the appropriate media, as well as Gram staining, should be performed prior to delivery of the specimen to the other departments for testing.

Answer 445

D In general, a 24-hour urine is unsuitable for culture; a first morning specimen is best for the recovery of mycobacteria in the urine.

Answer 446

B Specimens submitted for culture and recovery of any bacteria should be submitted without fixatives

Answer 447

D EIEC produces dysentery similar to that by Shigella, with invasion and destruction of the intestinal mucosal epithelium. Leukocytes are seen on the Gram-stained smear. Adults who travel to foreign countries, especially Mexico, are at greatest risk.

Answer 448

A K. oxytoca is similar to K. pneumoniae except that the indole test is positive for K. oxytoca.

Answer 449

B Y. enterocolitica has been associated with fatal bacteremia and septic shock from contaminated blood transfusion products. The motility at room temperature is a clue to this identification

Answer 450

B E. coli O157:H7 is usually the most common isolate from bloody stools of the EHEC group, which results from undercooked beef. These strains are waterborne and foodborne, and the infections from E. coli O157:H7 are greatest during the summer months in temperate climates.

Answer 451

A Y. enterocolitica is responsible for diseases in younger persons. Blood and leukocytes can be present in stools. Patients (usually teens) exhibiting appendicitislike symptoms with lactose-negative colonies growing on MacConkey agar (small colonies at 24 hours, but larger colonies at 48 hours if incubated at room temperature) should be tested for the growth of Y. enterocolitica.

Answer 452

A S. maltophilia is the third most frequently isolated nonfermentative gram-negative rod in the clinical laboratory. Patients with CF are at greater risk for infections because of previous antimicrobial treatment and recurrent pneumonia and because

Answer 453

D E. corrodens is part of the normal flora of the human mouth and typically “pits” the agar. This organism is capnophilic (needing increased CO2).

Answer 454

B C. canimorsus is associated with septicemia or meningitis following dog bites. All Capnocytophaga strains are capnophilic, facultative anaerobic, gram-negative, slender or filamentous rods with tapered ends

Answer 455

B Both E. coli O157:H7 and Shigella spp. are invasive and cause bloody stools

Answer 456

A S. aureus is an opportunistic human pathogen. A wound or ulcer infected with S. aureus that is left untreated is especially detrimental to a patient with diabetes

Answer 457

C MRSA isolates are usually tested for susceptibility or resistance to vancomycin, a glycopeptide

Answer 458

B Group B streptococci (S. agalactiae) are important pathogens and can cause serious neonatal infections. Women who are found to be heavily colonized vaginally with S. agalactiae pose a threat to the newborn, especially within the first few days after delivery. The infection acquired by the infant is associated with pneumonia

Answer 459

C Enterococcus (Streptococcus) faecalis is the cause of up to 20% of the bacterial endocarditis cases and is the most commonly encountered species in this condition

Answer 460

A N. gonorrhoeae can be presumptively identified from a male patient (only) from Gram staining and growth on selective agar. In female patients, the normal flora from a urethral swab may appear to be N. gonorrhoeae (gram-negative diplococci) but may be part of the normal flora, such as Veillonella spp. (anaerobic gram-negative cocci resembling N. gonorrhoeae).

Answer 461

C G. vaginalis, a gram-negative or gram-variable pleomorphic coccobacillus, causes bacterial vaginosis, but is also present as part of the normal vaginal flora of women of reproductive age with a normal vaginal examination. “Clue cells” are vaginal epithelial cells with gram-negative or gram-variable coccobacilli attached to them.

Answer 462

A L. monocytogenes is catalase positive and displays a “tumbling” motility at room temperature. Streptococcus spp. are catalase negative and nonmotile.

Answer 463

A M. tuberculosis is niacin positive and nonphotochromogenic. This organism takes up to 3 weeks to grow on selective agar.

Answer 464

A A swarmer on blood agar would most likely be a Proteus spp. A lactose nonfermenter and swarmer that is often isolated from UTIs is P. mirabilis.

Answer 465

B P. aeruginosa has a distinctive grape odor. The best tests to differentiate it from Acinetobacter spp. are growth at 42°C, oxidase and motility which give the following results

Answer 466

C H. haemolyticus requires both X and V factors for growth and lyses horse erythrocytes.

Answer 467

A C. perfringens is an anaerobic gram-positive rod that is often isolated from the tissue of patients with gas gangrene (myonecrosis). Spore production is not usually seen with this organism, which may also stain gram-negative.

Answer 468

B B. fragilis is the most frequently isolated gram-negative anaerobic bacillus. It is resistant to many antibiotics. A good screening agar is a 20% bile plate that does not support the growth of Prevotella spp. or Porphyromonas spp.

Answer 469

B Breakpoint susceptibility testing is done by selecting only two appropriate drug concentrations for testing. If the results show growth at both concentrations, then resistance is indicated; growth only at the lower concentration signifies an intermediate result; no growth at either concentration is interpreted as susceptible.

Answer 470

B Elizabethkingia (formerly Chryseobacterium) meningosepticum is a well-known cause of neonatal meningitis. It grows well on chocolate agar, producing yellowpigmented colonies

Answer 471

A L. pneumophila is the cause of pneumonia and can occur as part of an epidemic sporadically or as a nosocomial infection or may be community acquired. The appearance of mottled, cut-glass colonies on buffered CYE agar under low power and the use of a DIF technique on sputum samples determine the presence of L. pneumophila. The most common environmental sites for recovery are shower heads, faucets, water tanks, and air-conditioning systems.

Answer 472

A Leuconostoc spp. are vancomycin-resistant opportunistic pathogens, and infection with them occurs after invasive procedures. The organisms are often recovered from positive neonatal blood culture specimens following colonization during delivery.

Answer 473

B Gemella spp. are often recovered from patients with endocarditis and meningitis. In the Gram-stained smear, they resemble staphylococci morphologically but are catalase negative.

Answer 474

A Leuconostoc spp. and Pediococcus spp. are vancomycin-resistant, catalase-negative, gram-positive aerobic organisms recovered from immunosuppressed patients.

Answer 475

D S. aureus is a usual cause of skin infections and a common cause of cellulitis, impetigo, postsurgical wounds, and scalded skin syndrome in infants.

Answer 476

B S. aureus is the cause of “scalded skin” syndrome in newborn infants. The production of a potent exotoxin (exfoliatin) causes the epidermis to slough off, leaving the newborn’s skin with a red, raw texture or a burned, scalded look.

Answer 477

A E. faecalis gives a positive reaction to the PYR test and is often implicated in UTIs. It is part of the normal flora of the female genitourinary tract and the human gastrointestinal tract. On 5% sheep blood agar, E. faecalis colonies may appear as nonhemolytic, α-hemolytic, or β-hemolytic colonies, depending on the strain

Answer 478

C S. pneumoniae colonies appear as α-hemolytic donut-shaped colonies on 5% sheep blood agar. The mucoid colonies may appear “wet” or “watery” because of the capsule surrounding the organism. The Gram stain smear reveals lancet-shaped gram-positive cocci in pairs surrounded by a clear area (the capsule). To differentiate the viridans streptococci from S. pneumoniae, the most appropriate test is the optochin disk test. S. pneumoniae on blood agar are susceptible to optochin, but viridans streptococci are resistant.

Answer 479

B H. pylori is not easily cultured for growth and identification. Tissue samples should be transported in appropriate media and tested immediately. Other means of successful identification are rapid urease test on biopsy material; urea breath test; and serological tests for the detection of antibodies to H. pylori by ELISA and IFA procedures

Answer 480

A L. monocytogenes colonies recovered from blood and CSF display a narrow zone of β-hemolysis on 5% sheep blood agar, which often mimics group B β-hemolytic streptococci. A catalase test and Gram staining will differentiate the two organisms. L. monocytogenes are catalase positive, motile (tumbling motility at room temperature), and rod shaped instead of being coccus shaped.

Answer 481

C C. diphtheriae, unlike other Corynebacterium spp., are not part of the normal flora of the human nasopharynx. Exposure through direct contact (respiratory or cutaneous lesions) is the most likely mode of transmission. Underdeveloped countries are the prime areas where exposure to C. diphtheriae occurs. The diagnosis is made more rapidly when the examining physician alerts the laboratory that diphtheria is suspected so that Tinsdale agar or other media containing tellurite salts can be used for culture. Corynebacterium will grow on blood and chocolate agars, but Tinsdale agar is the preferred culture medium because the potassium tellurite in the agar causes C. diphtheriae to produce brown colonies surrounded by a brown halo. The halo effect is seen with C. diphtheriae, C. ulcerans, and C. pseudotuberculosis, but not with other Corynebacterium or with other pigmented colonies growing on Tinsdale agar, such as Streptococcus or Staphylococcus spp

Answer 482

B G. vaginalis is part of the normal flora (anorectal) of adults and children. Clue cells (vaginal epithelial cells with gram-variable coccobacilli on the cell margins) are seen in vaginal washings and the organism grows slowly on chocolate agar, 5% sheep blood agar, and V-agar.

Answer 483

A N. asteroides is a gram-positive, beaded, long bacillus and is partially acid fast. It is an intracellular pathogen that grows in human cells. Immunocompromised patients are susceptible to infections, especially pulmonary infections, which then disseminate to other organs, often proving fatal

Answer 484

A Early detection in pregnant women is very important when dealing with L. monocytogenes. If it is not detected and treated, infection of the fetus, resulting in stillbirth, abortion, or premature birth may result. Detection can also be made postpartum by culturing the CSF, blood, amniotic fluid, and respiratory secretions of the neonate

Answer 485

D The overgrowth of C. difficile in the bowel is the cause of antimicrobial-associated colitis. Culturing for C. difficile is less specific for infection than toxin testing, but is the more sensitive method, along with NAT, to detect possible disease related to C.

Answer 486

B P. acnes is a diphtheroid (pleomorphic rod) that may appear to branch on the Gramstained smear. It is one of the most common organisms isolated from blood cultures and is often a contaminant. Abundant propionic acid is produced by GLC.

Answer 487

A C. septicum is often recovered from patients with malignancies or other diseases of the colon, especially the cecum. The following chart defines the swarming Clostridium spp.

Answer 488

B The B. fragilis group is a dominant part of the indigenous flora of the large bowel and is recovered most commonly from postoperative abdominal fluids. The B. fragilis group is more resistant to antibiotics and is not pigmented. Prevotella and Porphyromonas spp. are pigmented.

Answer 489

A A slender gram-negative rod with pointed ends that does not grow on 20% bile agar rules out B. fragilis group and indicates F. nucleatum.

Answer 490

B C. albicans is the common cause of oral thrush involving the mucocutaneous membranes of the mouth. C. albicans is part of the normal flora of skin, mucous membranes, and the gastrointestinal tract.

Answer 491

D A. fumigatus is the cause of aspergillosis and involves the organism colonizing the mucous plugs in the lung. This is called allergic aspergillosis and is characterized by a high titer of IgE antibody to Aspergillus. Invasive aspergillosis seen in patients with neutropenia exhibits sinusitis and is disseminated throughout the body.

Answer 492

A T. mentagrophytes, a common cause of athletes’ foot, is sometimes confused with T. rubrum, the most common dermatophyte to infect humans. The differential tests are shown in the following chart

Answer 493

C C. neoformans produces brown colonies on birdseed agar, is urease positive, and produces only blastospores. Immunosuppressed patients are susceptible to this organism.

Answer 494

A CMV infection in young adults causes a self-limiting mononucleosis syndrome. CMV infections are common and usually self-limiting, except in neonates and immunosuppressed patients, in whom they may cause a life-threatening situation.

Answer 495

B With the permission of the patient (state law may require him or her to sign a consent form) and after counseling of the student nurse, the appropriate course of action is to test the patient for HIV by using a screening test (EIA or ELISA). The student should also be tested for baseline values. If the HIV test result is positive for the patient, the student is administered the appropriate antiviral drug(s) immediately or within 2 hours of the incident. Confirmatory testing should be done after an initial positive HIV test result.

Answer 496

B Herpes genitalis is an infection caused by HSV-2. Symptomatic primary herpes by HSV-2 is responsible for about 85% of herpes infections. HSV-1 (causing the other 15%) does not involve recurring infections of herpes and causes fever blisters. HSV-2 causes 99% of recurrent genital herpes.

Answer 497

D If the patient has malaria and has been taking prophylaxis (often sporadically), the number of parasites on the blood smear will be reduced and examination of routine thick and thin blood films should be more exhaustive. Furthermore, specific geographic travel history may help to determine whether chloroquine-resistant P. falciparum may be a factor. Also, if the patient is immunologically naïve (no prior exposure to malaria), these patients usually come into an emergency room with a very low parasitemia and no periodicity; thus malaria may not even be suspected

Answer 498

C One of the coccidian parasites, C. cayetanensis, has been implicated in cases of human diarrhea. The recommended stains are modified acid-fast stains, and the organisms are quite variable in their staining characteristics. The color can range from clear (resembling wrinkled cellophane) to red-purple. The oocysts are immature when passed (no internal morphology) and they measure about 8 to 10 μm

Answer 499

A It is now well established that E. histolytica is being used to designate the true pathogenic species, whereas E. dispar is now being used to designate nonpathogenic species. Unless trophozoites containing ingested RBCs (E. histolytica) are seen, the two organisms cannot be differentiated on the basis of morphology. On the basis of this information, there are now two separate species, only one of which (E. histolytica) is pathogenic. Because this patient is asymptomatic, the organisms seen in the fecal smears are probably E. dispar (nonpathogen); the laboratory report should have said “Entamoeba histolytica/Entamoeba dispar”—unable to differentiate on the basis of morphology unless trophozoites are seen to contain ingested RBCs (E. histolytica— true pathogen). Immunoassays and real time PCR can differentiate the true pathogen, E. histolytica, from nonpathogenic species of Entamoeba, and are more sensitive and specific than microscopic examination alone.

Answer 500

B It is well known that G. lamblia (G. intestinalis, G. duodenalis) trophozoites adhere to the intestinal mucosal surface by means of the sucking disk. Although a patient may have giardiasis and be symptomatic, confirmation of the infection from stool examinations may require more than the routine three stools or may require the examination of duodenal contents. These organisms are shed very sporadically. Two fecal immunoassays may be required to diagnose giardiasis; this also relates to sporadic shedding. The need for two fecal immunoassays does not apply to cryptosporidiosis; a single immunoassay is sufficien

Answer 501

C Although infection with S. stercoralis may have been acquired in Central America many years before, the patient may have remained asymptomatic while the infection was maintained at a low level in the body via the autoinfective portion of the life cycle. As the patient became more immunosuppressed (steroids), the life cycle began to reactivate with penetration of the larvae through the intestinal wall (abdominal pain) and larval migration through the lungs (pneumonia), and the patient may have presented with evidence of sepsis (often with gram-negative bacteria carried with the larvae as they penetrate the intestinal wall). In patients who become immunosuppressed, the life cycle of Strongyloides may be reactivated, and this results in serious illness (e.g., hyperinfection syndrome, disseminated disease); this can occur many years after the initial infection and after the patient has left the endemic area.