Bacteriology: 7.2 Enterobacteriaceae Flashcards
Biochemically, the Enterobacteriaceae are gram-negative rods that:
Ferment glucose, reduce nitrate to nitrite, and are oxidase negative
The family – consists of more than 100 species and represents the most commonly encountered isolates in clinical specimens.
Enterobacteriaceae
All Enterobacteriaceae ferment – and are – negative and nonsporulating. Most Enterobacteriaceae are motile, but the genera Shigella and Klebsiella are not.
glucose, oxidase -
All Enterobacteriaceae ferment glucose and are oxidase negative and nonsporulating. Most Enterobacteriaceae are motile, but the genera – and –are not.
Shigella and Klebsiella
The ortho-nitrophenyl-β-galactopyranoside (ONPG) test is most useful when differentiating:
Shigella spp. from some strains of Escherichia coli
The – test detects β-galactosidase activity and is most useful in distinguishing late lactose fermenters from lactose nonfermenters.
ONPG test
Some strains of E. coli are slow lactose fermenters and may be confused with – spp., which do not ferment lactose.
E. coli are ONPG positive while –spp. are ONPG negative.
Shigella
The Voges-Proskauer (VP) test detects which end product of glucose fermentation?
Acetoin
– or – , an end product of glucose fermentation, is converted to diacetyl after the addition of the VP reagents (α-naphthol and 40% potassium hydroxide [KOH]). Diacetyl is seen as a red- to pink-colored complex.
Acetoin or carbinol
Acetoin or carbinol, an end product of glucose fermentation, is converted to – after the addition of the VP reagents (α-naphthol and 40% potassium hydroxide [KOH]). Diacetyl is seen as a red- to pink-colored complex.
diacetyl
Acetoin or carbinol, an end product of glucose fermentation, is converted to diacetyl after the addition of the VP reagents (–). Diacetyl is seen as a red- to pink-colored complex.
α-naphthol and 40% potassium hydroxide [KOH]
Diacetyl is seen as a — to —colored complex.
red- to pink
At which pH does the methyl red (MR) test become positive?
4.5
Both – tests detect acid production from the fermentation of glucose.
MR and VP
positive– test 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).
MR
A positive Simmons citrate test is seen as a
Blue color in the medium after 24 hours of incubation at 35°C
The – 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.
Simmons citrate test
The Simmons citrate test determines if an organism can utilize citrate as the sole source of carbon. The medium turns –, 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.
blue
In the test for urease production, ammonia reacts to form which product?
Ammonium carbonate
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) – at pH–
pink at pH 8.0
Which of the following reagents is added to detect the production of indole?
p-Dimethylaminobenzaldehyde
The indole test detects the conversion of tryptophan (present in the media) to indole by the enzyme –.
tryptophanase
– is detected by the reaction with the aldehyde group of p-dimethylaminobenzaldehyde (the active reagent in Kovac’s and Ehrlich’s reagents) in acid, forming a red complex.
Indole
Decarboxylation of the amino acids lysine, ornithine, and arginine results in the formation of:
Amines
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 – to –
yellow to purple
Lysine iron agar (LIA) showing a purple slant and a blackened butt indicates:
Salmonella spp.
– is used as an aid for the identification of Salmonella species. It contains phenylalanine, lysine, glucose, thiosulfate, ferric ammonium citrate, and bromcresol purple.
LIA
Salmonella produce H2S from –. This reduces ferric ammonium citrate, forming ferrous sulfate and causing the butt to blacken.
thiosulfate
Salmonella also decarboxylate lysine to produce alkaline amines, giving the slant its purple color and differentiating it from – spp., which are lysine decarboxylase negative.
Citrobacter
Putrescine is an alkaline amine product of which bacterial enzyme?
Ornithine decarboxylase
– is the amine product of the decarboxylation of ornithine
Putrescine
Which genera are positive for phenylalanine deaminase?
Morganella, Providencia, and Proteus
Phenylalanine deaminase oxidatively deaminates phenylalanine, forming phenylpyruvic acid. When a solution of ferric chloride is added, the iron reacts with phenylpyruvic acid, forming a —colored complex.
green
– 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–positive as well.
Phenylalanine deaminase
Kligler iron agar (KIA) differs from triple-sugar iron agar (TSI) in the:
Use of sucrose in the medium
Both KIA and TSI contain 10-fold more – than glucose, peptone, and phenol red to detect acid production (turns yellow) and sodium thiosulfate and ferrous ammonium sulfate to detect H2S production.
lactose
TSI contains – and KIA does not.
sucrose
Organisms fermenting either sucrose or lactose will turn the slant of the agar tube –.
yellow
some organisms (e.g., many species of Cedecea, Citrobacter, Edwardsiella, and Serratia) will produce a – slant on TSI but a – slant on KIA.
yellow, red
The malonate test is most useful in differentiating which members of the Enterobacteriaceae?
Salmonella subgroups 2, 3 (the former Arizona)
The – test determines whether an organism can utilize sodium malonate as the sole source of carbon. – is broken down, forming alkaline metabolites that raise the pH of the broth above 7.6.
malonate
malonate test:
This causes bromthymol blue to turn from green to –.
deep blue (Prussian blue)
malonate test:
E. coli, Shigella, and most Salmonella are malonate negative, whereas – and – (formerly Arizona) subgroups 2, 3a, and 3b are positive. Proteus, Providencia, Serratia, and Yersinia are also malonate negative
Enterobacter and Salmonella
Which genera of the Enterobacteriaceae are known to cause diarrhea and are considered enteric pathogens?
Escherichia, Salmonella, Shigella, and Yersinia
(4) are responsible for the majority of enteric diarrhea cases attributable to the Enterobacteriaceae family.
Escherichia, Salmonella, Shigella, and Yersinia
An isolate of E. coli recovered from the stool of a patient with severe bloody diarrhea should be tested for which sugar before sending it to a reference laboratory for serotyping?
Sorbitol (fermentation)
An isolate of E. coli recovered from a stool culture in hemorrhagic colitis can be definitely identified only by –.
serotyping
The strain of E. coli responsible for hemorrhagic colitis is – and is usually negative for sorbitol fermentation. Colonies of this strain of E. coli appear colorless on MacConkey agar with sorbitol added.
O157:H7
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 – on MacConkey agar with sorbitol added.
colorless
Care must be taken when identifying biochemical isolates of Shigella because serological cross reactions occur with:
E. coli
Serological confirmation of Shigella isolates is based upon – antigen typing.
O antigen
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 – serotype.
Alkalescens-Dispar
Which species of Shigella is most commonly associated with diarrheal disease in the United States?
S. sonnei
The Shigella spp. are lactose – 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.
nonfermenters
– is the species most often isolated from diarrhea cases in the United States. It is more active biochemically than the other species owing to ornithine decarboxylase and β-galactosidase activity.
S. sonnei
Which of the following tests best differentiates Shigella species from E. coli?
Lactose, indole, ONPG, and motility
–, positive for lactose, indole, and ONPG are usually motile. Shigella species do not ferment lactose or produce indole, lack β-galactosidase, and are nonmotile.
E. coli
E. coli, positive for lactose, indole, and ONPG are usually motile. – species do not ferment lactose or produce indole, lack β-galactosidase, and are nonmotile.
Shigella
Which genera of Enterobacteriaceae are usually nonmotile at 36°C?
Shigella, Klebsiella, and Yersinia
Shigella spp. and Klebsiella spp. are for the most part nonmotile. – can be motile at 22°C but is nonmotile at 36°C.
Yersinia
Other members of the Enterobacteriaceae that have been isolated from human specimens and are usually – include Leminorella, Rahnella, and Tatumella.
nonmotile
Fever, abdominal cramping, watery stools, and fluid and electrolyte loss preceded by bloody stools 2-3 days before is characteristic of shigellosis but may also result from infection with:
Campylobacter spp.
–spp. and –spp. are both causes of diarrhea, abdominal pain, fever, and sometimes vomiting. Blood is present in the stools of patients infected with –as a result of invasion and penetration of the bowel. Young children may also exhibit bloody stools when infected with –.
Shigella spp. and Campylobacter spp
Cold enrichment of feces (incubation at 4°C) in phosphate-buffered saline prior to subculture onto enteric media enhances the recovery of:
Y. enterocolitica
– 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.
Cold enrichment
Cold enrichment : In addition to Yersinia, the technique has been used to enhance recovery of – from specimens containing other bacteria.
Listeria monocytogenes
Which group of tests, along with colonial morphology on primary media, aids most in the rapid identification of the Enterobacteriaceae?
Indole, oxidase, MacConkey, and blood agar plates
All Enterobacteriaceae are oxidase –.
negative
– 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.
E. coli
– swarm on blood agar and are indole negative.
Proteus spp.
A routine, complete stool culture procedure should include media for the isolation of E. coli O157:H7 as well as:
Salmonella, Shigella, Yersinia, Campylobacter, and Staphylococcus aureus
are usually not included in a routine stool culture. If Vibrio spp. are suspected, a special request should be included.
V. cholerae and C. difficile
– culture requires special media (CCFA) that inhibit other anaerobic flora and facultative anaerobic flora and should be requested specifically if symptoms warrant.
C. difficile
– agar with – will allow the E. coli O157:H7 to be recovered.
MacConkey agar with sorbitol
–spp. can be detected on a regular MacConkey agar plate.
Yersinia
Which group of tests best identifies the Morganella and Proteus genera?
Motility, urease, and phenylalanine deaminase
–and –spp. are motile, produce urease, and deaminate phenylalanine
Morganella and Proteus
Which group of tests best differentiates Enterobacter aerogenes from Edwardsiella tarda?
Hydrogen sulfide (H2S) production, sucrose fermentation, indole, and VP
Enterobacter sakazakii can best be differentiated from Enterobacter cloacae by which of the following characteristics?
Yellow pigmentation and negative sorbitol fermentation
– is 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. sakazakii).
E. sakazakii
E. sakazakii is called a yellow-pigmented E. cloacae and is best differentiated from E. cloacae by – fermentation (95% positive for E. cloacae and 0% for E. sakazakii).
sorbitol
E. cloacae is usually positive for –and –(65% and 75%, respectively) and E. sakazakii is usually negative (1% and
< 20%, respectively). Both species are usually motile and arginine dihydrolase positive.
urease and malonate
