8.1 Specimen Collection, Media, and Methods Flashcards
The aseptic collection of blood cultures requires that the skin be cleansed with:
A. 2% iodine and then 70% alcohol solution
B. 70% alcohol and then 2% iodine or an iodophor
C. 70% alcohol and then 95% alcohol
D. 95% alcohol only
B. 70% alcohol and then 2% iodine or an iodophor
To attain asepsis of the skin, 70% alcohol followed by 2% iodine is used for obtaining blood cultures.
When cleansing the skin with alcohol and then iodine for the collection of a blood culture, the iodine (or iodophor) should remain intact on the skin for at least:
A. 10 seconds
B. 30 seconds
C. 60 seconds
D. 5 minutes
C. 60 seconds
The iodine should remain on the skin for 1 minute because instant antisepsis does not occur when cleansing the skin for a blood culture.
What is the purpose of adding 0.025% to 0.050% sodium polyanethol sulfonate (SPS) to nutrient broth media for the collection of blood cultures?
A. It inhibits phagocytosis and complement
B. It promotes formation of a blood clot
C. It enhances growth of anaerobes
D. It functions as a preservative
A. It inhibits phagocytosis and complement
SPS is used in most commercial blood culture products because it functions as an anticoagulant and prevent phagocytosis and complement activation. In addition, SPS neutralizes aminoglycoside antibiotics. Addition of SPS may inhibit some Neisseria and Peptostreptococcus, but this can be reversed with 1.2% gelatin.
A flexible calcium alginate nasopharyngeal swab is the collection device of choice for recovery of which organism from the nasopharynx?
A. Staphylococcus aureus
B. Streptococcus pneumoniae
C. Corynebacterium diphtheriae
D. Bacteroides fragilis
C. Corynebacterium diphtheriae
C.diphtheriae must be recovered from the deep layers of the pseudomembrane that forms in the nasopharyngeal area. A flexible calcium alginate nasopharyngeal swab is the best choice for collecting a specimen from the posterior nares and pharynx.
Semisolid transport media, such as Amies, Stuart, or Cary-Blair, are suitable for the transport of swabs for culture of most pathogens except:
A. Neisseria gonorrhoeae
B. Enterobacteriaceae
C. Campylobacter fetus
D. Streptococcus pneumoniae
A. Neisseria gonorrhoeae
Specimens for culture of N. gonorrhoeae are best if plated immediately or transported in a medium containing activated charcoal to absorb inhibitory substances that hinder their recovery from the specimens.
Select the method of choice for recovery of anaerobic bacteria from a deep abscess.
A. Cotton fiber swab of the abscess area
B. Skin snip of the surface tissue
C. Needle aspirate after surface decontamination
D. Swab of the scalpel used for debridement
C. Needle aspirate after surface decontamination
Anaerobic specimens are easily contaminated with organisms present on the skin or mucosal surfaces when a swab is used. Needle aspiration of an abscess following surface decontamination provides the least exposure to ambient oxygen.
Select the primary and differential media of choice for recovery of most fecal pathogens.
A. MacConkey, blood, birdseed, and Campylobacter (Campy) agars
B. Hektoen, MacConkey, MacConkey-Sorbitol, Campy blood, colistin-nalidixic acid (CNA)
C. CNA and Christensen urea agars and thioglycollate media
D. Blood, Campy, Mueller-Hinton agars, and thioglycollate media
B. Hektoen, MacConkey, MacConkey-Sorbitol, Campy blood, colistin-nalidixic acid (CNA)
Hektoen agar selectively isolates pathogenic coliforms, especially Salmonella and Shigella. MacConkey agar differentiates lactose fermenters from nonfermenters. MacConkey with sorbitol isolates *Escherichia coli 0157:H7. CNA agar contains antibiotics that prohibit growth of gram-negative coliforms but not gram-positive cocci. Campy blood agar contains the antibiotics cephalothin, trimethoprim, vancomycin, polymyxin B, and amphotericin B to prevent growth of Enterobacteriaceae, *Pseudomonas spp., and fungi.
Select the media of choice for recovery of Vibrio cholerae from a stool specimen.
A. MacConkey agar and thioglycollate media
B. Thiosulfate-citrate-bile-sucrose (TCBS) agar and alkaline peptone water (APW) broth
C. Blood agar and SEL broth
D. CNA agar
B. Thiosulfate-citrate-bile-sucrose (TCBS) agar and alkaline peptone water (APW) broth
TCBS agar is used to grow V.cholerae, which appear as yellow colonies as a result of the use of both citrate and sucrose. APW is used as an enrichment broth and should be subcultured to TCBs agar for further evaluation of Vibrio colonies.
CNA agar is used primarily for the recovery of:
A. Neisseria species
B. Enterobacteriaceae
C. Pseudomonas aeruginose
D. Staphylococcus aureus
D. Staphylococcus aureus
CNA agar inhibits the growth of gram-negative bacteria and is used to isolate gram-positive cocci from specimens. This medium is especially used for stool and wound cultures because these may contain large numbers of gram-negative rods.
In the United States, most blood agar plates are prepared with 5% or 10% red blood cells (RBCs) obtained from:
A. Sheep
B. Horses
C. Humans
D. Dogs
A. Sheep
Sheep RBCs are used in blood agar plates because they are readily available and less inhibitory than cells of other species. The type of hemolysis is determined by the source of RBCs. Sheep RBCs are chosen because of the characteristically clear hemolysis produced by Beta-hemolysins. Sheep blood does not support the growth of Haemophilus haemolyticus, eliminating the possibility of confusing it with Beta-hemolytic streptococci in throat culture.
All of the following are appropriate when attempting to isolate N. gonorrhoeae from a genital specimen except:
A. Transport the genital swab in charcoal transport medium
B. Plate the specimen directly on modified Thayer-Martin (MTM) medium
C. Plate the specimen directly on New York City or Martin-Lewis agar
D. Culture specimens in ambient oxygen at 37 C
D. Culture specimens in ambient oxygen at 37 C
MTM, New York City, and Martin-Lewis agars contain blood factors needed to support the growth of *N. gonorrhoeae as well as antibiotics that prevent growth of normal genital flora. Cultures must be incubated in 3% to 7% carbon dioxide (CO2) at 35C. Cultures should be held a minimum of 48 hours before being considered negative.
Chocolate agar and MTM agar are used for the recovery of:
A. Haemophilus spp. and Neisseria spp., respectively
B. Haemophilus spp. and N. gonorrhoeae, respectively
C. Neisseria spp. and Streptococcus spp., respectively
D. Streptococcus spp. and Staphylococcus spp., respectively
B. Haemophilus spp. and N. gonorrhoeae, respectively
Chocolate agar provides X factor (hemin) and V factor (nicotinamide adenine dinucleotide [NAD] required for the growth of Haemophilus spp. Thayer-Martin medium is a chocolate agar containing the antibiotics that permit isolation of N. gonorrhoeae in specimens containing large numbers of gram-negative bacteria, including commensal Neisseria spp.
Cycloserine-cefoxitin-fructose agar (CCFA) is used for the recovery of:
A. Yersinia enterocolitica
B. Yersinia intermedia
C. Clostridium perfringens
D. Clostridum difficile
D. Clostridum difficile
CCFA is used for recovery of C. difficile from stool cultures. Cycloserine and cefoxitin inhibit growth of gram-negative coliforms in the stool specimen. *C. difficile ferments fructose, forming acid that, in the presence of neutral red, causes the colonies to become yellow.
Deoxycholate agar (DCA) is useful for the isolation of:
A. Enterobacteriaceae
B. Enterococcus spp.
C. Staphylococcus spp.
D. Neisseria spp.
A. Enterobacteriaceae
DCA inhibits gram-positive organisms. N.gonorrhoeae and Neisseria meningitidis are too fastidious to grow on DCS. Citrate and deoxycholate salts inhibit growth of gram-positive bacteria. The media contain lactose and neutral red, allowing differentiation of lactose fermenters (pink colonies) from nonfermenters (colorless).
Xylose lysine deoxycholate (XLD) agar is a highly selective medium used for the recovery of which bacteria?
A. *Staphylococcus spp. from normal flora
B. Yersinia spp. that do not grow on Hektoen agar
C. Enterobacteriaceae from gastrointestinal specimens
D. Streptococcus spp. from stool cultures
C. Enterobacteriaceae from gastrointestinal specimens
XLD agar is selective for gram-negative coliforms because of a high concentration (0.25%) of deoxycholate, which inhibits gram-positive bacteria. In addition, XLD is differential for Shigella and Salmonella spp. The medium contains xylose, lactose, and sucrose, which are fermented by most normal intestinal coliforms producing yellow colonies. Shigella does not ferment the sugars and produces red (or clear) colonies. Salmonella spp. ferment xylose; however, they also decarboxylate lysine in the medium, causing production of ammonia. Therefore, Salmonella first appear yellow but become red. Some Salmonella produce hydrogen sulfide (H2S) from sodium thiosulfate and therefore appear as red colonies with black centers.
SITUATION: Several attendees of a medical conference in the Gulf coast area became ill after frequenting a seafood restaurant. A presumptive identification of V. cholerae was made after stool specimens from several subjects grew clear colonies on MacConkey agar and yellow colonies on TCBS agar. Which key tests would help eliminate Aeromonas and Plesiomonas spp.?
A. Mannitol fermentation, Na+ requirement
B. Oxidase, motility
C. Oxidase, nitrate
D. Hemolysis on blood agar, catalase
A. Mannitol fermentation, Na+ requirement
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.
