ASCP MLS Microbiology || Questions and 100%
Verified Answers.
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 correct answers B. In order 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 sec
B. 30 sec
C. 60 sec
D. 5 min correct answers C. The iodine should remain on the skin for 1 min because instant
antisepsis does not occur when cleansing the skin for a blood culture.
What is the purpose of adding 0.025%-0.050% sodium polyanetholsulfonate (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 correct answers A. SPS is used in most commercial blood
culture products because it functions as an anticoagulant and prevents 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 correct answers C. 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 correct answers A. 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.
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 débridement correct answers C. 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, Campy, colistin-nalidixic acid (CNA) agars
C. CNA and Christensen urea agars and thioglycollate media
D. Blood, Campy, Mueller-Hinton agars, and thioglycollate media correct answers B. Hektoen
agar selectively isolates pathogenic coliforms, especially Salmonella and Shigella. MacConkey
agar differentiates lactose fermenters from nonfermenters. CNA agar contains antibiotics that
prohibit growth of gram-negative coliforms but not gram-positive cocci. Campy 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 selenite-F (SEL) broth
D. CNA agar correct answers B. TCBS agar is used to grow Vibrio 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.
Colistin-nalidixic acid agar (CNA) is used primarily for the recovery of:
A. Neisseria species
B. Enterobacteriaceae
,C. Pseudomonas aeruginosa
D. Staphylococcus aureus correct answers D. CNA agar inhibits the growth of gram-negative
bacteria and is used to isolate gram-positive cocci from specimens. This medium is especially
useful 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 correct answers A. 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 β-hemolytic streptococci, Staphylococcus, and other pathogens producing β-
hemolysins. Sheep blood does not support the growth of Haemophilus haemolyticus, eliminating
the possibility of confusing it with β-hemolytic streptococci in throat cultures.
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 on modified Thayer-Martin (MTM) medium
C. Plate the specimen on New York City or Martin-Lewis agar
D. Culture specimens in ambient oxygen at 37°C correct answers D. 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%-
7% CO2 at 35°C. Cultures should be held a minimum of 48 hours before being considered
negative.
Chocolate agar and modified Thayer-Martin 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 correct answers B. Chocolate agar
provides X factor (hemin) and V factor (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. Clostridium difficile correct answers D. 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. correct answers A. DCA inhibits gram-positive organisms. N. gonorrhoeae and
Neisseria meningitidis are too fastidious to grow on DCA. 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 correct answers C. 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.
A sheep blood agar plate is used as a primary isolation medium when all of the following
organisms are to be recovered from a wound specimen except:
A. β-Hemolytic streptococci and coagulase-positive staphylococci
B. Haemophilus influenzae and Haemophilus parainfluenzae
C. Proteus spp. and Escherichia coli
D. Pseudomonas spp. and Acinetobacter spp. correct answers B. Both gram-positive cocci and
gram-negative bacilli will grow on blood agar plates, but the medium is used in conjunction with
a selective medium such as CNA agar for gram-positive cocci and MacConkey agar for gram-
negative bacilli. H. influenzae requires X and V factors, and H. parainfluenzae requires V factor;
the primary isolation medium for Haemophilus is chocolate agar.
Prereduced and vitamin K1-supplemented blood agar plates are recommended isolation media
for:
