ASCP Microbiology 2 — Questions and Answers
Question 1: Which culture medium is used for isolation of Neisseria gonorrhoeae?
- MacConkey agar
- Thayer-Martin agar (Correct answer)
- Mannitol salt agar
- TCBS agar
Correct answer: Thayer-Martin agar
Modified Thayer-Martin agar contains antibiotics (vancomycin, colistin, nystatin) to select for Neisseria species from genital specimens.
Question 2: Acid-fast bacilli seen on Ziehl-Neelsen stain are most consistent with:
- Clostridium difficile
- Mycobacterium tuberculosis (Correct answer)
- Nocardia asteroides only
- Bacillus anthracis
Correct answer: Mycobacterium tuberculosis
Mycobacterium tuberculosis retains carbol fuchsin after acid-alcohol decolorization due to its mycolic acid-rich cell wall.
Question 3: A positive oxidase test (purple color) differentiates which organism from Enterobacteriaceae?
- Staphylococcus aureus
- Pseudomonas aeruginosa (Correct answer)
- Escherichia coli
- Enterococcus faecalis
Correct answer: Pseudomonas aeruginosa
Pseudomonas aeruginosa is oxidase-positive (contains cytochrome c oxidase), while all Enterobacteriaceae are oxidase-negative.
Question 4: Which toxin produced by Clostridium difficile causes pseudomembranous colitis?
- Exotoxin A only
- Toxin A (enterotoxin) and Toxin B (cytotoxin) (Correct answer)
- Endotoxin
- Shiga toxin
Correct answer: Toxin A (enterotoxin) and Toxin B (cytotoxin)
C. difficile produces Toxin A (enterotoxin) and Toxin B (cytotoxin), both detected in stool EIA or PCR testing for diagnosis.
Question 5: India ink preparation is used to visualize the capsule of which organism in CSF?
- Haemophilus influenzae
- Cryptococcus neoformans (Correct answer)
- Neisseria meningitidis
- Listeria monocytogenes
Correct answer: Cryptococcus neoformans
India ink stain creates a dark background, highlighting the large capsule of Cryptococcus neoformans as a clear halo.
Question 6: Which antibiotic resistance mechanism involves enzymatic destruction of the beta-lactam ring?
- Efflux pumps
- Target site modification (altered PBPs)
- Beta-lactamase production (Correct answer)
- Decreased outer membrane permeability
Correct answer: Beta-lactamase production
Beta-lactamases hydrolyze the beta-lactam ring of penicillins and cephalosporins, rendering them inactive.
ASCP Microbiology 2
Which culture medium is used for isolation of Neisseria gonorrhoeae?