CMLA Microbiology Basics 5 — Questions and Answers
Question 1: Which viral hepatitis is transmitted primarily through the fecal-oral route?
- Hepatitis B
- Hepatitis C
- Hepatitis A (Correct answer)
- Hepatitis D
Correct answer: Hepatitis A
Hepatitis A virus (HAV) is transmitted via contaminated food and water (fecal-oral route), unlike hepatitis B and C which spread parenterally.
Question 2: What is the function of a mordant in the Gram staining procedure?
- Decolorize the primary stain
- Fix the crystal violet–iodine complex to the cell wall (Correct answer)
- Provide contrast via counterstaining
- Lyse the bacterial cell membrane
Correct answer: Fix the crystal violet–iodine complex to the cell wall
Gram's iodine acts as a mordant, forming an insoluble crystal violet–iodine complex that becomes trapped within the thick peptidoglycan of Gram-positive cell walls.
Question 3: Clostridium difficile is best detected in stool specimens using which laboratory method?
- Culture on blood agar
- Gram stain of the specimen
- EIA or PCR for toxins A and B (Correct answer)
- Wet mount microscopy
Correct answer: EIA or PCR for toxins A and B
Enzyme immunoassay (EIA) for C. difficile toxins A/B or PCR-based molecular testing for toxin genes are the preferred diagnostic methods for C. difficile infection.
Question 4: The oxidase test detects the presence of which enzyme in bacteria?
- Catalase
- Coagulase
- Cytochrome c oxidase (Correct answer)
- Urease
Correct answer: Cytochrome c oxidase
The oxidase test uses a reagent (tetramethyl-p-phenylenediamine) that turns purple when reduced by cytochrome c oxidase, an enzyme in the electron transport chain.
Question 5: Which organism is a common cause of nosocomial (hospital-acquired) urinary tract infections and is known for its characteristic grape-like odor?
- Escherichia coli
- Pseudomonas aeruginosa (Correct answer)
- Klebsiella pneumoniae
- Proteus mirabilis
Correct answer: Pseudomonas aeruginosa
Pseudomonas aeruginosa produces pyocyanin and has a distinctive grape-like or fruity odor; it is a leading cause of healthcare-associated infections, especially in immunocompromised patients.
Question 6: Which stain is used to visualize Cryptococcus neoformans capsule in cerebrospinal fluid?
- Gram stain
- Acid-fast stain
- India ink preparation (Correct answer)
- Giemsa stain
Correct answer: India ink preparation
India ink (or nigrosin) creates a dark background that outlines the large polysaccharide capsule of Cryptococcus neoformans as a clear halo in CSF specimens.
Question 7: Beta-lactamase production by bacteria causes resistance by:
- Altering the bacterial ribosome target
- Enzymatically destroying the beta-lactam ring of antibiotics (Correct answer)
- Pumping the antibiotic out via efflux
- Changing outer membrane porin proteins
Correct answer: Enzymatically destroying the beta-lactam ring of antibiotics
Beta-lactamases hydrolyze the beta-lactam ring that is essential for the antibiotic's activity, rendering penicillins and cephalosporins inactive before they can inhibit cell wall synthesis.
CMLA Microbiology Basics 5
Which viral hepatitis is transmitted primarily through the fecal-oral route?