PCAT Microbiology 2 — Questions and Answers
Question 1: Which bacterial structure is the primary determinant of Gram stain results?
- Capsule thickness
- Peptidoglycan layer thickness (Correct answer)
- Lipopolysaccharide content
- Flagella composition
Correct answer: Peptidoglycan layer thickness
Gram-positive bacteria have a thick peptidoglycan layer that retains crystal violet, while Gram-negative bacteria have a thin layer that is decolorized.
Question 2: A bacterium that can grow at refrigerator temperatures (4°C) is classified as a:
- Thermophile
- Mesophile
- Psychrophile (Correct answer)
- Halophile
Correct answer: Psychrophile
Psychrophiles (cold-loving organisms) thrive at low temperatures ranging from -5°C to 20°C, with an optimum around 15°C.
Question 3: Which mechanism allows bacteria to acquire antibiotic resistance genes from the environment through DNA uptake?
- Transduction
- Conjugation
- Transformation (Correct answer)
- Transposition
Correct answer: Transformation
Transformation involves uptake of naked DNA from the environment; naturally competent bacteria can incorporate these genes into their chromosome.
Question 4: The Kirby-Bauer disk diffusion test measures:
- Minimum bactericidal concentration
- Minimum inhibitory concentration
- Antibiotic susceptibility via zone of inhibition (Correct answer)
- Rate of bacterial killing
Correct answer: Antibiotic susceptibility via zone of inhibition
The Kirby-Bauer test places antibiotic-impregnated disks on an inoculated agar plate; the diameter of the clear zone indicates susceptibility.
Question 5: Which viral enzyme enables RNA viruses to replicate their genome without using DNA as an intermediate?
- DNA polymerase
- RNA-dependent RNA polymerase (Correct answer)
- Reverse transcriptase
- Integrase
Correct answer: RNA-dependent RNA polymerase
RNA-dependent RNA polymerase (RdRp) uses an RNA template to synthesize new RNA strands, allowing RNA viruses to replicate independently of DNA.
Question 6: Aspergillus fumigatus is an opportunistic pathogen that most commonly causes serious infections in patients with:
- Type 2 diabetes
- Severe neutropenia (Correct answer)
- Chronic kidney disease
- HIV with CD4 > 500
Correct answer: Severe neutropenia
Neutrophils are the primary defense against Aspergillus; severe neutropenia (as in chemotherapy patients) dramatically increases risk of invasive aspergillosis.
Question 7: Which of the following is a characteristic feature of prions that distinguishes them from viruses?
- They can replicate outside host cells
- They contain only protein and no nucleic acid (Correct answer)
- They are sensitive to UV radiation
- They trigger antibody production
Correct answer: They contain only protein and no nucleic acid
Prions are misfolded proteins (PrP^Sc) that lack nucleic acid entirely, yet can propagate by inducing normal proteins to misfold.
PCAT Microbiology 2
Which bacterial structure is the primary determinant of Gram stain results?