PCAT Microbiology 4 — Questions and Answers
Question 1: Which bacterium uses a type III secretion system to inject effector proteins directly into host cell cytoplasm?
- Clostridium difficile
- Salmonella enterica (Correct answer)
- Staphylococcus aureus
- Mycobacterium tuberculosis
Correct answer: Salmonella enterica
Salmonella uses its type III secretion system (injectisome) to deliver effector proteins into epithelial cells, triggering actin rearrangement and bacterial uptake.
Question 2: In a continuous culture system (chemostat), bacterial growth rate is controlled by:
- Temperature adjustment
- pH manipulation
- Dilution rate of limiting nutrient (Correct answer)
- Oxygen tension
Correct answer: Dilution rate of limiting nutrient
In a chemostat, the dilution rate determines the growth rate because the limiting nutrient is supplied at a fixed rate, maintaining steady-state culture conditions.
Question 3: Which characteristic distinguishes a temperate bacteriophage from a lytic bacteriophage?
- Ability to infect Gram-negative bacteria
- Ability to integrate into the host chromosome as a prophage (Correct answer)
- Presence of a lipid envelope
- Use of RNA as genetic material
Correct answer: Ability to integrate into the host chromosome as a prophage
Temperate phages can integrate their DNA into the bacterial chromosome (becoming a prophage) and remain dormant, while lytic phages always lyse the host immediately.
Question 4: Clostridium botulinum toxin causes flaccid paralysis by:
- Blocking acetylcholine receptors at the neuromuscular junction
- Cleaving SNARE proteins to prevent acetylcholine release (Correct answer)
- Activating chloride channels in motor neurons
- Inhibiting acetylcholinesterase activity
Correct answer: Cleaving SNARE proteins to prevent acetylcholine release
Botulinum toxin is a zinc-dependent protease that cleaves SNARE proteins (SNAP-25, synaptobrevin, syntaxin), preventing vesicle fusion and acetylcholine release.
Question 5: Which Plasmodium species can form hypnozoites in the liver, enabling relapse months after initial infection?
- Plasmodium falciparum
- Plasmodium malariae
- Plasmodium vivax (Correct answer)
- Plasmodium knowlesi
Correct answer: Plasmodium vivax
P. vivax (and P. ovale) form dormant hypnozoites in hepatocytes; primaquine is required to eliminate them and prevent relapse.
Question 6: Which type of bacterial toxin requires endocytosis and intracellular processing to exert its effect on the host cell?
- Superantigen
- Membrane-disrupting toxin
- A-B toxin (Correct answer)
- Lipopolysaccharide endotoxin
Correct answer: A-B toxin
A-B toxins consist of a binding (B) subunit that facilitates endocytosis and an active (A) subunit that exerts enzymatic activity intracellularly after processing.
Question 7: Resistance to beta-lactam antibiotics via production of beta-lactamase enzymes can be overcome by administering:
- Vancomycin
- Clavulanic acid (Correct answer)
- Azithromycin
- Rifampin
Correct answer: Clavulanic acid
Clavulanic acid is a beta-lactamase inhibitor that irreversibly binds and inactivates beta-lactamase, protecting co-administered beta-lactam antibiotics.
PCAT Microbiology 4
Which bacterium uses a type III secretion system to inject effector proteins directly into host cell cytoplasm?