Microbiology practice questions
Fifteen questions on bacterial structure, staining, antibiotics, viruses and immunity, the core of most health-science microbiology courses. Pick an answer to see the explanation.
Written by the Quizard team. Last updated
Microbiology for nursing and health students links structure to consequence: a cell wall explains a stain and an antibiotic, a spore explains a cleaning rule. These questions focus on those links rather than long lists of organisms.
| Gram-positive | Gram-negative | |
|---|---|---|
| Peptidoglycan layer | Thick | Thin |
| Outer membrane | None | Yes, with lipopolysaccharide |
| Color after Gram stain | Purple | Pink or red |
| Examples | Staphylococcus aureus, Streptococcus | Escherichia coli, Pseudomonas aeruginosa |
Questions
0 of 15 answered
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1. Why do Gram-positive bacteria stain purple?
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Answer: Their thick peptidoglycan layer holds the crystal violet. The thick wall traps the crystal violet-iodine complex, so alcohol does not wash it out.
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2. Endotoxin (lipopolysaccharide) is part of the cell wall of which organisms?
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Answer: Gram-negative bacteria. LPS sits in the outer membrane of Gram-negative bacteria and can trigger fever and septic shock.
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3. What are spherical bacteria called?
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Answer: Cocci. Cocci are spheres, bacilli are rods and spirilla are spirals.
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4. Which structure lets some bacteria survive boiling and disinfectants for long periods?
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Answer: Endospores. Clostridium and Bacillus species form endospores, which are why spores need sterilization rather than simple disinfection.
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5. Which organism has no cell wall, so penicillin does not affect it?
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Answer: Mycoplasma. Penicillins target cell wall synthesis, and Mycoplasma has no cell wall to target.
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6. How do penicillins kill bacteria?
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Answer: They block cell wall (peptidoglycan) synthesis. Beta-lactams bind penicillin-binding proteins and stop the cross-linking of peptidoglycan.
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7. How do viruses differ from bacteria?
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Answer: They can only replicate inside a host cell. Viruses use the host cell’s machinery to replicate, which is also why antibacterial antibiotics do not work on them.
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8. Which organism causes tuberculosis?
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Answer: Mycobacterium tuberculosis. M. tuberculosis is a slow-growing, acid-fast bacillus spread through the air.
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9. The acid-fast stain is mainly used to identify which bacteria?
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Answer: Mycobacteria. The waxy mycolic acid in mycobacterial walls holds the stain even after an acid-alcohol wash.
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10. What kind of organism is Candida albicans?
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Answer: A fungus (yeast). Candida is a yeast that causes thrush and, in vulnerable patients, invasive infection.
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11. What kind of immunity does a vaccine produce?
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Answer: Active artificial immunity. The body makes its own antibodies and memory cells (active) in response to a deliberate exposure (artificial).
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12. Which cells produce antibodies?
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Answer: Plasma cells. Plasma cells develop from activated B cells and secrete antibodies.
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13. Which antibody class can cross the placenta?
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Answer: IgG. Maternal IgG crosses the placenta and protects the newborn for its first months.
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14. How does sterilization differ from disinfection?
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Answer: It destroys all microbial life, including spores. Disinfection reduces most pathogens on objects; sterilization, for example by autoclave, destroys everything, spores included.
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15. Why is MRSA resistant to beta-lactam antibiotics?
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Answer: It makes an altered penicillin-binding protein (PBP2a). The mecA gene codes for PBP2a, which beta-lactams bind poorly, so cell wall synthesis continues.
For exam study only, not clinical guidance. Doses, reference ranges and protocols vary, so follow your course, your local policy and current references. Spotted a mistake? Tell us.
Further reading
Your exam is written from your slides. Practise from them.
Drop in a microbiology lecture and Quizard writes questions like these from it, links each one to its slide, and brings back the ones you miss.