Microbiology Practice Questions With Answers
Twelve microbiology practice questions with answers and explanations, from the Gram stain to complement and generation time, from Encodr's free deck.
These twelve questions are pulled straight from Encodr's free Microbiology deck, one or two from each of eleven different units. They run from the first week of the course (stains and cell walls) to the last (immunity and hypersensitivity), so you can use them as a quick check on where you stand.
The method matters as much as the questions. Cover the answer, commit to a response in your head or on paper, and only then read the answer and explanation. Trying to retrieve an answer, even when you get it wrong, does more for memory than reading it again; the testing effect explains why. Anything you miss goes on a list to review again in a couple of days. The calculation question below uses the generation-time formula, and the bacterial growth calculator lets you check your arithmetic on similar problems.
1. Foundations and Microscopy
After a completed Gram stain, what color do gram-negative bacteria appear?
- A. Purple
- B. Blue
- C. Pink
- D. Green
Answer: C. Pink
Why: Gram-negative cells lose the crystal violet-iodine complex during decolorization and pick up the pink/red safranin counterstain.
2. The Microbial Cell
In gram-negative bacteria, what molecule embedded in the outer membrane can act as an endotoxin?
- A. Lipopolysaccharide (LPS)
- B. Teichoic acid
- C. Peptidoglycan
- D. Mycolic acid
Answer: A. Lipopolysaccharide (LPS)
Why: LPS in the gram-negative outer membrane acts as an endotoxin, and its exact composition varies by species.
3. Acellular Pathogens
During the bacteriophage lytic cycle's penetration step, what actually enters the bacterial cell?
Answer: Only the viral genome; the capsid and tail stay outside.
Why: Penetration is the genome-injection step of the lytic cycle, which is why the capsid and tail stay outside the cell. The cycle runs attachment, penetration, biosynthesis, maturation, release.
4. Microbial Growth and Culture
During lag phase, does cell number change, and what are the cells actually doing?
Answer: Cell number does not change, but cells are metabolically active, growing larger, and preparing molecular machinery for division.
Why: Lag phase is the first of the four growth-curve phases (lag, log, stationary, death). Cell number stays flat while the cells prepare to divide, and exponential doubling only starts in log phase.
5. Microbial Growth and Culture
Using generation time g = t / n: a culture underwent 3 generations over 120 minutes. Give the generation time in minutes, as a whole number.
Answer: 40
Why: Generation time g = t / n = 120 minutes / 3 generations = 40 minutes per generation.
6. Control of Microbial Growth
At roughly what aqueous concentration are alcohols like ethanol most effective as disinfectants?
- A. 50%
- B. 70%
- C. 90%
- D. 100%
Answer: B. 70%
Why: Alcohols are most effective at roughly 70% aqueous concentration; higher concentrations up to 100% are less effective because some water is needed for the protein-denaturation reaction.
7. Antimicrobial Drugs
What is the difference between a bacteriostatic drug and a bactericidal drug?
Answer: A bacteriostatic drug stops bacteria from growing and reproducing but relies on the host immune system to finish clearing the infection, while a bactericidal drug directly kills the bacteria.
Why: The suffixes carry the meaning: -cidal kills, -static only inhibits growth. A bacteriostatic drug leaves the final clearing of the infection to the immune system.
8. Mechanisms of Pathogenicity
Toxic shock syndrome toxin (Staphylococcus aureus) causes excessive, nonspecific stimulation of immune cells and a potentially lethal cytokine storm. Which exotoxin class does it belong to?
- A. Superantigens
- B. A-B toxins
- C. Membrane-disrupting toxins
- D. Endotoxins
Answer: A. Superantigens
Why: Superantigens like toxic shock syndrome toxin cause excessive, nonspecific stimulation of immune cells, triggering a cytokine storm that can be lethal.
9. Disease and Epidemiology
A health department counts every NEW case of a foodborne illness reported during a two-week outbreak. Which epidemiological measure are they calculating?
- A. Prevalence
- B. Incidence
- C. Mortality rate
- D. Endemic status
Answer: B. Incidence
Why: Incidence is the number of NEW cases arising in a population during a specified period - exactly what's being counted here.
10. Innate Immunity
Which complement pathway is triggered by the spontaneous hydrolysis of complement protein C3 into C3a and C3b, without needing a specific antibody first?
- A. Classical pathway
- B. Alternative pathway
- C. Lectin pathway
- D. Antibody-dependent pathway
Answer: B. Alternative pathway
Why: The alternative pathway is triggered by spontaneous hydrolysis of C3 into C3a and C3b, with C3b attaching directly to microbial surfaces.
11. Adaptive Immunity
Which host cells carry MHC class I molecules?
- A. Only professional antigen-presenting cells
- B. All nucleated cells (mature red blood cells lack them)
- C. Only cells actively infected by a pathogen
- D. All circulating blood cells, including mature red blood cells
Answer: B. All nucleated cells (mature red blood cells lack them)
Why: MHC I molecules are found on all nucleated cells; mature red blood cells lack them because they are not nucleated.
12. Immune Disorders and Testing
In type II (cytotoxic) hypersensitivity, what do IgG and IgM antibodies bind to trigger host cell destruction?
- A. Circulating immune complexes already formed in serum
- B. Antigens on the surface of host cells
- C. Allergen bound to mast cell IgE
- D. T-cell receptors on cytotoxic T cells
Answer: B. Antigens on the surface of host cells
Why: Type II hypersensitivity involves IgG and IgM binding antigens already present on a host cell's surface, triggering complement activation or ADCC.
What to do with your score
If you missed two or more, find the unit headings above that match and revisit those chapters in the full Microbiology deck. The deck schedules each card for you, so the ones you miss come back sooner. For a map of what the course covers, see what's in microbiology, and for a study plan by unit, see how to study for microbiology. The growth-curve and counting questions have their own walkthrough in the bacterial growth curve and generation time.
Encodr turns this into a habit: study anything in a feed, and it schedules the rest.
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