Microbiology MCQs With Answers and Explanations

Thirty-one microbiology multiple choice questions with answers and explanations, covering the whole of general microbiology: the scope and history of the subject, microscopy and staining, prokaryotic and eukaryotic cell structure and physiology, and a range of mixed topics. Write down your answers first, then check them against the key and explanations at the end.

Scope and history of microbiology

1. A scientist who studies the frequency and distribution of diseases is a:

a. Mycologist
b. Immunologist
c. Etiologist
d. Epidemiologist
e. Ecologist

2. The biggest obstacle to the early acceptance and development of microbiology was:

a. Lack of effective vaccines
b. Lack of sterile containers
c. The theory of spontaneous generation
d. Absence of debilitating diseases before the seventeenth century
e. Use of aseptic technique

3. The germ theory of disease states that:

a. Microorganisms that invade other organisms can cause disease in those organisms
b. Microorganisms can spontaneously arise in debilitated hosts
c. Microorganisms do not cause infectious diseases
d. Not all microorganisms are harmful
e. All microorganisms are beneficial to humans in some way

Microscopy, units, and staining

4. Of the following, the smallest unit of measurement is the:

a. Millimeter
b. Micrometer
c. Centimeter
d. Decimeter
e. Nanometer

5. The average wavelength of light visible to the human eye is about:

a. 800 nm
b. 200 nm
c. 550 nm
d. 100 nm
e. 420 nm

6. In a light microscope, what function does the condenser serve?

a. Focuses the light rays onto the eye
b. Magnifies the light rays after they pass through the sample
c. Focuses the light rays onto the sample
d. Increases the light intensity e. Reduces glare

7. The total magnification of a microscope is calculated by:

a. Adding the objective lens and ocular lens magnification powers
b. Multiplying the objective lens and ocular lens magnification powers
c. Multiplying the objective lens and condenser lens magnification powers
d. Squaring the objective lens power
e. None of the above

8. Which stain is frequently used to identify Mycobacterium and other bacteria whose cell walls contain high amounts of lipid?

a. Gram stain
b. Schaeffer-Fulton stain
c. Acid-fast stain
d. Giemsa stain
e. Spore stain

9. Which microscopic technique provides three-dimensional images of a bacterial cell?

a. Transmission electron microscopy
b. Scanning electron microscopy
c. Negative staining microscopy
d. Dark-field microscopy
e. Fluorescent microscopy

10. The transmission electron microscope has the greatest resolving power because it uses an electron beam instead of light. The electron beam is used because:

a. Electrons have longer wavelengths than light waves
b. Electrons do not penetrate the sample
c. Light waves are less visible
d. Electrons have shorter wavelengths than light waves
e. Electrons are less invasive

Cell structure and physiology

11. The average radius of a prokaryotic cell is:

a. 0.5-2.0 μm
b. 1.0-4.0 μm
c. 1.0-10.0 μm
d. 5.0-10.0 μm
e. 10.0-30.0 μm

12. Viewing a soil sample under the light microscope, you see a rod-shaped cell with no nucleus. The only correct assumption you can make so far is that:

a. It is a eukaryote
b. It is gram-positive
c. It contains peptidoglycan
d. It contains mitochondria
e. It is part of a multicellular microorganism

13. Which of the following is likely to contain structures composed of N-acetylmuramic acid and N-acetylglucosamine?

a. Spheroplasts
b. Mycoplasmas
c. Escherichia coli
d. Protoplasts
e. Amoebas

14. The endotoxin of gram-negative bacteria is a result of the presence of:

a. Peptidoglycan
b. Lipopolysaccharide
c. Polypeptide
d. Steroids
e. Calcified proteins

15. Bacterial cells are more resistant to osmotic shock than eukaryotic cells because they:

a. Contain a cell wall composed of cellulose
b. Contain osmoregulating porins
c. Actively block water molecules from entering the cell
d. Are selectively permeable
e. Contain a cell wall composed of peptidoglycan

16. Which of the following describes the prokaryotic cell membrane?

a. Selectively permeable
b. Regulates the passage of materials into and out of the cell
c. Contains proteins and phospholipids
d. Contains metabolic enzymes
e. All of these

17. Bacterial fimbriae on the outer cell surface are used for:

a. Cellular activity
b. Sexual reproduction
c. Cell wall synthesis
d. Adherence to surfaces
e. Adherence and exchange of genetic information

18. Antibiotics that inhibit or inactivate cellular ribosomes will directly cause the loss of which function?

a. ATP production
b. DNA replication
c. Phagocytosis
d. Protein synthesis
e. Cell division

19. Chloroplasts actively carry out photosynthesis. Which of the following is NOT a characteristic of them?

a. Are found in plant and algal cells
b. Contain the pigment chlorophyll
c. Are membrane-enclosed
d. Are found in photosynthetic prokaryotic cells
e. Are found only in eukaryotic cells

20. Which of the following is true of phagocytosis?

a. Is not energy-dependent
b. Requires a concentration gradient
c. Occurs in both prokaryotic and eukaryotic cells
d. None of the above
e. A larger cell engulfs a smaller cell, which ends up inside an internal vacuole

21. All of the following pertain to mitosis except:

a. New cells receive one copy of each parent cell chromosome
b. It is preceded by DNA replication
c. It results in gamete production
d. A spindle apparatus guides chromosome movement
e. It is a process of nuclear division

Mixed topics

22. Which of the following drugs is used to treat fungal infections?

a. Erythromycin
b. Penicillin
c. Amphotericin B
d. Quinine
e. None of the above

23. Pili (fimbriae) are thin, short appendages extending from certain bacteria. They are involved in:

a. Movement
b. Attachment
c. DNA transfer
d. a and b
e. b and c

24. Which organism is used as an index of fecal pollution of drinking water supplies?

a. Rotavirus
b. E. coli
c. Salmonella species
d. Hepatitis E virus

25. Protective mechanisms used by bacteria to survive in the host may include:

a. Capsules
b. M protein
c. Various enzymes
d. All of the above

26. Endotoxins are heat-stable lipopolysaccharide complexes in the bacterial cell wall. Which of the following statements regarding endotoxin is correct?

a. All endotoxins have the same effect regardless of bacterial source
b. They are found in both gram-positive and gram-negative cell walls
c. They are part of the cell wall of gram-positive bacteria only
d. None of the above statements are correct

27. Ethanol is a commonly used disinfectant. Which concentration of ethanol is most effective?

a. 100%
b. 70%
c. 50%
d. 30%

28. Which of the following can be reservoirs for human infection?

a. Food and water
b. Humans
c. Animals
d. All of the above

29. A typical bacterial growth curve consists of four phases. Which is the correct sequence?

a. Exponential, lag, stationary, death
b. Lag, exponential, stationary, death
c. Stationary, exponential, lag, death
d. Lag, stationary, exponential, death

30. Which of the following are true about viruses?

a. They are not composed of cells
b. As virions, they cannot metabolize nutrients
c. They alone cannot reproduce themselves
d. They contain DNA or RNA
e. All of the above

31. The correct sequence for the viral life cycle is:

a. Attachment, biosynthesis, penetration, maturation, release
b. Penetration, biosynthesis, attachment, maturation, release
c. Attachment, penetration, biosynthesis, maturation, release
d. Biosynthesis, attachment, penetration, maturation, release

Answer key

1. d. Epidemiologist
2. c. The theory of spontaneous generation
3. a. Microorganisms that invade other organisms can cause disease in those organisms
4. e. Nanometer
5. c. 550 nm
6. c. Focuses the light rays onto the sample
7. b. Multiplying the objective lens and ocular lens magnification powers
8. c. Acid-fast stain
9. b. Scanning electron microscopy
10. d. Electrons have shorter wavelengths than light waves
11. b. 1.0-4.0 μm
12. c. It contains peptidoglycan
13. c. Escherichia coli
14. b. Lipopolysaccharide
15. e. Contain a cell wall composed of peptidoglycan
16. e. All of these
17. d. Adherence to surfaces
18. d. Protein synthesis
19. d. Are found in photosynthetic prokaryotic cells
20. e. A larger cell engulfs a smaller cell, which ends up inside an internal vacuole
21. c. It results in gamete production
22. c. Amphotericin B
23. e. b and c (attachment and DNA transfer)
24. b. E. coli
25. d. All of the above
26. d. None of the above statements are correct
27. b. 70%
28. d. All of the above
29. b. Lag, exponential, stationary, death
30. e. All of the above
31. c. Attachment, penetration, biosynthesis, maturation, release

Why these are the answers

1. Epidemiologist. Epidemiology is the study of how often diseases occur and how they are distributed across populations. The distractors describe other roles: a mycologist studies fungi, an immunologist the immune system, and an etiologist the cause of a disease rather than its distribution.

2. The theory of spontaneous generation. The long-held belief that living things could arise spontaneously from non-living matter blocked the understanding that specific microbes cause disease and spoilage. Only when Pasteur and others disproved it could microbiology advance. Aseptic technique, by contrast, was part of the solution, not the obstacle.

3. Microorganisms that invade other organisms can cause disease. The germ theory is the foundational idea that specific microbes cause specific infectious diseases, replacing older notions such as miasma and spontaneous generation. The other statements either restate spontaneous generation or deny the microbial cause of disease.

4. Nanometer. Of the units listed, the nanometer (one billionth of a meter) is by far the smallest, which is why it is used for viruses and subcellular structures. The others are all larger, with the micrometer used for bacteria. (Even smaller units such as the picometer exist, but among these choices the nanometer is smallest.)

5. 550 nm. Visible light spans roughly 400 to 700 nm, and about 550 nm (green) sits near the middle, often quoted as the average. This matters in microscopy because wavelength sets the limit of resolution: a light microscope cannot resolve objects much smaller than about half the wavelength used, roughly 0.2 micrometers.

6. Focuses the light rays onto the sample. The condenser is the lens system beneath the stage that gathers and focuses light from the source onto the specimen for even, bright illumination. It does not magnify the image; that is the job of the objective and ocular lenses.

7. Multiplying the objective lens and ocular lens magnification powers. Total magnification is the product, not the sum, of the two lenses, so a 40x objective with a 10x eyepiece gives 400x. The condenser is not part of this calculation.

8. Acid-fast stain. The acid-fast (Ziehl-Neelsen) stain is used for Mycobacterium and other organisms with waxy, lipid-rich (mycolic acid) walls that resist ordinary stains. The organism holds the acid-fast dye even after an acid-alcohol wash, which is what "acid-fast" means.

9. Scanning electron microscopy. The SEM scans the specimen's surface with an electron beam and builds a three-dimensional image of its surface. The TEM, by contrast, passes electrons through a thin section to show internal structure in two dimensions. For a 3D surface view, SEM is the answer.

10. Electrons have shorter wavelengths than light waves. Resolving power is limited by the wavelength used, and shorter wavelengths resolve finer detail. Electrons have far shorter wavelengths than visible light, so an electron microscope resolves structures thousands of times smaller than a light microscope can.

11. 1.0-4.0 μm. Prokaryotic cells are small, on the order of a few micrometers, far smaller than most eukaryotic cells. Their small size gives a high surface-area-to-volume ratio that aids rapid nutrient exchange and growth. The larger ranges among the options describe eukaryotic cells.

12. It contains peptidoglycan. A rod-shaped cell with no nucleus is a prokaryote (a bacterium), and the safe conclusion is that it has a bacterial cell wall containing peptidoglycan. The other options assume too much: the Gram reaction cannot be told from shape alone, and eukaryotic features (mitochondria, multicellularity) are ruled out by the absence of a nucleus.

13. Escherichia coli. N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) are the two sugars that alternate to form the peptidoglycan backbone of the bacterial cell wall, so a normal bacterium such as E. coli contains them. Spheroplasts and protoplasts have had their walls removed, mycoplasmas naturally lack a wall, and amoebas are eukaryotes without peptidoglycan.

14. Lipopolysaccharide. Endotoxin is lipopolysaccharide (LPS), a component of the outer membrane of gram-negative bacteria, with its toxic activity in the lipid A portion. This is why endotoxin is found in gram-negative organisms and not in gram-positive ones, which lack the outer membrane.

15. Contain a cell wall composed of peptidoglycan. The rigid peptidoglycan wall protects bacteria from bursting under osmotic stress: when water rushes in, the strong mesh resists the swelling that would rupture a wall-less cell. This is why agents that damage peptidoglycan (like penicillin) or cells that lack a wall (like mycoplasmas) are osmotically fragile.

16. All of these. Every option describes a real property of the prokaryotic cell membrane: it is selectively permeable, it regulates what enters and leaves the cell, it is built of phospholipids and proteins, and, because prokaryotes have no mitochondria, it also houses metabolic enzymes (for energy production, such as the electron transport chain). Since all four are true, the answer is "all of these."

17. Adherence to surfaces. Fimbriae are short, numerous surface appendages whose main job is attachment, letting bacteria stick to host cells and surfaces as a first step in colonization. They are distinct from the sex pilus (which transfers DNA) and from flagella (which drive movement). (See also question 23, which considers pili and fimbriae together.)

18. Protein synthesis. Ribosomes are the machines of protein synthesis, so drugs that inhibit the ribosome directly block protein production. Because bacterial (70S) ribosomes differ from human (80S) ribosomes, these antibiotics can target bacteria selectively.

19. Are found in photosynthetic prokaryotic cells. This is the exception. Chloroplasts are membrane-bound organelles found only in eukaryotes (plants and algae). Photosynthetic prokaryotes such as cyanobacteria carry out photosynthesis without chloroplasts, using infolded membranes instead.

20. A larger cell engulfs a smaller cell, which ends up inside an internal vacuole. Phagocytosis is the engulfment of a particle or cell into a vacuole (phagosome) for digestion. It is energy-dependent, does not rely on a simple concentration gradient, and is a feature of eukaryotic cells, not prokaryotes.

21. It results in gamete production. This is the exception. Mitosis produces two genetically identical daughter cells for growth and repair; gamete production requires meiosis. The other statements are true of mitosis: each new cell gets one copy of each chromosome, it follows DNA replication, a spindle guides the chromosomes, and it is a form of nuclear division.

22. Amphotericin B. Amphotericin B is an antifungal that binds ergosterol in the fungal membrane and forms pores. The others target different organisms: erythromycin and penicillin are antibacterials, and quinine is an antimalarial. Antifungals are a distinct class because fungi are eukaryotes and share more machinery with human cells.

23. b and c (attachment and DNA transfer). Pili and fimbriae are surface appendages with two main roles: common fimbriae mediate attachment (adhesion) to surfaces, and the specialized sex pilus mediates DNA transfer during conjugation. They are not organs of movement; that is the flagellum. (Question 17 focuses specifically on the adherence role of fimbriae.)

24. E. coli. Escherichia coli is the standard indicator organism for fecal contamination of water: it lives in the intestine and is easy to detect, so its presence signals recent fecal pollution and possible intestinal pathogens. The true pathogens listed are harder to test for routinely, which is why an easily detected indicator is used.

25. All of the above. Bacteria use several survival tools: capsules resist phagocytosis, the streptococcal M protein interferes with complement and phagocytosis, and various enzymes (coagulase, hyaluronidase, IgA proteases) help the organism evade or spread. All three are genuine mechanisms.

26. None of the above statements are correct. Every one of the first three statements is false. Endotoxin (LPS) is found in gram-negative bacteria only, so it is not in gram-positive walls (b and c are wrong). And endotoxins do not all have the same effect regardless of source: LPS structure varies among species and its toxicity differs (some LPS are non-toxic), so a is wrong too. With a, b, and c all false, the answer is "none of the above." (The shared feature of endotoxins is the general lipid A mechanism, not identical effects.)

27. 70 percent. Ethanol is most effective at about 70 percent, not 100 percent. Some water is needed for ethanol to denature microbial proteins efficiently; absolute (100 percent) ethanol dehydrates the outer layers and coagulates a protective surface film, so it works less well. For alcohol disinfection, 70 percent beats 100 percent.

28. All of the above. A reservoir is where an infectious agent normally lives and multiplies. Reservoirs can be environmental (food and water), human (carriers and cases), or animal (zoonotic). Since all three are genuine reservoirs, the inclusive answer is correct.

29. Lag, exponential, stationary, death. The growth curve runs in order: lag (cells adapt), exponential/log (rapid doubling), stationary (growth balanced by death as nutrients run low), and death/decline. The sequence follows a population settling into a new environment, growing, then outstripping its resources.

30. All of the above. Each statement is true of viruses: they are acellular, a virion cannot carry out its own metabolism, a virus cannot reproduce without a host cell, and every virus has a nucleic acid genome that is either DNA or RNA (never both). Together these define what separates viruses from cellular organisms.

31. Attachment, penetration, biosynthesis, maturation, release. The viral replication cycle proceeds in this order: the virus attaches to a host receptor, penetrates (enters), directs biosynthesis of new components, assembles them during maturation, and releases new virions to infect other cells. The logic: get in, take over, build copies, assemble, get out.