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TEXTBOOK OF DIAGNOSTIC MICROBIOLOGY PREP 2026 QUESTIONS WITH
VERIFIED ANSWERS DETAILED RATIONALES GRADED A+
Part I: Bacterial Cell Structure, Physiology, Genetics, and Host Interaction
1. Which of the following structures is found in prokaryotic cells but absent in
eukaryotic cells?
A. Mitochondria
B. Peptidoglycan cell wall
C. Golgi apparatus
D. Endoplasmic reticulum
Rationale: Peptidoglycan is a unique polymer of the bacterial cell wall and is not
found in eukaryotic cells. Mitochondria, Golgi, and endoplasmic reticulum are
eukaryotic organelles.
2. A bacterial cell that lacks a cell wall and therefore cannot be Gram-stained is
best described as which of the following?
A. Mycoplasma species
B. Staphylococcus aureus
C. Escherichia coli
D. Bacillus subtilis
Rationale: Mycoplasma lacks a peptidoglycan cell wall, so it does not stain with
Gram stain and is intrinsically resistant to beta-lactam antibiotics.
3. Which of the following best describes the function of bacterial pili in the
infectious process?
A. They provide motility by rotating like flagella.
B. They mediate adherence to host epithelial surfaces.
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C. They are the primary site of peptidoglycan synthesis.
D. They store genetic material during starvation.
Rationale: Pili (fimbriae) are surface appendages that promote attachment to host
cells, a critical first step in colonization and infection. Motility is a function of
flagella.
4. A bacterium that grows optimally at 37°C but can also grow at refrigerator
temperature is classified as which of the following?
A. Psychrotroph
B. Strict thermophile
C. Obligate anaerobe
D. Capnophile
Rationale: Psychrotrophs grow at low temperatures and are a concern in
refrigerated food and blood products. Thermophiles require high temperatures,
and capnophiles require elevated CO₂.
5. Which of the following is the most important mechanism by which bacteria
acquire new virulence genes from unrelated species?
A. Binary fission
B. Horizontal gene transfer
C. Sporulation
D. Endospore formation
Rationale: Horizontal gene transfer—via transformation, transduction, or
conjugation—allows bacteria to acquire virulence factors, antibiotic resistance
genes, and metabolic capabilities from other organisms.
6. The normal flora of the human colon is best described by which of the following
statements?
A. It is composed exclusively of aerobic gram-positive cocci.
B. It is a complex community dominated by anaerobic bacteria.
C. It is sterile until the age of 2 years.
D. It consists only of Lactobacillus species.
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Rationale: The colon harbors a dense, diverse microbiota in which anaerobes such
as Bacteroides species vastly outnumber aerobes and facultative organisms.
7. Which of the following is a key virulence factor of Streptococcus pneumoniae
that inhibits phagocytosis?
A. Catalase
B. Polysaccharide capsule
C. Coagulase
D. Urease
Rationale: The polysaccharide capsule of S. pneumoniae is antiphagocytic and is
the basis for serotyping and conjugate vaccines. Catalase and coagulase are not
features of pneumococci.
8. A laboratory technologist is processing a specimen for anaerobic culture. Which
of the following is the most critical preanalytical factor?
A. Using a swab with a wooden shaft
B. Minimizing exposure of the specimen to oxygen
C. Refrigerating the specimen for 24 hours before culture
D. Diluting the specimen in sterile water
Rationale: Anaerobes are killed by oxygen exposure, so specimens must be
collected and transported in anaerobic transport systems and processed promptly.
9. Which of the following best explains why broad-spectrum antibiotic use can
lead to Clostridioides difficile infection?
A. The antibiotic directly activates C. difficile spores.
B. It disrupts normal colonic flora, allowing C. difficile to overgrow.
C. It increases gastric acid production.
D. It enhances the toxin-producing ability of the organism.
Rationale: Antibiotics suppress competing normal flora, creating a niche for toxin-
producing C. difficile to proliferate and cause colitis.
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10. Which of the following is the primary purpose of a biological safety cabinet in
the diagnostic microbiology laboratory?
A. To incubate cultures at 37°C
B. To protect the worker, the environment, and the specimen from aerosols and
contamination
C. To sterilize inoculating loops
D. To centrifuge specimens safely
Rationale: Biological safety cabinets provide primary containment through HEPA-
filtered airflow, protecting personnel and samples during manipulations that
generate aerosols.
Part II: Specimen Collection, Transport, and Processing
11. A sputum specimen that appears watery and clear on visual inspection should
be handled in which of the following ways?
A. Processed immediately because the patient has a productive cough.
B. Rejected because it likely represents saliva, not lower respiratory secretions.
C. Centrifuged to concentrate organisms before culture.
D. Mixed with formalin for preservation.
Rationale: A watery, clear specimen is saliva and will yield oropharyngeal flora
rather than pulmonary pathogens. A true sputum specimen should be viscous and
purulent.
12. Which of the following is the most appropriate transport medium for a stool
specimen suspected of containing Salmonella species?
A. Cary-Blair transport medium
B. Anaerobic transport vial
C. Amies medium with charcoal
D. Stuart’s medium without charcoal
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TEXTBOOK OF DIAGNOSTIC MICROBIOLOGY PREP 2026 QUESTIONS WITH
VERIFIED ANSWERS DETAILED RATIONALES GRADED A+
Part I: Bacterial Cell Structure, Physiology, Genetics, and Host Interaction
1. Which of the following structures is found in prokaryotic cells but absent in
eukaryotic cells?
A. Mitochondria
B. Peptidoglycan cell wall
C. Golgi apparatus
D. Endoplasmic reticulum
Rationale: Peptidoglycan is a unique polymer of the bacterial cell wall and is not
found in eukaryotic cells. Mitochondria, Golgi, and endoplasmic reticulum are
eukaryotic organelles.
2. A bacterial cell that lacks a cell wall and therefore cannot be Gram-stained is
best described as which of the following?
A. Mycoplasma species
B. Staphylococcus aureus
C. Escherichia coli
D. Bacillus subtilis
Rationale: Mycoplasma lacks a peptidoglycan cell wall, so it does not stain with
Gram stain and is intrinsically resistant to beta-lactam antibiotics.
3. Which of the following best describes the function of bacterial pili in the
infectious process?
A. They provide motility by rotating like flagella.
B. They mediate adherence to host epithelial surfaces.
1|Page
,Page 2 of 77
C. They are the primary site of peptidoglycan synthesis.
D. They store genetic material during starvation.
Rationale: Pili (fimbriae) are surface appendages that promote attachment to host
cells, a critical first step in colonization and infection. Motility is a function of
flagella.
4. A bacterium that grows optimally at 37°C but can also grow at refrigerator
temperature is classified as which of the following?
A. Psychrotroph
B. Strict thermophile
C. Obligate anaerobe
D. Capnophile
Rationale: Psychrotrophs grow at low temperatures and are a concern in
refrigerated food and blood products. Thermophiles require high temperatures,
and capnophiles require elevated CO₂.
5. Which of the following is the most important mechanism by which bacteria
acquire new virulence genes from unrelated species?
A. Binary fission
B. Horizontal gene transfer
C. Sporulation
D. Endospore formation
Rationale: Horizontal gene transfer—via transformation, transduction, or
conjugation—allows bacteria to acquire virulence factors, antibiotic resistance
genes, and metabolic capabilities from other organisms.
6. The normal flora of the human colon is best described by which of the following
statements?
A. It is composed exclusively of aerobic gram-positive cocci.
B. It is a complex community dominated by anaerobic bacteria.
C. It is sterile until the age of 2 years.
D. It consists only of Lactobacillus species.
2|Page
,Page 3 of 77
Rationale: The colon harbors a dense, diverse microbiota in which anaerobes such
as Bacteroides species vastly outnumber aerobes and facultative organisms.
7. Which of the following is a key virulence factor of Streptococcus pneumoniae
that inhibits phagocytosis?
A. Catalase
B. Polysaccharide capsule
C. Coagulase
D. Urease
Rationale: The polysaccharide capsule of S. pneumoniae is antiphagocytic and is
the basis for serotyping and conjugate vaccines. Catalase and coagulase are not
features of pneumococci.
8. A laboratory technologist is processing a specimen for anaerobic culture. Which
of the following is the most critical preanalytical factor?
A. Using a swab with a wooden shaft
B. Minimizing exposure of the specimen to oxygen
C. Refrigerating the specimen for 24 hours before culture
D. Diluting the specimen in sterile water
Rationale: Anaerobes are killed by oxygen exposure, so specimens must be
collected and transported in anaerobic transport systems and processed promptly.
9. Which of the following best explains why broad-spectrum antibiotic use can
lead to Clostridioides difficile infection?
A. The antibiotic directly activates C. difficile spores.
B. It disrupts normal colonic flora, allowing C. difficile to overgrow.
C. It increases gastric acid production.
D. It enhances the toxin-producing ability of the organism.
Rationale: Antibiotics suppress competing normal flora, creating a niche for toxin-
producing C. difficile to proliferate and cause colitis.
3|Page
, Page 4 of 77
10. Which of the following is the primary purpose of a biological safety cabinet in
the diagnostic microbiology laboratory?
A. To incubate cultures at 37°C
B. To protect the worker, the environment, and the specimen from aerosols and
contamination
C. To sterilize inoculating loops
D. To centrifuge specimens safely
Rationale: Biological safety cabinets provide primary containment through HEPA-
filtered airflow, protecting personnel and samples during manipulations that
generate aerosols.
Part II: Specimen Collection, Transport, and Processing
11. A sputum specimen that appears watery and clear on visual inspection should
be handled in which of the following ways?
A. Processed immediately because the patient has a productive cough.
B. Rejected because it likely represents saliva, not lower respiratory secretions.
C. Centrifuged to concentrate organisms before culture.
D. Mixed with formalin for preservation.
Rationale: A watery, clear specimen is saliva and will yield oropharyngeal flora
rather than pulmonary pathogens. A true sputum specimen should be viscous and
purulent.
12. Which of the following is the most appropriate transport medium for a stool
specimen suspected of containing Salmonella species?
A. Cary-Blair transport medium
B. Anaerobic transport vial
C. Amies medium with charcoal
D. Stuart’s medium without charcoal
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