WGU D311 – Microbiology Objective Assessment
Practice Exam the Actual OA | 150 Verified-Style
Questions with Answers & Rationales
Overview:
This comprehensive practice exam is designed to closely mirror the Western Governors
University (WGU) D311 Microbiology Objective Assessment.
It includes 150 multiple-choice questions that reflect the difficulty, style, and topic
distribution of the real exam.
Coverage Areas:
1. Microbial Cell Structure & Function – Prokaryotic vs. Eukaryotic, cell walls,
organelles
2. Microbial Growth & Control – Nutritional requirements, physical/chemical control
methods
3. Microbial Genetics & Molecular Biology – DNA replication, transcription, translation,
mutation, gene transfer
4. Metabolism & Enzymology – Catabolism, anabolism, ATP generation
5. Immunology & Host Defenses – Innate and adaptive immunity, antibodies, antigen
processing
6. Infectious Diseases & Pathogenicity – Bacterial, viral, fungal, and parasitic diseases;
epidemiology
7. Laboratory Techniques & Safety – Microscopy, staining, aseptic technique, culture
methods
Which cellular structure is the primary site of ATP production in bacteria?
A. Nucleus
B. Mitochondrion
C. Plasma (cytoplasmic) membrane
D. Ribosome
Answer: C. Plasma (cytoplasmic) membrane.
Rationale: Bacteria lack mitochondria; oxidative phosphorylation occurs across the
plasma membrane.
Gram-positive bacteria differ from Gram-negative bacteria primarily by having:
A. Outer membrane with lipopolysaccharide
B. Thick peptidoglycan layer and teichoic acids
,C. Periplasmic space
D. Porin proteins in outer membrane
Answer: B. Thick peptidoglycan layer and teichoic acids.
Rationale: Gram-positive have thick peptidoglycan with teichoic acids; Gram-
negatives have outer membrane/LPS.
Which of the following structures is used by bacteria to attach to host cells?
A. Flagella
B. Pili (fimbriae)
C. Endospore
D. Capsule
Answer: B. Pili (fimbriae).
Rationale: Pili/fimbriae mediate attachment; capsules can aid adherence but pili are
specialized for attachment.
What is the primary function of bacterial endospores?
A. Reproduction
B. Nutrient absorption
C. Survival under adverse conditions
D. Protein synthesis
Answer: C. Survival under adverse conditions.
Rationale: Endospores are dormant, highly resistant forms for survival.
Which enzyme degrades penicillin by hydrolyzing the β-lactam ring?
A. DNA polymerase
B. β-lactamase (penicillinase)
C. Transpeptidase
D. Topoisomerase
Answer: B. β-lactamase (penicillinase).
Rationale: β-lactamases hydrolyze the β-lactam ring, inactivating penicillins.
The process by which bacteria acquire new genetic material from the
environment is called:
A. Transduction
B. Conjugation
C. Transformation
D. Binary fission
Answer: C. Transformation.
Rationale: Transformation = uptake of naked DNA from environment.
, Which immune cell is the main producer of antibodies?
A. T helper cells
B. B lymphocytes (plasma cells)
C. Macrophages
D. NK cells
Answer: B. B lymphocytes (plasma cells).
Rationale: Activated B cells differentiate into plasma cells that secrete antibodies.
A culture grows at 4°C but not at 37°C. What term describes this organism?
A. Mesophile
B. Thermophile
C. Psychrophile
D. Halophile
Answer: C. Psychrophile.
Rationale: Psychrophiles grow at low temperatures (around 0–15°C).
Which stain differentiates Mycobacterium due to high mycolic acid content?
A. Gram stain
B. Acid-fast stain (Ziehl-Neelsen)
C. Endospore stain
D. Capsule stain
Answer: B. Acid-fast stain (Ziehl-Neelsen).
Rationale: Acid-fast stain detects mycolic acids in Mycobacterium cell walls.
Viral genomes that integrate into host DNA are called:
A. Lytic phages
B. Lysogenic prophages (temperate phages)
C. Naked viruses
D. Retroviruses only
Answer: B. Lysogenic prophages (temperate phages).
Rationale: Temperate phages integrate genome as prophage in lysogeny.
Which mechanism best explains antigenic shift in influenza viruses?
A. Point mutation in HA gene
B. Reassortment of segmented genome between strains
C. Recombination with host DNA
D. Reverse transcription errors
Answer: B. Reassortment of segmented genome between strains.
Rationale: Shift = major change via reassortment of segmented RNA segments.
, Which of the following antibiotics inhibits cell wall synthesis by binding to
penicillin-binding proteins (PBPs)?
A. Tetracycline
B. Penicillin
C. Chloramphenicol
D. Ciprofloxacin
Answer: B. Penicillin.
Rationale: β-lactams inhibit PBPs, blocking peptidoglycan cross-linking.
A catalase-positive organism added to hydrogen peroxide produces bubbles.
What does catalase do?
A. Converts H₂O₂ to water and oxygen
B. Produces H₂O₂ from water
C. Breaks down carbohydrates
D. Synthesizes ATP
Answer: A. Converts H₂O₂ to water and oxygen.
Rationale: Catalase breaks down hydrogen peroxide into water and oxygen gas
(bubbles).
Which of these is an obligate intracellular pathogen?
A. Staphylococcus aureus
B. Chlamydia trachomatis
C. Pseudomonas aeruginosa
D. Bacillus anthracis
Answer: B. Chlamydia trachomatis.
Rationale: Chlamydia requires host cells to replicate (obligate intracellular).
The lag phase in bacterial growth curve is characterized by:
A. Rapid cell division
B. No net increase in cell number as bacteria adapt
C. Death phase predominates
D. Maximum metabolic activity and division rate
Answer: B. No net increase in cell number as bacteria adapt.
Rationale: Lag = adaptation phase; cells prepare for growth without dividing much.
Which molecule is the primary genetic material for many viruses (e.g., SARS-
CoV-2)?
A. Double-stranded DNA
B. Single-stranded RNA (positive-sense)
C. Protein only
Practice Exam the Actual OA | 150 Verified-Style
Questions with Answers & Rationales
Overview:
This comprehensive practice exam is designed to closely mirror the Western Governors
University (WGU) D311 Microbiology Objective Assessment.
It includes 150 multiple-choice questions that reflect the difficulty, style, and topic
distribution of the real exam.
Coverage Areas:
1. Microbial Cell Structure & Function – Prokaryotic vs. Eukaryotic, cell walls,
organelles
2. Microbial Growth & Control – Nutritional requirements, physical/chemical control
methods
3. Microbial Genetics & Molecular Biology – DNA replication, transcription, translation,
mutation, gene transfer
4. Metabolism & Enzymology – Catabolism, anabolism, ATP generation
5. Immunology & Host Defenses – Innate and adaptive immunity, antibodies, antigen
processing
6. Infectious Diseases & Pathogenicity – Bacterial, viral, fungal, and parasitic diseases;
epidemiology
7. Laboratory Techniques & Safety – Microscopy, staining, aseptic technique, culture
methods
Which cellular structure is the primary site of ATP production in bacteria?
A. Nucleus
B. Mitochondrion
C. Plasma (cytoplasmic) membrane
D. Ribosome
Answer: C. Plasma (cytoplasmic) membrane.
Rationale: Bacteria lack mitochondria; oxidative phosphorylation occurs across the
plasma membrane.
Gram-positive bacteria differ from Gram-negative bacteria primarily by having:
A. Outer membrane with lipopolysaccharide
B. Thick peptidoglycan layer and teichoic acids
,C. Periplasmic space
D. Porin proteins in outer membrane
Answer: B. Thick peptidoglycan layer and teichoic acids.
Rationale: Gram-positive have thick peptidoglycan with teichoic acids; Gram-
negatives have outer membrane/LPS.
Which of the following structures is used by bacteria to attach to host cells?
A. Flagella
B. Pili (fimbriae)
C. Endospore
D. Capsule
Answer: B. Pili (fimbriae).
Rationale: Pili/fimbriae mediate attachment; capsules can aid adherence but pili are
specialized for attachment.
What is the primary function of bacterial endospores?
A. Reproduction
B. Nutrient absorption
C. Survival under adverse conditions
D. Protein synthesis
Answer: C. Survival under adverse conditions.
Rationale: Endospores are dormant, highly resistant forms for survival.
Which enzyme degrades penicillin by hydrolyzing the β-lactam ring?
A. DNA polymerase
B. β-lactamase (penicillinase)
C. Transpeptidase
D. Topoisomerase
Answer: B. β-lactamase (penicillinase).
Rationale: β-lactamases hydrolyze the β-lactam ring, inactivating penicillins.
The process by which bacteria acquire new genetic material from the
environment is called:
A. Transduction
B. Conjugation
C. Transformation
D. Binary fission
Answer: C. Transformation.
Rationale: Transformation = uptake of naked DNA from environment.
, Which immune cell is the main producer of antibodies?
A. T helper cells
B. B lymphocytes (plasma cells)
C. Macrophages
D. NK cells
Answer: B. B lymphocytes (plasma cells).
Rationale: Activated B cells differentiate into plasma cells that secrete antibodies.
A culture grows at 4°C but not at 37°C. What term describes this organism?
A. Mesophile
B. Thermophile
C. Psychrophile
D. Halophile
Answer: C. Psychrophile.
Rationale: Psychrophiles grow at low temperatures (around 0–15°C).
Which stain differentiates Mycobacterium due to high mycolic acid content?
A. Gram stain
B. Acid-fast stain (Ziehl-Neelsen)
C. Endospore stain
D. Capsule stain
Answer: B. Acid-fast stain (Ziehl-Neelsen).
Rationale: Acid-fast stain detects mycolic acids in Mycobacterium cell walls.
Viral genomes that integrate into host DNA are called:
A. Lytic phages
B. Lysogenic prophages (temperate phages)
C. Naked viruses
D. Retroviruses only
Answer: B. Lysogenic prophages (temperate phages).
Rationale: Temperate phages integrate genome as prophage in lysogeny.
Which mechanism best explains antigenic shift in influenza viruses?
A. Point mutation in HA gene
B. Reassortment of segmented genome between strains
C. Recombination with host DNA
D. Reverse transcription errors
Answer: B. Reassortment of segmented genome between strains.
Rationale: Shift = major change via reassortment of segmented RNA segments.
, Which of the following antibiotics inhibits cell wall synthesis by binding to
penicillin-binding proteins (PBPs)?
A. Tetracycline
B. Penicillin
C. Chloramphenicol
D. Ciprofloxacin
Answer: B. Penicillin.
Rationale: β-lactams inhibit PBPs, blocking peptidoglycan cross-linking.
A catalase-positive organism added to hydrogen peroxide produces bubbles.
What does catalase do?
A. Converts H₂O₂ to water and oxygen
B. Produces H₂O₂ from water
C. Breaks down carbohydrates
D. Synthesizes ATP
Answer: A. Converts H₂O₂ to water and oxygen.
Rationale: Catalase breaks down hydrogen peroxide into water and oxygen gas
(bubbles).
Which of these is an obligate intracellular pathogen?
A. Staphylococcus aureus
B. Chlamydia trachomatis
C. Pseudomonas aeruginosa
D. Bacillus anthracis
Answer: B. Chlamydia trachomatis.
Rationale: Chlamydia requires host cells to replicate (obligate intracellular).
The lag phase in bacterial growth curve is characterized by:
A. Rapid cell division
B. No net increase in cell number as bacteria adapt
C. Death phase predominates
D. Maximum metabolic activity and division rate
Answer: B. No net increase in cell number as bacteria adapt.
Rationale: Lag = adaptation phase; cells prepare for growth without dividing much.
Which molecule is the primary genetic material for many viruses (e.g., SARS-
CoV-2)?
A. Double-stranded DNA
B. Single-stranded RNA (positive-sense)
C. Protein only
