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,BIOD 171 Final Exam 2026 | (Essential Microbiology) Complete Questions and Answers Graded A+
2026/2027 Examination
Total Questions: 200
Instructions:
• Answer all questions.
• Select the single best answer.
• Each question has four answer choices: A, B, C, and D.
• Select only ONE answer for each question.
SECTION 1: Microbial Cell Structure, Function, and Taxonomy
Questions 1–25
Q1. Which structural feature is universally present in all bacterial cells and serves as the primary
target for lysozyme?
A. Outer membrane
B. Peptidoglycan layer
C. Teichoic acid
D. Lipopolysaccharide
Correct Answer: B. Peptidoglycan layer
Rationale: Lysozyme cleaves the β-1,4 glycosidic bonds between N-acetylglucosamine and N-
acetylmuramic acid in peptidoglycan. This layer is present in all bacteria (though modified in some),
making it the correct target. Teichoic acids are only in Gram-positives, LPS is only in Gram-negatives,
and the outer membrane is absent in Gram-positives.
Q2. A microbiologist observes an organism that lacks a nucleus, has a cell wall containing
pseudomurein, and produces methane. This organism is best classified as a:
A. Gram-negative bacterium
B. Gram-positive bacterium
C. Methanogenic archaeon
D. Fungal species
Correct Answer: C. Methanogenic archaeon
Rationale: Pseudomurein is a peptidoglycan-like polymer found specifically in methanogenic
archaea. Archaea lack nuclei (prokaryotic) but differ from bacteria in cell wall composition. Methane
production is a hallmark of methanogens, which are archaea.
Q3. Which of the following best explains why the Gram stain differentiates bacteria into two
major groups?
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,A. Differences in ribosome size
B. Differences in cytoplasmic membrane lipid composition
C. Differences in peptidoglycan thickness and outer membrane presence
D. Differences in DNA replication machinery
Correct Answer: C. Differences in peptidoglycan thickness and outer membrane presence
Rationale: Gram-positive cells have a thick peptidoglycan layer that retains crystal violet-iodine
complex after alcohol decolorization. Gram-negative cells have a thin peptidoglycan layer and an
outer membrane; the alcohol dissolves the outer membrane and washes out the crystal violet,
allowing safranin counterstain to be seen.
Q4. A patient presents with a wound infection caused by a bacterium that grows in long chains
and is catalase-negative. Which genus is most likely responsible?
A. Staphylococcus
B. Streptococcus
C. Bacillus
D. Clostridium
Correct Answer: B. Streptococcus
Rationale: Streptococci are Gram-positive cocci arranged in chains and are catalase-negative.
Staphylococci are catalase-positive and arranged in clusters. Bacillus and Clostridium are rod-shaped
and catalase-positive (Bacillus) or catalase-negative but spore-forming rods (Clostridium).
Q5. Which component of the bacterial flagellum functions as the motor that rotates the
filament?
A. Filament
B. Hook
C. Basal body
D. Pilus
Correct Answer: C. Basal body
Rationale: The basal body is embedded in the cell envelope and contains the motor proteins
(MotA/MotB) that generate torque using the proton motive force. The filament is the propeller, the
hook is the flexible coupling, and pili are separate appendages for attachment.
Q6. An organism is discovered that grows optimally at 85°C in deep-sea hydrothermal vents.
Which term best describes this organism?
A. Psychrophile
B. Mesophile
C. Thermophile
D. Hyperthermophile
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, Correct Answer: D. Hyperthermophile
Rationale: Hyperthermophiles grow optimally at temperatures above 80°C. Thermophiles grow
at 45–80°C, mesophiles at 20–45°C, and psychrophiles at below 20°C. The 85°C optimum places this
organism in the hyperthermophile category.
Q7. Which of the following is a key difference between prokaryotic and eukaryotic ribosomes
that is exploited by antibiotics?
A. Prokaryotic ribosomes are 80S; eukaryotic are 70S
B. Prokaryotic ribosomes are 70S; eukaryotic are 80S
C. Prokaryotic ribosomes lack RNA; eukaryotic contain RNA
D. Prokaryotic ribosomes are membrane-bound; eukaryotic are free
Correct Answer: B. Prokaryotic ribosomes are 70S; eukaryotic are 80S
Rationale: Prokaryotic ribosomes are 70S (50S + 30S subunits), while eukaryotic ribosomes are
80S (60S + 40S). Antibiotics such as aminoglycosides and macrolides target the 70S ribosome
selectively, minimizing host toxicity.
Q8. A bacterial cell is placed in a hypertonic solution. What is the most likely immediate
consequence?
A. Cell lysis due to water influx
B. Plasmolysis due to water efflux
C. No change because bacteria lack a cell wall
D. Increased turgor pressure
Correct Answer: B. Plasmolysis due to water efflux
Rationale: In a hypertonic environment, water moves out of the cell by osmosis, causing the
cytoplasmic membrane to shrink away from the cell wall (plasmolysis). Lysis occurs in hypotonic
solutions. Bacteria have cell walls, so they do not lyse immediately in hypertonic conditions.
Q9. Which of the following structures is responsible for the endotoxic properties of Gram-
negative bacteria?
A. Peptidoglycan
B. Lipopolysaccharide
C. Teichoic acid
D. Capsule
Correct Answer: B. Lipopolysaccharide
Rationale: Lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria contains
lipid A, which is the endotoxin that triggers fever, shock, and inflammation. Peptidoglycan and
teichoic acids are not endotoxins, and capsules are antiphagocytic but not endotoxic.
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,BIOD 171 Final Exam 2026 | (Essential Microbiology) Complete Questions and Answers Graded A+
2026/2027 Examination
Total Questions: 200
Instructions:
• Answer all questions.
• Select the single best answer.
• Each question has four answer choices: A, B, C, and D.
• Select only ONE answer for each question.
SECTION 1: Microbial Cell Structure, Function, and Taxonomy
Questions 1–25
Q1. Which structural feature is universally present in all bacterial cells and serves as the primary
target for lysozyme?
A. Outer membrane
B. Peptidoglycan layer
C. Teichoic acid
D. Lipopolysaccharide
Correct Answer: B. Peptidoglycan layer
Rationale: Lysozyme cleaves the β-1,4 glycosidic bonds between N-acetylglucosamine and N-
acetylmuramic acid in peptidoglycan. This layer is present in all bacteria (though modified in some),
making it the correct target. Teichoic acids are only in Gram-positives, LPS is only in Gram-negatives,
and the outer membrane is absent in Gram-positives.
Q2. A microbiologist observes an organism that lacks a nucleus, has a cell wall containing
pseudomurein, and produces methane. This organism is best classified as a:
A. Gram-negative bacterium
B. Gram-positive bacterium
C. Methanogenic archaeon
D. Fungal species
Correct Answer: C. Methanogenic archaeon
Rationale: Pseudomurein is a peptidoglycan-like polymer found specifically in methanogenic
archaea. Archaea lack nuclei (prokaryotic) but differ from bacteria in cell wall composition. Methane
production is a hallmark of methanogens, which are archaea.
Q3. Which of the following best explains why the Gram stain differentiates bacteria into two
major groups?
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,A. Differences in ribosome size
B. Differences in cytoplasmic membrane lipid composition
C. Differences in peptidoglycan thickness and outer membrane presence
D. Differences in DNA replication machinery
Correct Answer: C. Differences in peptidoglycan thickness and outer membrane presence
Rationale: Gram-positive cells have a thick peptidoglycan layer that retains crystal violet-iodine
complex after alcohol decolorization. Gram-negative cells have a thin peptidoglycan layer and an
outer membrane; the alcohol dissolves the outer membrane and washes out the crystal violet,
allowing safranin counterstain to be seen.
Q4. A patient presents with a wound infection caused by a bacterium that grows in long chains
and is catalase-negative. Which genus is most likely responsible?
A. Staphylococcus
B. Streptococcus
C. Bacillus
D. Clostridium
Correct Answer: B. Streptococcus
Rationale: Streptococci are Gram-positive cocci arranged in chains and are catalase-negative.
Staphylococci are catalase-positive and arranged in clusters. Bacillus and Clostridium are rod-shaped
and catalase-positive (Bacillus) or catalase-negative but spore-forming rods (Clostridium).
Q5. Which component of the bacterial flagellum functions as the motor that rotates the
filament?
A. Filament
B. Hook
C. Basal body
D. Pilus
Correct Answer: C. Basal body
Rationale: The basal body is embedded in the cell envelope and contains the motor proteins
(MotA/MotB) that generate torque using the proton motive force. The filament is the propeller, the
hook is the flexible coupling, and pili are separate appendages for attachment.
Q6. An organism is discovered that grows optimally at 85°C in deep-sea hydrothermal vents.
Which term best describes this organism?
A. Psychrophile
B. Mesophile
C. Thermophile
D. Hyperthermophile
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, Correct Answer: D. Hyperthermophile
Rationale: Hyperthermophiles grow optimally at temperatures above 80°C. Thermophiles grow
at 45–80°C, mesophiles at 20–45°C, and psychrophiles at below 20°C. The 85°C optimum places this
organism in the hyperthermophile category.
Q7. Which of the following is a key difference between prokaryotic and eukaryotic ribosomes
that is exploited by antibiotics?
A. Prokaryotic ribosomes are 80S; eukaryotic are 70S
B. Prokaryotic ribosomes are 70S; eukaryotic are 80S
C. Prokaryotic ribosomes lack RNA; eukaryotic contain RNA
D. Prokaryotic ribosomes are membrane-bound; eukaryotic are free
Correct Answer: B. Prokaryotic ribosomes are 70S; eukaryotic are 80S
Rationale: Prokaryotic ribosomes are 70S (50S + 30S subunits), while eukaryotic ribosomes are
80S (60S + 40S). Antibiotics such as aminoglycosides and macrolides target the 70S ribosome
selectively, minimizing host toxicity.
Q8. A bacterial cell is placed in a hypertonic solution. What is the most likely immediate
consequence?
A. Cell lysis due to water influx
B. Plasmolysis due to water efflux
C. No change because bacteria lack a cell wall
D. Increased turgor pressure
Correct Answer: B. Plasmolysis due to water efflux
Rationale: In a hypertonic environment, water moves out of the cell by osmosis, causing the
cytoplasmic membrane to shrink away from the cell wall (plasmolysis). Lysis occurs in hypotonic
solutions. Bacteria have cell walls, so they do not lyse immediately in hypertonic conditions.
Q9. Which of the following structures is responsible for the endotoxic properties of Gram-
negative bacteria?
A. Peptidoglycan
B. Lipopolysaccharide
C. Teichoic acid
D. Capsule
Correct Answer: B. Lipopolysaccharide
Rationale: Lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria contains
lipid A, which is the endotoxin that triggers fever, shock, and inflammation. Peptidoglycan and
teichoic acids are not endotoxins, and capsules are antiphagocytic but not endotoxic.
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