Brock Biology of Microorganisms, 15th Edition
– Comprehensive Practice Exam.
Domain 1: Microbial Evolution, Diversity & Cell Structure (15 Questions)
1. Which structural component is exclusively present in the envelope of a Gram-negative
bacterium, contributing to its resistance against hydrophobic antibiotics and its ability to
trigger septic shock?
A. Thick peptidoglycan layer
B. Lipopolysaccharide (LPS) outer membrane
C. Teichoic acid cross-links
D. Periplasmic space
Answer: B
Rationale: The Gram-negative cell envelope is characterized by a thin peptidoglycan layer
surrounded by an outer membrane containing Lipopolysaccharide (LPS). The Lipid A portion
of LPS is an endotoxin that can trigger septic shock, and the outer membrane acts as a
permeability barrier. [CORRECT]
2. A microbiologist performs a Gram stain on a clinical isolate. Upon microscopic examination,
the cells appear as purple cocci in clusters. Which structural feature is responsible for
retaining the crystal violet-iodine complex during the decolorization step?
A. The presence of a periplasmic space.
B. A thick, multilayered peptidoglycan matrix.
C. An outer membrane composed of lipopolysaccharide.
D. A capsule composed of polysaccharide slime.
Answer: B
Rationale: Gram-positive bacteria possess a thick peptidoglycan layer that traps the crystal
violet-iodine complex during the alcohol decolorization, keeping the cells purple. [CORRECT]
,3. In a comparative analysis of cell envelope architecture, which feature is unique to Gram-
negative bacteria and absent in Gram-positive bacteria?
A. A cytoplasmic membrane containing phospholipids.
B. A rigid cell wall containing muramic acid.
C. A periplasmic space located between the cytoplasmic membrane and an outer membrane.
D. Surface proteins anchored to the cell wall.
Answer: C
Rationale: While both domains have a cytoplasmic membrane, Gram-negative bacteria have a
distinct periplasmic space containing a thin peptidoglycan layer situated between the inner
and outer membranes. Gram-positive bacteria have a thick peptidoglycan layer exterior to the
cytoplasmic membrane but lack the outer membrane defining the periplasm. [CORRECT]
4. A laboratory experiment involves treating a bacterial culture with lysozyme. The Gram-
positive cells lyse immediately, while the Gram-negative cells remain intact initially. Which
structural difference explains this observation?
A. Gram-negative cells have an LPS outer membrane that shields the underlying
peptidoglycan from lysozyme attack.
B. Gram-positive cells have teichoic acids that attract lysozyme.
C. Gram-negative cells have a thicker peptidoglycan layer that resists enzymatic digestion.
D. Gram-positive cells lack a cytoplasmic membrane.
Answer: A
Rationale: The outer membrane of Gram-negative bacteria acts as a permeability barrier,
preventing large molecules like lysozyme from easily accessing the thin peptidoglycan layer
underneath. Gram-positive bacteria have exposed peptidoglycan, making them susceptible to
lysis. [CORRECT]
5. According to the endosymbiotic theory, which specific evidence supports the origin of
mitochondria from an ancient alphaproteobacterium?
A. Mitochondria possess a linear genome similar to the host nucleus.
,B. Mitochondria contain 80S ribosomes identical to eukaryotic cytoplasmic ribosomes.
C. Mitochondria have their own circular DNA and 70S ribosomes, showing homology to
bacterial ribosomes.
D. Mitochondria are surrounded by a single membrane derived from the host cell.
Answer: C
Rationale: The key evidence for the endosymbiotic origin of mitochondria is the presence of a
circular DNA genome and 70S ribosomes within the organelle, which are structurally and
phylogenetically similar to those found in bacteria, rather than the linear DNA and 80S
ribosomes of the eukaryotic nucleus. [CORRECT]
6. Which physiological trait of mitochondria is a vestige of their bacterial ancestry regarding
genome maintenance?
A. Binary fission independent of the cell cycle.
B. Sensitivity to tetracycline antibiotics.
C. The presence of a nucleoid region not bounded by a membrane.
D. Utilization of oxygen for respiration.
Answer: B
Rationale: Because mitochondria originated from bacteria, their ribosomes (70S) are sensitive
to antibiotics like tetracycline that target bacterial protein synthesis, while eukaryotic 80S
cytoplasmic ribosomes are generally unaffected. [CORRECT]
7. Chloroplasts in photosynthetic eukaryotes are hypothesized to have originated from a
cyanobacterial ancestor. Which feature is homologous between chloroplasts and
cyanobacteria?
A. The presence of chlorophyll a and photosystem II architecture.
B. A double-membrane envelope structure exclusively.
C. The ability to fix nitrogen.
D. The presence of peptidoglycan in the chloroplast wall.
Answer: A
, Rationale: Chloroplasts and cyanobacteria both contain chlorophyll a and similar
photosynthetic thylakoid membrane systems, providing strong evidence for the
endosymbiotic event involving a photosynthetic prokaryote. [CORRECT]
8. When Escherichia coli is placed in a gradient of an attractant (such as glucose), which
mechanism allows the bacterium to migrate toward the highest concentration?
A. Increasing the frequency of "tumbles" when moving up the gradient.
B. Suppressing "tumbles" and extending "runs" when moving up the gradient.
C. Rotating the flagella clockwise to induce smooth swimming.
D. Reversing the direction of flagellar rotation to back away from the attractant.
Answer: B
Rationale: In chemotaxis, bacteria extend the duration of "runs" (counterclockwise flagellar
rotation) when moving toward an attractant, effectively biasing their random movement
toward the favorable environment. [CORRECT]
9. The bacterial flagellar motor switches rotation direction in response to chemotactic signals.
What is the physical result of clockwise (CW) rotation of the flagellum in a peritrichously
flagellated bacterium?
A. Smooth swimming in a straight line.
B. A coordinated "run" toward an attractant.
C. A chaotic "tumble" resulting in reorientation of the cell.
D. Increased speed of forward movement.
Answer: C
Rationale: Clockwise rotation causes the flagellar bundle to fly apart, resulting in a "tumble"
where the cell stops moving forward and reorients its direction. [CORRECT]
10. A graduate student isolates an organism from a hot spring. Analysis reveals the organism
lacks peptidoglycan in its cell wall and possesses membrane lipids with ether linkages. To
which domain does this organism belong?
– Comprehensive Practice Exam.
Domain 1: Microbial Evolution, Diversity & Cell Structure (15 Questions)
1. Which structural component is exclusively present in the envelope of a Gram-negative
bacterium, contributing to its resistance against hydrophobic antibiotics and its ability to
trigger septic shock?
A. Thick peptidoglycan layer
B. Lipopolysaccharide (LPS) outer membrane
C. Teichoic acid cross-links
D. Periplasmic space
Answer: B
Rationale: The Gram-negative cell envelope is characterized by a thin peptidoglycan layer
surrounded by an outer membrane containing Lipopolysaccharide (LPS). The Lipid A portion
of LPS is an endotoxin that can trigger septic shock, and the outer membrane acts as a
permeability barrier. [CORRECT]
2. A microbiologist performs a Gram stain on a clinical isolate. Upon microscopic examination,
the cells appear as purple cocci in clusters. Which structural feature is responsible for
retaining the crystal violet-iodine complex during the decolorization step?
A. The presence of a periplasmic space.
B. A thick, multilayered peptidoglycan matrix.
C. An outer membrane composed of lipopolysaccharide.
D. A capsule composed of polysaccharide slime.
Answer: B
Rationale: Gram-positive bacteria possess a thick peptidoglycan layer that traps the crystal
violet-iodine complex during the alcohol decolorization, keeping the cells purple. [CORRECT]
,3. In a comparative analysis of cell envelope architecture, which feature is unique to Gram-
negative bacteria and absent in Gram-positive bacteria?
A. A cytoplasmic membrane containing phospholipids.
B. A rigid cell wall containing muramic acid.
C. A periplasmic space located between the cytoplasmic membrane and an outer membrane.
D. Surface proteins anchored to the cell wall.
Answer: C
Rationale: While both domains have a cytoplasmic membrane, Gram-negative bacteria have a
distinct periplasmic space containing a thin peptidoglycan layer situated between the inner
and outer membranes. Gram-positive bacteria have a thick peptidoglycan layer exterior to the
cytoplasmic membrane but lack the outer membrane defining the periplasm. [CORRECT]
4. A laboratory experiment involves treating a bacterial culture with lysozyme. The Gram-
positive cells lyse immediately, while the Gram-negative cells remain intact initially. Which
structural difference explains this observation?
A. Gram-negative cells have an LPS outer membrane that shields the underlying
peptidoglycan from lysozyme attack.
B. Gram-positive cells have teichoic acids that attract lysozyme.
C. Gram-negative cells have a thicker peptidoglycan layer that resists enzymatic digestion.
D. Gram-positive cells lack a cytoplasmic membrane.
Answer: A
Rationale: The outer membrane of Gram-negative bacteria acts as a permeability barrier,
preventing large molecules like lysozyme from easily accessing the thin peptidoglycan layer
underneath. Gram-positive bacteria have exposed peptidoglycan, making them susceptible to
lysis. [CORRECT]
5. According to the endosymbiotic theory, which specific evidence supports the origin of
mitochondria from an ancient alphaproteobacterium?
A. Mitochondria possess a linear genome similar to the host nucleus.
,B. Mitochondria contain 80S ribosomes identical to eukaryotic cytoplasmic ribosomes.
C. Mitochondria have their own circular DNA and 70S ribosomes, showing homology to
bacterial ribosomes.
D. Mitochondria are surrounded by a single membrane derived from the host cell.
Answer: C
Rationale: The key evidence for the endosymbiotic origin of mitochondria is the presence of a
circular DNA genome and 70S ribosomes within the organelle, which are structurally and
phylogenetically similar to those found in bacteria, rather than the linear DNA and 80S
ribosomes of the eukaryotic nucleus. [CORRECT]
6. Which physiological trait of mitochondria is a vestige of their bacterial ancestry regarding
genome maintenance?
A. Binary fission independent of the cell cycle.
B. Sensitivity to tetracycline antibiotics.
C. The presence of a nucleoid region not bounded by a membrane.
D. Utilization of oxygen for respiration.
Answer: B
Rationale: Because mitochondria originated from bacteria, their ribosomes (70S) are sensitive
to antibiotics like tetracycline that target bacterial protein synthesis, while eukaryotic 80S
cytoplasmic ribosomes are generally unaffected. [CORRECT]
7. Chloroplasts in photosynthetic eukaryotes are hypothesized to have originated from a
cyanobacterial ancestor. Which feature is homologous between chloroplasts and
cyanobacteria?
A. The presence of chlorophyll a and photosystem II architecture.
B. A double-membrane envelope structure exclusively.
C. The ability to fix nitrogen.
D. The presence of peptidoglycan in the chloroplast wall.
Answer: A
, Rationale: Chloroplasts and cyanobacteria both contain chlorophyll a and similar
photosynthetic thylakoid membrane systems, providing strong evidence for the
endosymbiotic event involving a photosynthetic prokaryote. [CORRECT]
8. When Escherichia coli is placed in a gradient of an attractant (such as glucose), which
mechanism allows the bacterium to migrate toward the highest concentration?
A. Increasing the frequency of "tumbles" when moving up the gradient.
B. Suppressing "tumbles" and extending "runs" when moving up the gradient.
C. Rotating the flagella clockwise to induce smooth swimming.
D. Reversing the direction of flagellar rotation to back away from the attractant.
Answer: B
Rationale: In chemotaxis, bacteria extend the duration of "runs" (counterclockwise flagellar
rotation) when moving toward an attractant, effectively biasing their random movement
toward the favorable environment. [CORRECT]
9. The bacterial flagellar motor switches rotation direction in response to chemotactic signals.
What is the physical result of clockwise (CW) rotation of the flagellum in a peritrichously
flagellated bacterium?
A. Smooth swimming in a straight line.
B. A coordinated "run" toward an attractant.
C. A chaotic "tumble" resulting in reorientation of the cell.
D. Increased speed of forward movement.
Answer: C
Rationale: Clockwise rotation causes the flagellar bundle to fly apart, resulting in a "tumble"
where the cell stops moving forward and reorients its direction. [CORRECT]
10. A graduate student isolates an organism from a hot spring. Analysis reveals the organism
lacks peptidoglycan in its cell wall and possesses membrane lipids with ether linkages. To
which domain does this organism belong?
