Microbiology BIO 420 Exam 1| Contains Questions
with Verified Correct Answers| Already Graded A+
1. A researcher isolates a bacterial strain that is resistant to multiple antibiotics. Whole-genome
sequencing reveals an integron carrying several gene cassettes. Which of the following best
describes the mechanism by which these gene cassettes are integrated into the integron?
A. Site-specific recombination mediated by an integrase that recognizes attC sites
B. Homologous recombination between repeated sequences flanking the cassettes
C. Transposition via a transposase that recognizes inverted repeats
D. Illegitimate recombination during DNA replication repair
Answer: A
Rationale: Integrons capture gene cassettes via site-specific recombination catalyzed by an integrase
(IntI) that recognizes attC sites in the cassettes and attI sites in the integron. Homologous recombination
(B) is not involved; transposition (C) is characteristic of transposons; illegitimate recombination (D) is
rare and not specific.
2. In a chemostat culture of Escherichia coli under glucose limitation, the dilution rate is set to 0.3
h¹. The Monod constant (K_s) for glucose is 0.02 g/L, and the maximum specific growth rate
(_max) is 0.6 h¹. What is the steady-state glucose concentration in the culture?
A. 0.01 g/L
B. 0.02 g/L
C. 0.03 g/L
D. 0.04 g/L
Answer: B
Rationale: At steady state in a chemostat, dilution rate (D) equals specific growth rate (¼). Using Monod
equation: = _max * S / (K_s + S). Rearranging: S = K_s * D / (_max - D) = 0.02 * 0.3 / (0.6 - 0.3) =
0..3 = 0.02 g/L. Thus the correct answer is B.
3. A patient with a prosthetic heart valve develops a persistent fever and blood cultures grow a
gram-positive coccus that is catalase-negative and optochin-resistant. The organism forms chains
in broth. Which of the following virulence factors is most likely responsible for the persistence of
this infection?
A. Protein A
B. Lipoteichoic acid
C. Dextran production
D. IgA protease
Answer: C
Rationale: The organism is likely a viridans group streptococcus (e.g., Streptococcus sanguinis), which
produces dextran (a polysaccharide) that facilitates adherence to prosthetic surfaces and formation of
biofilms. Protein A is from Staphylococcus aureus; lipoteichoic acid is common but not specific; IgA
Page 1
,protease is produced by Neisseria and Haemophilus.
4. Which of the following best explains why certain archaea can thrive at pH 1-2 while maintaining
a near-neutral intracellular pH?
A. They possess a highly impermeable cell membrane composed of tetraether lipids that reduce proton influx
B. They actively pump protons out via a reversed ATP synthase operating as a proton pump
C. They accumulate high concentrations of potassium ions to buffer internal pH
D. They express acid-stable proteins that function optimally at low pH
Answer: A
Rationale: Acidophilic archaea have cell membranes with tetraether lipids that form a monolayer with
low proton permeability, minimizing H entry. While some use proton pumps, the primary adaptation is
membrane impermeability (A). Reverse ATP synthase (B) is used by alkaliphiles to pump H in; K
accumulation (C) is not a major pH buffer; proteins (D) are stable but do not explain internal pH
maintenance.
5. A researcher studies a bacterial toxin that catalyzes the transfer of an ADP-ribose moiety from
NAD to the subunit of a heterotrimeric G protein. This modification locks the G protein in its
active GTP-bound form. Which of the following cellular processes would be most directly affected?
A. Inhibition of adenylate cyclase leading to decreased cAMP
B. Constitutive activation of adenylate cyclase leading to increased cAMP
C. Blockade of calcium channels in the endoplasmic reticulum
D. Activation of phospholipase C and IP3 production
Answer: B
Rationale: The toxin likely acts on Gs±, which stimulates adenylate cyclase. ADP-ribosylation locks Gs±
in the active form, causing persistent activation of adenylate cyclase and elevated cAMP. This is
characteristic of cholera toxin. Inhibition of adenylate cyclase (A) would occur if Gi were targeted;
calcium channels (C) and phospholipase C (D) are regulated by different G proteins.
6. In a laboratory, a bacterial culture is exposed to a bacteriophage. After incubation, the culture
clears but then regrows. Isolated colonies from the regrown culture are resistant to the same
phage. Which of the following mechanisms most likely explains the acquired resistance?
A. Mutations in the phage receptor on the bacterial surface
B. Acquisition of a plasmid encoding a restriction-modification system
C. Integration of the phage genome into the bacterial chromosome as a prophage
D. Induction of a CRISPR-Cas system that targets the phage DNA
Answer: A
Rationale: Regrowth after phage infection often results from selection of mutants that lack or alter the
phage receptor, preventing adsorption. Restriction-modification (B) may provide resistance but is less
common in regrowth; prophage integration (C) would not cause resistance to superinfection by the same
phage (it does but via repressor, not mutation); CRISPR-Cas (D) is adaptive but requires prior exposure,
and the scenario describes a single exposure.
Page 2
,7. A researcher is studying a novel bacterium that uses light energy to drive ATP synthesis but does
not produce oxygen. The organism contains bacteriochlorophylls and uses hydrogen sulfide (HS)
as an electron donor. Which of the following best describes the metabolic mode of this bacterium?
A. Oxygenic photosynthesis
B. Anoxygenic photosynthesis
C. Chemolithoautotrophy
D. Photoorganoheterotrophy
Answer: B
Rationale: The use of bacteriochlorophylls and H ‚S as electron donor, with no oxygen production,
indicates anoxygenic photosynthesis, typical of purple and green sulfur bacteria. Oxygenic
photosynthesis (A) uses chlorophyll a and water, producing O. Chemolithoautotrophy (C) uses inorganic
chemicals for energy, not light. Photoorganoheterotrophy (D) uses organic compounds as electron
donors, not HS.
8. Which of the following statements best explains why Mycobacterium tuberculosis is able to
survive within macrophages despite the acidic and hydrolytic environment of the phagolysosome?
A. It produces a polysaccharide capsule that neutralizes lysosomal enzymes
B. It inhibits phagosome-lysosome fusion through secretion of a phosphatase
C. It expresses efflux pumps that expel protons from the phagosome
D. It has a thick peptidoglycan layer that resists acid degradation
Answer: B
Rationale: M. tuberculosis secretes a phosphatase (e.g., SapM) that dephosphorylates
phosphatidylinositol 3-phosphate, blocking recruitment of proteins needed for phagosome-lysosome
fusion. Capsule (A) is not primary; efflux pumps (C) are not known for proton removal; peptidoglycan
(D) is not exceptionally acid-resistant and does not prevent fusion.
9. A laboratory technician accidentally creates a culture condition where the dissolved oxygen
concentration is 0.2 mg/L. Several bacterial species are present. Which of the following metabolic
pathways would be most severely inhibited in an obligate aerobe under this condition?
A. Glycolysis
B. Tricarboxylic acid cycle
C. Pentose phosphate pathway
D. Beta-oxidation of fatty acids
Answer: B
Rationale: The tricarboxylic acid (TCA) cycle requires oxygen as the terminal electron acceptor in the
electron transport chain to regenerate NAD and FAD. Under low oxygen, the TCA cycle halts due to lack
of oxidized cofactors. Glycolysis (A) and pentose phosphate pathway (C) do not directly require oxygen;
beta-oxidation (D) also depends on the ETC for reoxidation of NADH and FADH, but the TCA cycle is
most directly affected.
Page 3
, 10. A patient with a urinary tract infection is treated with a fluoroquinolone. After several days,
the infection recurs, and the isolated bacteria show a 16-fold increase in MIC for ciprofloxacin.
Sequencing reveals a mutation in the gene encoding DNA gyrase. Which of the following additional
mutations would most likely confer high-level resistance?
A. Mutation in the marR gene leading to overexpression of AcrAB-TolC efflux pump
B. Mutation in the parC gene encoding topoisomerase IV
C. Acquisition of a plasmid encoding a ciprofloxacin-modifying enzyme
D. Mutation in the outer membrane porin OmpF reducing drug uptake
Answer: B
Rationale: In gram-negative bacteria, high-level fluoroquinolone resistance typically requires mutations
in both DNA gyrase (gyrA) and topoisomerase IV (parC). The initial mutation in gyrase reduces
susceptibility; an additional mutation in parC further increases resistance. Efflux pump overexpression
(A) or porin loss (D) contribute but are not as potent as target site mutations. Plasmid-mediated
modifying enzymes (C) are rare for fluoroquinolones.
11. A research team isolates a novel bacterium from a deep-sea hydrothermal vent. It grows
optimally at 80°C, pH 5, and requires sulfur for respiration. Which of the following metabolic
strategies is most likely utilized by this organism?
A. Oxygenic photosynthesis using water as electron donor
B. Chemolithoautotrophy oxidizing H2S and reducing O2
C. Sulfur reduction coupled to hydrogen oxidation
D. Fermentation of glucose to ethanol and CO2
Answer: C
Rationale: The organism's growth conditions (high temperature, acidic pH, sulfur requirement) suggest it
is a hyperthermophilic archaeon that uses sulfur as a terminal electron acceptor. Many such organisms
couple H2 oxidation to S0 reduction, producing H2S. Option A is incorrect because oxygenic
photosynthesis is not thermophilic and uses water. Option B requires oxygen, which is scarce at vents.
Option D is fermentative and does not involve sulfur.
12. A bacterial strain has a mutation in the gene encoding the sigma factor RpoS. Which of the
following phenotypes is most likely affected?
A. Constitutive expression of heat shock proteins
B. Inability to form biofilms
C. Loss of flagellar motility
D. Constitutive SOS response
Answer: B
Rationale: RpoS (ÃS) is the master regulator of the general stress response in E. coli and related
bacteria, controlling genes for biofilm formation, stress resistance, and stationary phase survival.
Mutation in rpoS impairs biofilm formation. Heat shock proteins are regulated by 32 (RpoH). Flagellar
motility requires 28 (FliA). SOS response is controlled by LexA and RecA.
Page 4
with Verified Correct Answers| Already Graded A+
1. A researcher isolates a bacterial strain that is resistant to multiple antibiotics. Whole-genome
sequencing reveals an integron carrying several gene cassettes. Which of the following best
describes the mechanism by which these gene cassettes are integrated into the integron?
A. Site-specific recombination mediated by an integrase that recognizes attC sites
B. Homologous recombination between repeated sequences flanking the cassettes
C. Transposition via a transposase that recognizes inverted repeats
D. Illegitimate recombination during DNA replication repair
Answer: A
Rationale: Integrons capture gene cassettes via site-specific recombination catalyzed by an integrase
(IntI) that recognizes attC sites in the cassettes and attI sites in the integron. Homologous recombination
(B) is not involved; transposition (C) is characteristic of transposons; illegitimate recombination (D) is
rare and not specific.
2. In a chemostat culture of Escherichia coli under glucose limitation, the dilution rate is set to 0.3
h¹. The Monod constant (K_s) for glucose is 0.02 g/L, and the maximum specific growth rate
(_max) is 0.6 h¹. What is the steady-state glucose concentration in the culture?
A. 0.01 g/L
B. 0.02 g/L
C. 0.03 g/L
D. 0.04 g/L
Answer: B
Rationale: At steady state in a chemostat, dilution rate (D) equals specific growth rate (¼). Using Monod
equation: = _max * S / (K_s + S). Rearranging: S = K_s * D / (_max - D) = 0.02 * 0.3 / (0.6 - 0.3) =
0..3 = 0.02 g/L. Thus the correct answer is B.
3. A patient with a prosthetic heart valve develops a persistent fever and blood cultures grow a
gram-positive coccus that is catalase-negative and optochin-resistant. The organism forms chains
in broth. Which of the following virulence factors is most likely responsible for the persistence of
this infection?
A. Protein A
B. Lipoteichoic acid
C. Dextran production
D. IgA protease
Answer: C
Rationale: The organism is likely a viridans group streptococcus (e.g., Streptococcus sanguinis), which
produces dextran (a polysaccharide) that facilitates adherence to prosthetic surfaces and formation of
biofilms. Protein A is from Staphylococcus aureus; lipoteichoic acid is common but not specific; IgA
Page 1
,protease is produced by Neisseria and Haemophilus.
4. Which of the following best explains why certain archaea can thrive at pH 1-2 while maintaining
a near-neutral intracellular pH?
A. They possess a highly impermeable cell membrane composed of tetraether lipids that reduce proton influx
B. They actively pump protons out via a reversed ATP synthase operating as a proton pump
C. They accumulate high concentrations of potassium ions to buffer internal pH
D. They express acid-stable proteins that function optimally at low pH
Answer: A
Rationale: Acidophilic archaea have cell membranes with tetraether lipids that form a monolayer with
low proton permeability, minimizing H entry. While some use proton pumps, the primary adaptation is
membrane impermeability (A). Reverse ATP synthase (B) is used by alkaliphiles to pump H in; K
accumulation (C) is not a major pH buffer; proteins (D) are stable but do not explain internal pH
maintenance.
5. A researcher studies a bacterial toxin that catalyzes the transfer of an ADP-ribose moiety from
NAD to the subunit of a heterotrimeric G protein. This modification locks the G protein in its
active GTP-bound form. Which of the following cellular processes would be most directly affected?
A. Inhibition of adenylate cyclase leading to decreased cAMP
B. Constitutive activation of adenylate cyclase leading to increased cAMP
C. Blockade of calcium channels in the endoplasmic reticulum
D. Activation of phospholipase C and IP3 production
Answer: B
Rationale: The toxin likely acts on Gs±, which stimulates adenylate cyclase. ADP-ribosylation locks Gs±
in the active form, causing persistent activation of adenylate cyclase and elevated cAMP. This is
characteristic of cholera toxin. Inhibition of adenylate cyclase (A) would occur if Gi were targeted;
calcium channels (C) and phospholipase C (D) are regulated by different G proteins.
6. In a laboratory, a bacterial culture is exposed to a bacteriophage. After incubation, the culture
clears but then regrows. Isolated colonies from the regrown culture are resistant to the same
phage. Which of the following mechanisms most likely explains the acquired resistance?
A. Mutations in the phage receptor on the bacterial surface
B. Acquisition of a plasmid encoding a restriction-modification system
C. Integration of the phage genome into the bacterial chromosome as a prophage
D. Induction of a CRISPR-Cas system that targets the phage DNA
Answer: A
Rationale: Regrowth after phage infection often results from selection of mutants that lack or alter the
phage receptor, preventing adsorption. Restriction-modification (B) may provide resistance but is less
common in regrowth; prophage integration (C) would not cause resistance to superinfection by the same
phage (it does but via repressor, not mutation); CRISPR-Cas (D) is adaptive but requires prior exposure,
and the scenario describes a single exposure.
Page 2
,7. A researcher is studying a novel bacterium that uses light energy to drive ATP synthesis but does
not produce oxygen. The organism contains bacteriochlorophylls and uses hydrogen sulfide (HS)
as an electron donor. Which of the following best describes the metabolic mode of this bacterium?
A. Oxygenic photosynthesis
B. Anoxygenic photosynthesis
C. Chemolithoautotrophy
D. Photoorganoheterotrophy
Answer: B
Rationale: The use of bacteriochlorophylls and H ‚S as electron donor, with no oxygen production,
indicates anoxygenic photosynthesis, typical of purple and green sulfur bacteria. Oxygenic
photosynthesis (A) uses chlorophyll a and water, producing O. Chemolithoautotrophy (C) uses inorganic
chemicals for energy, not light. Photoorganoheterotrophy (D) uses organic compounds as electron
donors, not HS.
8. Which of the following statements best explains why Mycobacterium tuberculosis is able to
survive within macrophages despite the acidic and hydrolytic environment of the phagolysosome?
A. It produces a polysaccharide capsule that neutralizes lysosomal enzymes
B. It inhibits phagosome-lysosome fusion through secretion of a phosphatase
C. It expresses efflux pumps that expel protons from the phagosome
D. It has a thick peptidoglycan layer that resists acid degradation
Answer: B
Rationale: M. tuberculosis secretes a phosphatase (e.g., SapM) that dephosphorylates
phosphatidylinositol 3-phosphate, blocking recruitment of proteins needed for phagosome-lysosome
fusion. Capsule (A) is not primary; efflux pumps (C) are not known for proton removal; peptidoglycan
(D) is not exceptionally acid-resistant and does not prevent fusion.
9. A laboratory technician accidentally creates a culture condition where the dissolved oxygen
concentration is 0.2 mg/L. Several bacterial species are present. Which of the following metabolic
pathways would be most severely inhibited in an obligate aerobe under this condition?
A. Glycolysis
B. Tricarboxylic acid cycle
C. Pentose phosphate pathway
D. Beta-oxidation of fatty acids
Answer: B
Rationale: The tricarboxylic acid (TCA) cycle requires oxygen as the terminal electron acceptor in the
electron transport chain to regenerate NAD and FAD. Under low oxygen, the TCA cycle halts due to lack
of oxidized cofactors. Glycolysis (A) and pentose phosphate pathway (C) do not directly require oxygen;
beta-oxidation (D) also depends on the ETC for reoxidation of NADH and FADH, but the TCA cycle is
most directly affected.
Page 3
, 10. A patient with a urinary tract infection is treated with a fluoroquinolone. After several days,
the infection recurs, and the isolated bacteria show a 16-fold increase in MIC for ciprofloxacin.
Sequencing reveals a mutation in the gene encoding DNA gyrase. Which of the following additional
mutations would most likely confer high-level resistance?
A. Mutation in the marR gene leading to overexpression of AcrAB-TolC efflux pump
B. Mutation in the parC gene encoding topoisomerase IV
C. Acquisition of a plasmid encoding a ciprofloxacin-modifying enzyme
D. Mutation in the outer membrane porin OmpF reducing drug uptake
Answer: B
Rationale: In gram-negative bacteria, high-level fluoroquinolone resistance typically requires mutations
in both DNA gyrase (gyrA) and topoisomerase IV (parC). The initial mutation in gyrase reduces
susceptibility; an additional mutation in parC further increases resistance. Efflux pump overexpression
(A) or porin loss (D) contribute but are not as potent as target site mutations. Plasmid-mediated
modifying enzymes (C) are rare for fluoroquinolones.
11. A research team isolates a novel bacterium from a deep-sea hydrothermal vent. It grows
optimally at 80°C, pH 5, and requires sulfur for respiration. Which of the following metabolic
strategies is most likely utilized by this organism?
A. Oxygenic photosynthesis using water as electron donor
B. Chemolithoautotrophy oxidizing H2S and reducing O2
C. Sulfur reduction coupled to hydrogen oxidation
D. Fermentation of glucose to ethanol and CO2
Answer: C
Rationale: The organism's growth conditions (high temperature, acidic pH, sulfur requirement) suggest it
is a hyperthermophilic archaeon that uses sulfur as a terminal electron acceptor. Many such organisms
couple H2 oxidation to S0 reduction, producing H2S. Option A is incorrect because oxygenic
photosynthesis is not thermophilic and uses water. Option B requires oxygen, which is scarce at vents.
Option D is fermentative and does not involve sulfur.
12. A bacterial strain has a mutation in the gene encoding the sigma factor RpoS. Which of the
following phenotypes is most likely affected?
A. Constitutive expression of heat shock proteins
B. Inability to form biofilms
C. Loss of flagellar motility
D. Constitutive SOS response
Answer: B
Rationale: RpoS (ÃS) is the master regulator of the general stress response in E. coli and related
bacteria, controlling genes for biofilm formation, stress resistance, and stationary phase survival.
Mutation in rpoS impairs biofilm formation. Heat shock proteins are regulated by 32 (RpoH). Flagellar
motility requires 28 (FliA). SOS response is controlled by LexA and RecA.
Page 4
