BIOL 271 MICROBIOLOGY ULTIMATE
FINAL EXAM BANK 200 MCQS +
ANSWERS & RATIONALES | LAB &
CLINICAL CORRELATIONS | A+
STUDY GUIDE 2026
BIOL 271 Microbiology with Lab: Final Exam Prep
2026 Undergraduate Microbiology
1. A microbiology student observes a bacterial cell under the microscope
and notes the absence of a membrane-bound nucleus, the presence of
70S ribosomes, and a cell wall containing peptidoglycan; which
classification BEST describes this organism?
A) Eukaryotic microbe
B) Archaea
C) Gram-negative bacterium
D) Prokaryotic bacterium
D Prokaryotic bacteria are characterized by the absence of a
membrane-bound nucleus, presence of 70S ribosomes, and a
peptidoglycan-containing cell wall; Archaea lack peptidoglycan
(contain pseudopeptidoglycan or other polymers), and eukaryotes have
membrane-bound organelles and 80S ribosomes.
2. During a Gram stain procedure, a student forgets to apply the iodine
mordant step; what is the MOST likely outcome when observing a mixed
culture of Staphylococcus aureus (Gram-positive) and Escherichia coli
(Gram-negative)?
A) All cells will appear purple
B) All cells will appear pink/red
C) Gram-positive cells will appear pink, Gram-negative will appear
purple
D) No cells will retain any stain
B Iodine forms a crystal violet-iodine complex that is trapped in the
thick peptidoglycan layer of Gram-positive cells; without iodine, the
primary stain is not fixed and is washed out during decolorization,
causing both Gram-positive and Gram-negative cells to take up the
counterstain (safranin) and appear pink/red.
,3. A bacterial culture is inoculated into nutrient broth and incubated at
37°C; growth is monitored by measuring optical density, and the curve
shows a prolonged period with no increase in cell number followed by
exponential growth; which phase of the bacterial growth curve does the
initial period represent?
A) Log (exponential) phase
B) Stationary phase
C) Lag phase
D) Death phase
C The lag phase is characterized by metabolic activity and cellular
preparation for division (enzyme synthesis, ribosome production)
without an increase in cell number; exponential growth follows once
cells are adapted to the new environment.
4. An antibiotic is discovered that inhibits the enzyme DNA gyrase in
bacteria; which cellular process is MOST directly affected, and why is
this a selective target?
A) Protein synthesis; bacterial ribosomes differ from eukaryotic
ribosomes
B) Cell wall synthesis; peptidoglycan is unique to bacteria
C) DNA replication; DNA gyrase is essential for bacterial DNA
supercoiling and is absent in humans
D) Folate synthesis; bacteria synthesize folate de novo while humans
obtain it from diet
C DNA gyrase (a type II topoisomerase) introduces negative supercoils
into bacterial DNA to relieve torsional stress during replication; it is
absent in human cells (which use different topoisomerases), making it
an excellent selective target for antibiotics like fluoroquinolones.
5. A patient presents with a urinary tract infection; urine culture grows a
lactose-fermenting, indole-positive, Gram-negative rod on MacConkey
agar; which organism is MOST likely responsible?
A) Pseudomonas aeruginosa
B) Klebsiella pneumoniae
C) Escherichia coli
D) Proteus mirabilis
C E. coli is the most common cause of UTIs and is characterized by
lactose fermentation (pink colonies on MacConkey), indole positivity
(tryptophanase enzyme), and Gram-negative rod morphology;
Klebsiella is lactose-fermenting but indole-negative; Pseudomonas is
,non-lactose fermenting; Proteus is indole-variable and swarms on
agar.
6. During a lab exercise, a student performs a catalase test on an
unknown Gram-positive coccus; bubbles form immediately upon
addition of hydrogen peroxide; which genus is MOST consistent with
this result?
A) Streptococcus
B) Enterococcus
C) Staphylococcus
D) Lactococcus
C The catalase test differentiates Staphylococcus (catalase-positive,
produces bubbles) from Streptococcus and Enterococcus (catalase-
negative, no bubbles); catalase breaks down hydrogen peroxide into
water and oxygen, protecting cells from oxidative damage.
7. A microbiology student isolates a bacterium that grows only in the
presence of oxygen and possesses superoxide dismutase and catalase
enzymes; which term BEST describes its oxygen requirement?
A) Obligate anaerobe
B) Facultative anaerobe
C) Microaerophile
D) Obligate aerobe
D Obligate aerobes require oxygen for growth and possess enzymes
(superoxide dismutase, catalase) to detoxify reactive oxygen species;
obligate anaerobes lack these enzymes and are killed by oxygen;
facultative anaerobes grow with or without oxygen; microaerophiles
require low oxygen concentrations.
8. A researcher is studying horizontal gene transfer in bacteria and
observes that a non-pathogenic strain acquires a toxin gene after
exposure to lysed pathogenic cells; which mechanism of gene transfer is
MOST likely responsible?
A) Conjugation via sex pilus
B) Transformation via uptake of free DNA
C) Transduction via bacteriophage vector
D) Transposition via mobile genetic elements
B Transformation involves the uptake of free environmental DNA
released from lysed cells; the non-pathogenic strain incorporated the
toxin gene from lysed pathogenic cells; conjugation requires cell-to-cell
contact; transduction requires a phage vector; transposition involves
movement of DNA within a genome.
, 9. A patient with a prosthetic heart valve develops fever and a new heart
murmur; blood cultures grow Gram-positive cocci in chains that are
catalase-negative and show alpha-hemolysis on blood agar; which
organism is MOST likely, and what virulence factor contributes to
endocarditis?
A) Staphylococcus aureus; protein A
B) Streptococcus pneumoniae; pneumolysin
C) Streptococcus sanguinis; adhesins that bind platelets and fibrin
D) Enterococcus faecalis; cytolysin
C Viridans group streptococci (e.g., S. sanguinis) are alpha-hemolytic,
catalase-negative Gram-positive cocci in chains and are common
causes of subacute bacterial endocarditis; they produce adhesins that
bind to platelet-fibrin aggregates on damaged heart valves, facilitating
colonization.
10. In a lab experiment, a student exposes bacterial cultures to UV light
for varying durations; survival curves show a sharp decline in viability
with increased exposure; which DNA repair mechanism is MOST
important for bacterial recovery from UV-induced damage?
A) Mismatch repair
B) Nucleotide excision repair (light-dependent photolyase or light-
independent UvrABC)
C) Base excision repair
D) Homologous recombination
B UV light causes thymine dimers that distort DNA; nucleotide excision
repair removes the damaged segment—photolyase uses visible light to
directly reverse dimers (photoreactivation), while UvrABC excises and
replaces the damaged nucleotides in the dark; mismatch repair corrects
replication errors, not UV damage.
11. A microbiology student performs an oxidase test on a Gram-negative
rod isolated from a wound; the filter paper turns purple within 10
seconds; which genus is MOST consistent with this result?
A) Escherichia
B) Pseudomonas
C) Klebsiella
D) Salmonella
B Pseudomonas aeruginosa is oxidase-positive (produces cytochrome c
oxidase), while Enterobacteriaceae (E. coli, Klebsiella, Salmonella) are
oxidase-negative; the oxidase test helps differentiate Pseudomonas
from common enteric Gram-negative rods.
FINAL EXAM BANK 200 MCQS +
ANSWERS & RATIONALES | LAB &
CLINICAL CORRELATIONS | A+
STUDY GUIDE 2026
BIOL 271 Microbiology with Lab: Final Exam Prep
2026 Undergraduate Microbiology
1. A microbiology student observes a bacterial cell under the microscope
and notes the absence of a membrane-bound nucleus, the presence of
70S ribosomes, and a cell wall containing peptidoglycan; which
classification BEST describes this organism?
A) Eukaryotic microbe
B) Archaea
C) Gram-negative bacterium
D) Prokaryotic bacterium
D Prokaryotic bacteria are characterized by the absence of a
membrane-bound nucleus, presence of 70S ribosomes, and a
peptidoglycan-containing cell wall; Archaea lack peptidoglycan
(contain pseudopeptidoglycan or other polymers), and eukaryotes have
membrane-bound organelles and 80S ribosomes.
2. During a Gram stain procedure, a student forgets to apply the iodine
mordant step; what is the MOST likely outcome when observing a mixed
culture of Staphylococcus aureus (Gram-positive) and Escherichia coli
(Gram-negative)?
A) All cells will appear purple
B) All cells will appear pink/red
C) Gram-positive cells will appear pink, Gram-negative will appear
purple
D) No cells will retain any stain
B Iodine forms a crystal violet-iodine complex that is trapped in the
thick peptidoglycan layer of Gram-positive cells; without iodine, the
primary stain is not fixed and is washed out during decolorization,
causing both Gram-positive and Gram-negative cells to take up the
counterstain (safranin) and appear pink/red.
,3. A bacterial culture is inoculated into nutrient broth and incubated at
37°C; growth is monitored by measuring optical density, and the curve
shows a prolonged period with no increase in cell number followed by
exponential growth; which phase of the bacterial growth curve does the
initial period represent?
A) Log (exponential) phase
B) Stationary phase
C) Lag phase
D) Death phase
C The lag phase is characterized by metabolic activity and cellular
preparation for division (enzyme synthesis, ribosome production)
without an increase in cell number; exponential growth follows once
cells are adapted to the new environment.
4. An antibiotic is discovered that inhibits the enzyme DNA gyrase in
bacteria; which cellular process is MOST directly affected, and why is
this a selective target?
A) Protein synthesis; bacterial ribosomes differ from eukaryotic
ribosomes
B) Cell wall synthesis; peptidoglycan is unique to bacteria
C) DNA replication; DNA gyrase is essential for bacterial DNA
supercoiling and is absent in humans
D) Folate synthesis; bacteria synthesize folate de novo while humans
obtain it from diet
C DNA gyrase (a type II topoisomerase) introduces negative supercoils
into bacterial DNA to relieve torsional stress during replication; it is
absent in human cells (which use different topoisomerases), making it
an excellent selective target for antibiotics like fluoroquinolones.
5. A patient presents with a urinary tract infection; urine culture grows a
lactose-fermenting, indole-positive, Gram-negative rod on MacConkey
agar; which organism is MOST likely responsible?
A) Pseudomonas aeruginosa
B) Klebsiella pneumoniae
C) Escherichia coli
D) Proteus mirabilis
C E. coli is the most common cause of UTIs and is characterized by
lactose fermentation (pink colonies on MacConkey), indole positivity
(tryptophanase enzyme), and Gram-negative rod morphology;
Klebsiella is lactose-fermenting but indole-negative; Pseudomonas is
,non-lactose fermenting; Proteus is indole-variable and swarms on
agar.
6. During a lab exercise, a student performs a catalase test on an
unknown Gram-positive coccus; bubbles form immediately upon
addition of hydrogen peroxide; which genus is MOST consistent with
this result?
A) Streptococcus
B) Enterococcus
C) Staphylococcus
D) Lactococcus
C The catalase test differentiates Staphylococcus (catalase-positive,
produces bubbles) from Streptococcus and Enterococcus (catalase-
negative, no bubbles); catalase breaks down hydrogen peroxide into
water and oxygen, protecting cells from oxidative damage.
7. A microbiology student isolates a bacterium that grows only in the
presence of oxygen and possesses superoxide dismutase and catalase
enzymes; which term BEST describes its oxygen requirement?
A) Obligate anaerobe
B) Facultative anaerobe
C) Microaerophile
D) Obligate aerobe
D Obligate aerobes require oxygen for growth and possess enzymes
(superoxide dismutase, catalase) to detoxify reactive oxygen species;
obligate anaerobes lack these enzymes and are killed by oxygen;
facultative anaerobes grow with or without oxygen; microaerophiles
require low oxygen concentrations.
8. A researcher is studying horizontal gene transfer in bacteria and
observes that a non-pathogenic strain acquires a toxin gene after
exposure to lysed pathogenic cells; which mechanism of gene transfer is
MOST likely responsible?
A) Conjugation via sex pilus
B) Transformation via uptake of free DNA
C) Transduction via bacteriophage vector
D) Transposition via mobile genetic elements
B Transformation involves the uptake of free environmental DNA
released from lysed cells; the non-pathogenic strain incorporated the
toxin gene from lysed pathogenic cells; conjugation requires cell-to-cell
contact; transduction requires a phage vector; transposition involves
movement of DNA within a genome.
, 9. A patient with a prosthetic heart valve develops fever and a new heart
murmur; blood cultures grow Gram-positive cocci in chains that are
catalase-negative and show alpha-hemolysis on blood agar; which
organism is MOST likely, and what virulence factor contributes to
endocarditis?
A) Staphylococcus aureus; protein A
B) Streptococcus pneumoniae; pneumolysin
C) Streptococcus sanguinis; adhesins that bind platelets and fibrin
D) Enterococcus faecalis; cytolysin
C Viridans group streptococci (e.g., S. sanguinis) are alpha-hemolytic,
catalase-negative Gram-positive cocci in chains and are common
causes of subacute bacterial endocarditis; they produce adhesins that
bind to platelet-fibrin aggregates on damaged heart valves, facilitating
colonization.
10. In a lab experiment, a student exposes bacterial cultures to UV light
for varying durations; survival curves show a sharp decline in viability
with increased exposure; which DNA repair mechanism is MOST
important for bacterial recovery from UV-induced damage?
A) Mismatch repair
B) Nucleotide excision repair (light-dependent photolyase or light-
independent UvrABC)
C) Base excision repair
D) Homologous recombination
B UV light causes thymine dimers that distort DNA; nucleotide excision
repair removes the damaged segment—photolyase uses visible light to
directly reverse dimers (photoreactivation), while UvrABC excises and
replaces the damaged nucleotides in the dark; mismatch repair corrects
replication errors, not UV damage.
11. A microbiology student performs an oxidase test on a Gram-negative
rod isolated from a wound; the filter paper turns purple within 10
seconds; which genus is MOST consistent with this result?
A) Escherichia
B) Pseudomonas
C) Klebsiella
D) Salmonella
B Pseudomonas aeruginosa is oxidase-positive (produces cytochrome c
oxidase), while Enterobacteriaceae (E. coli, Klebsiella, Salmonella) are
oxidase-negative; the oxidase test helps differentiate Pseudomonas
from common enteric Gram-negative rods.
