1
BIO 171 FINAL EXAM 2026/2027 | Microbiology | 100%
Correct Answers | Latest Update | Portage Learning | Pass
Guaranteed - A+ Graded
Section 1: Microbial Cell Structure & Function (Q1-12)
Q1. A clinical microbiologist performs a Gram stain on a bacterial sample from a
patient with pneumonia. After the decolorization step, the bacteria appear purple.
Which structural component is responsible for retaining the crystal violet-iodine
complex?
A. Lipopolysaccharide in the outer membrane
B. Peptidoglycan layer with teichoic acids
C. Periplasmic space containing enzymes
D. Lipid bilayer of the plasma membrane
Correct Answer: B
Rationale: Gram-positive bacteria retain crystal violet because their thick
peptidoglycan layer (20-80 nm) with embedded teichoic acids traps the crystal violet-
iodine complex during decolorization with alcohol/acetone. The alcohol dehydrates
the thick peptidoglycan, shrinking pores and preventing dye escape. Option A (LPS)
is found in Gram-negative outer membranes, not Gram-positive. Option C
(periplasmic space) is more prominent in Gram-negative bacteria. Option D (plasma
membrane lipid bilayer) is present in all cells but does not retain Gram stain.
[CORRECT]
Q2. A student observes bacterial cells under an electron microscope and notes the
absence of a membrane-bound nucleus, mitochondria, and endoplasmic reticulum.
However, the cells contain 70S ribosomes and a nucleoid region. Which cell type is
being described?
A. Eukaryotic fungal cell
B. Prokaryotic bacterial cell
,2
C. Eukaryotic protozoan cell
D. Viral particle
Correct Answer: B
Rationale: The absence of membrane-bound organelles (nucleus, mitochondria, ER)
combined with 70S ribosomes (50S + 30S subunits) and a nucleoid (non-membrane-
bound DNA region) are definitive characteristics of prokaryotic cells. Option A is
incorrect because fungi have 80S ribosomes and membrane-bound organelles.
Option C is incorrect because protozoa are eukaryotic with complex organelles.
Option D is incorrect because viruses lack ribosomes entirely and are not cells.
[CORRECT]
Q3. During Gram staining of a mixed bacterial culture, some cells appear pink/red
while others remain purple. Which step in the Gram stain procedure is MOST critical
for this differential outcome?
A. Primary staining with crystal violet
B. Mordant application with iodine
C. Decolorization with alcohol or acetone
D. Counterstaining with safranin
Correct Answer: C
Rationale: The decolorization step is the critical differential step in Gram staining.
Gram-negative bacteria have a thin peptidoglycan layer (2-7 nm) surrounded by an
outer membrane containing LPS; the alcohol dissolves the outer membrane and
washes the crystal violet-iodine complex from the thin peptidoglycan, leaving them
colorless until counterstained pink with safranin. Gram-positive bacteria retain the
complex due to their thick peptidoglycan. Option A (crystal violet) stains all cells
purple initially. Option B (iodine) fixes the dye but does not differentiate. Option D
(safranin) only colors previously decolorized cells. [CORRECT]
,3
Q4. A bacterium isolated from a deep wound is found to be resistant to heat,
radiation, and desiccation. Microscopic examination reveals thick-walled, dormant
structures. Which cellular structure allows this survival mechanism?
A. Capsule
B. Endospore
C. Plasmid
D. Inclusion body
Correct Answer: B
Rationale: Endospores are highly resistant, dormant structures formed by certain
Gram-positive bacteria (Bacillus, Clostridium) during nutrient deprivation. They
contain dipicolinic acid and small acid-soluble proteins (SASPs) that protect DNA,
and their thick peptidoglycan cortex and protein coats provide resistance to heat,
radiation, chemicals, and desiccation for years or decades. Option A (capsule)
provides protection against phagocytosis but not extreme heat. Option C (plasmid)
carries accessory genes but provides no physical resistance. Option D (inclusion
body) stores nutrients, not resistance structures. [CORRECT]
Q5. A microbiologist is comparing two bacterial species. Species A has a thick
peptidoglycan layer with teichoic acids and no outer membrane. Species B has a thin
peptidoglycan layer, an outer membrane with LPS, and a periplasmic space. Which
statement correctly describes Species B?
A. Species B is Gram-positive and contains endotoxin in its cell wall
B. Species B is Gram-negative and contains endotoxin in its outer membrane
C. Species B is Gram-positive and lacks endotoxin entirely
D. Species B is Gram-negative and contains exotoxin in its outer membrane
Correct Answer: B
Rationale: Species B exhibits classic Gram-negative characteristics: thin
peptidoglycan, outer membrane with lipopolysaccharide (LPS), and periplasmic
space. The LPS in the outer membrane serves as endotoxin, released when the cell
lyses, causing fever, shock, and inflammation via TLR4 activation. Option A incorrectly
identifies Gram-positive structure. Option C incorrectly identifies Gram-positive and
, 4
denies endotoxin presence. Option D confuses endotoxin (LPS, integral to outer
membrane) with exotoxin (secreted proteins). [CORRECT]
Q6. A bacterial cell is placed in a hypertonic solution containing 20% NaCl. What will
happen to the cell, and which structural component primarily prevents excessive
water loss in some bacteria?
A. The cell will burst; the cell wall prevents lysis
B. The cell will undergo plasmolysis; the capsule helps retain water
C. The cell will undergo plasmolysis; the cell wall prevents complete collapse
D. The cell will remain unchanged; the plasma membrane regulates all osmotic stress
Correct Answer: C
Rationale: In a hypertonic solution, water moves out of the bacterial cell by osmosis,
causing the plasma membrane to pull away from the cell wall (plasmolysis). The rigid
cell wall (peptidoglycan) prevents complete cellular collapse and rupture, though
metabolic activity ceases. Option A is incorrect because cells shrink, not burst, in
hypertonic solutions. Option B incorrectly attributes water retention to the capsule
(which prevents desiccation but not plasmolysis). Option D is incorrect because
hypertonic environments always affect cells; the cell wall is essential for structural
integrity. [CORRECT]
Q7. A student is examining bacterial morphology and observes spherical cells
arranged in grape-like clusters. Which term describes this arrangement, and which
bacterial genus is a classic example?
A. Streptococcus; Streptococcus pyogenes
B. Staphylococcus; Staphylococcus aureus
C. Diplococcus; Neisseria meningitidis
D. Sarcinae; Micrococcus luteus
Correct Answer: B
Rationale: Staphylococcus (from Greek "staphyle" meaning bunch of grapes)
BIO 171 FINAL EXAM 2026/2027 | Microbiology | 100%
Correct Answers | Latest Update | Portage Learning | Pass
Guaranteed - A+ Graded
Section 1: Microbial Cell Structure & Function (Q1-12)
Q1. A clinical microbiologist performs a Gram stain on a bacterial sample from a
patient with pneumonia. After the decolorization step, the bacteria appear purple.
Which structural component is responsible for retaining the crystal violet-iodine
complex?
A. Lipopolysaccharide in the outer membrane
B. Peptidoglycan layer with teichoic acids
C. Periplasmic space containing enzymes
D. Lipid bilayer of the plasma membrane
Correct Answer: B
Rationale: Gram-positive bacteria retain crystal violet because their thick
peptidoglycan layer (20-80 nm) with embedded teichoic acids traps the crystal violet-
iodine complex during decolorization with alcohol/acetone. The alcohol dehydrates
the thick peptidoglycan, shrinking pores and preventing dye escape. Option A (LPS)
is found in Gram-negative outer membranes, not Gram-positive. Option C
(periplasmic space) is more prominent in Gram-negative bacteria. Option D (plasma
membrane lipid bilayer) is present in all cells but does not retain Gram stain.
[CORRECT]
Q2. A student observes bacterial cells under an electron microscope and notes the
absence of a membrane-bound nucleus, mitochondria, and endoplasmic reticulum.
However, the cells contain 70S ribosomes and a nucleoid region. Which cell type is
being described?
A. Eukaryotic fungal cell
B. Prokaryotic bacterial cell
,2
C. Eukaryotic protozoan cell
D. Viral particle
Correct Answer: B
Rationale: The absence of membrane-bound organelles (nucleus, mitochondria, ER)
combined with 70S ribosomes (50S + 30S subunits) and a nucleoid (non-membrane-
bound DNA region) are definitive characteristics of prokaryotic cells. Option A is
incorrect because fungi have 80S ribosomes and membrane-bound organelles.
Option C is incorrect because protozoa are eukaryotic with complex organelles.
Option D is incorrect because viruses lack ribosomes entirely and are not cells.
[CORRECT]
Q3. During Gram staining of a mixed bacterial culture, some cells appear pink/red
while others remain purple. Which step in the Gram stain procedure is MOST critical
for this differential outcome?
A. Primary staining with crystal violet
B. Mordant application with iodine
C. Decolorization with alcohol or acetone
D. Counterstaining with safranin
Correct Answer: C
Rationale: The decolorization step is the critical differential step in Gram staining.
Gram-negative bacteria have a thin peptidoglycan layer (2-7 nm) surrounded by an
outer membrane containing LPS; the alcohol dissolves the outer membrane and
washes the crystal violet-iodine complex from the thin peptidoglycan, leaving them
colorless until counterstained pink with safranin. Gram-positive bacteria retain the
complex due to their thick peptidoglycan. Option A (crystal violet) stains all cells
purple initially. Option B (iodine) fixes the dye but does not differentiate. Option D
(safranin) only colors previously decolorized cells. [CORRECT]
,3
Q4. A bacterium isolated from a deep wound is found to be resistant to heat,
radiation, and desiccation. Microscopic examination reveals thick-walled, dormant
structures. Which cellular structure allows this survival mechanism?
A. Capsule
B. Endospore
C. Plasmid
D. Inclusion body
Correct Answer: B
Rationale: Endospores are highly resistant, dormant structures formed by certain
Gram-positive bacteria (Bacillus, Clostridium) during nutrient deprivation. They
contain dipicolinic acid and small acid-soluble proteins (SASPs) that protect DNA,
and their thick peptidoglycan cortex and protein coats provide resistance to heat,
radiation, chemicals, and desiccation for years or decades. Option A (capsule)
provides protection against phagocytosis but not extreme heat. Option C (plasmid)
carries accessory genes but provides no physical resistance. Option D (inclusion
body) stores nutrients, not resistance structures. [CORRECT]
Q5. A microbiologist is comparing two bacterial species. Species A has a thick
peptidoglycan layer with teichoic acids and no outer membrane. Species B has a thin
peptidoglycan layer, an outer membrane with LPS, and a periplasmic space. Which
statement correctly describes Species B?
A. Species B is Gram-positive and contains endotoxin in its cell wall
B. Species B is Gram-negative and contains endotoxin in its outer membrane
C. Species B is Gram-positive and lacks endotoxin entirely
D. Species B is Gram-negative and contains exotoxin in its outer membrane
Correct Answer: B
Rationale: Species B exhibits classic Gram-negative characteristics: thin
peptidoglycan, outer membrane with lipopolysaccharide (LPS), and periplasmic
space. The LPS in the outer membrane serves as endotoxin, released when the cell
lyses, causing fever, shock, and inflammation via TLR4 activation. Option A incorrectly
identifies Gram-positive structure. Option C incorrectly identifies Gram-positive and
, 4
denies endotoxin presence. Option D confuses endotoxin (LPS, integral to outer
membrane) with exotoxin (secreted proteins). [CORRECT]
Q6. A bacterial cell is placed in a hypertonic solution containing 20% NaCl. What will
happen to the cell, and which structural component primarily prevents excessive
water loss in some bacteria?
A. The cell will burst; the cell wall prevents lysis
B. The cell will undergo plasmolysis; the capsule helps retain water
C. The cell will undergo plasmolysis; the cell wall prevents complete collapse
D. The cell will remain unchanged; the plasma membrane regulates all osmotic stress
Correct Answer: C
Rationale: In a hypertonic solution, water moves out of the bacterial cell by osmosis,
causing the plasma membrane to pull away from the cell wall (plasmolysis). The rigid
cell wall (peptidoglycan) prevents complete cellular collapse and rupture, though
metabolic activity ceases. Option A is incorrect because cells shrink, not burst, in
hypertonic solutions. Option B incorrectly attributes water retention to the capsule
(which prevents desiccation but not plasmolysis). Option D is incorrect because
hypertonic environments always affect cells; the cell wall is essential for structural
integrity. [CORRECT]
Q7. A student is examining bacterial morphology and observes spherical cells
arranged in grape-like clusters. Which term describes this arrangement, and which
bacterial genus is a classic example?
A. Streptococcus; Streptococcus pyogenes
B. Staphylococcus; Staphylococcus aureus
C. Diplococcus; Neisseria meningitidis
D. Sarcinae; Micrococcus luteus
Correct Answer: B
Rationale: Staphylococcus (from Greek "staphyle" meaning bunch of grapes)
