BIO 242: Microbiology Examination 2
Course Code: BIO 242 | Total Questions: 75 | Verified Solutions and Detailed Explanations
Section 1: Bacterial Structure, Cell Walls, and Staining
1. Which cellular component is found uniquely in the cell walls of Gram-positive bacteria
and provides antigenic specificity?
A. Lipopolysaccharide (LPS)
B. Teichoic acids
C. Porin proteins
D. Mycolic acid
Correct Answer: B. Teichoic acids
Rationale: Teichoic acids and lipoteichoic acids are polyol phosphate polymers embedded
in the thick peptidoglycan wall of Gram-positive bacteria. Lipopolysaccharide (LPS) is unique
to the outer membrane of Gram-negative bacteria, and mycolic acid is unique to acid-fast
bacteria.
2. During Gram staining, what is the critical role of 95% ethyl alcohol?
A. Primary stain fixative
B. Decolorizer that dissolves lipid-rich outer membranes
C. Counterstain that turns cells pink
D. Mordant that forms insoluble complexes with crystal violet
Correct Answer: B. Decolorizer that dissolves lipid-rich outer membranes
Rationale: Alcohol acts as the decolorizing agent. In Gram-negative cells, it dissolves the
outer lipid membrane and leaves thin peptidoglycan porous, allowing Crystal Violet-Iodine
complexes to wash out. In Gram-positive cells, it dehydrates thick peptidoglycan, trapping
the dye.
3. What chemical lipid component renders Mycobacterium tuberculosis resistant to
standard Gram staining, requiring an Acid-Fast stain instead?
A. Dipicolinic acid
B. Mycolic acid
C. Lipid A
D. Peptidoglycan cross-links
Correct Answer: B. Mycolic acid
, Rationale: Acid-fast organisms like Mycobacterium possess waxy cell walls rich in mycolic
acids (long-chain fatty acids) that resist penetration by water-soluble dyes like crystal violet.
Carbolfuchsin with heat/penetrant is required.
4. Bacterial endospores exhibit extreme resistance to heat, radiation, and desiccation
primarily due to high concentrations of which chemical compound in their core?
A. Calcium dipicolinate
B. Poly-beta-hydroxybutyrate
C. N-acetylmuramic acid
D. D-alanine dimer
Correct Answer: A. Calcium dipicolinate
Rationale: Endospores contain high levels of dipicolinic acid complexed with calcium ions
($Ca^{2+}$-DPA), which dehydrates the spore core, stabilizes DNA and proteins, and
confers heat/chemical resistance.
5. Which structure enables bacteria to exchange plasmid DNA directly between cells via
conjugation?
A. Flagellum
B. Fimbriae
C. Sex pilus (F-pilus)
D. Capsule
Correct Answer: C. Sex pilus (F-pilus)
Rationale: The sex pilus, encoded by the F plasmid in donor ($F^+$) cells, forms a
conjugation bridge attaching to a recipient ($F^-$) cell to facilitate horizontal transfer of
single-stranded plasmid DNA.
6. What is the primary functional difference between bacterial fimbriae and flagella?
A. Fimbriae drive rotary motility; flagella mediate attachment
B. Fimbriae facilitate surface attachment; flagella drive locomotion
C. Fimbriae transfer genetic material; flagella store nutrients
D. Fimbriae protect against phagocytosis; flagella synthesize ATP
Correct Answer: B. Fimbriae facilitate surface attachment; flagella drive locomotion
Rationale: Fimbriae are short hair-like protein appendages used for adhering to host
epithelial surfaces or biofilms. Flagella are long helical protein filaments that rotate to drive
bacterial chemotaxis and motility.
, 7. Penicillin selectively inhibits bacterial growth by targeting which specific enzyme
involved in peptidoglycan synthesis?
A. DNA gyrase
B. Transpeptidase (Penicillin-Binding Protein)
C. RNA polymerase
D. Topoisomerase IV
Correct Answer: B. Transpeptidase (Penicillin-Binding Protein)
Rationale: Beta-lactam antibiotics like penicillin structurally mimic D-Ala-D-Ala and
irreversibly bind to transpeptidase enzymes, preventing peptide cross-linking of NAM
subunits and causing cell lysis under osmotic pressure.
8. Which bacterial cell wall subunit molecule contains a pentapeptide chain attached to
its sugar backbone?
A. N-acetylglucosamine (NAG)
B. N-acetylmuramic acid (NAM)
C. Glycerol phosphate
D. Lipid A
Correct Answer: B. N-acetylmuramic acid (NAM)
Rationale: Peptidoglycan consists of alternating NAG and NAM glycan chains. The short
peptide chain (tetrapeptide/pentapeptide) is attached specifically to the lactic acid moiety of
NAM.
9. Bacteria that lack cell walls entirely and are membrane-stabilized by sterols belong to
which genus?
A. Chlamydia
B. Mycoplasma
C. Rickettsia
D. Treponema
Correct Answer: B. Mycoplasma
Rationale: Mycoplasma species (e.g., Mycoplasma pneumoniae) naturally lack a
peptidoglycan cell wall and incorporate host sterols into their cell membrane for structural
rigidity, making them intrinsically resistant to beta-lactam antibiotics.
Course Code: BIO 242 | Total Questions: 75 | Verified Solutions and Detailed Explanations
Section 1: Bacterial Structure, Cell Walls, and Staining
1. Which cellular component is found uniquely in the cell walls of Gram-positive bacteria
and provides antigenic specificity?
A. Lipopolysaccharide (LPS)
B. Teichoic acids
C. Porin proteins
D. Mycolic acid
Correct Answer: B. Teichoic acids
Rationale: Teichoic acids and lipoteichoic acids are polyol phosphate polymers embedded
in the thick peptidoglycan wall of Gram-positive bacteria. Lipopolysaccharide (LPS) is unique
to the outer membrane of Gram-negative bacteria, and mycolic acid is unique to acid-fast
bacteria.
2. During Gram staining, what is the critical role of 95% ethyl alcohol?
A. Primary stain fixative
B. Decolorizer that dissolves lipid-rich outer membranes
C. Counterstain that turns cells pink
D. Mordant that forms insoluble complexes with crystal violet
Correct Answer: B. Decolorizer that dissolves lipid-rich outer membranes
Rationale: Alcohol acts as the decolorizing agent. In Gram-negative cells, it dissolves the
outer lipid membrane and leaves thin peptidoglycan porous, allowing Crystal Violet-Iodine
complexes to wash out. In Gram-positive cells, it dehydrates thick peptidoglycan, trapping
the dye.
3. What chemical lipid component renders Mycobacterium tuberculosis resistant to
standard Gram staining, requiring an Acid-Fast stain instead?
A. Dipicolinic acid
B. Mycolic acid
C. Lipid A
D. Peptidoglycan cross-links
Correct Answer: B. Mycolic acid
, Rationale: Acid-fast organisms like Mycobacterium possess waxy cell walls rich in mycolic
acids (long-chain fatty acids) that resist penetration by water-soluble dyes like crystal violet.
Carbolfuchsin with heat/penetrant is required.
4. Bacterial endospores exhibit extreme resistance to heat, radiation, and desiccation
primarily due to high concentrations of which chemical compound in their core?
A. Calcium dipicolinate
B. Poly-beta-hydroxybutyrate
C. N-acetylmuramic acid
D. D-alanine dimer
Correct Answer: A. Calcium dipicolinate
Rationale: Endospores contain high levels of dipicolinic acid complexed with calcium ions
($Ca^{2+}$-DPA), which dehydrates the spore core, stabilizes DNA and proteins, and
confers heat/chemical resistance.
5. Which structure enables bacteria to exchange plasmid DNA directly between cells via
conjugation?
A. Flagellum
B. Fimbriae
C. Sex pilus (F-pilus)
D. Capsule
Correct Answer: C. Sex pilus (F-pilus)
Rationale: The sex pilus, encoded by the F plasmid in donor ($F^+$) cells, forms a
conjugation bridge attaching to a recipient ($F^-$) cell to facilitate horizontal transfer of
single-stranded plasmid DNA.
6. What is the primary functional difference between bacterial fimbriae and flagella?
A. Fimbriae drive rotary motility; flagella mediate attachment
B. Fimbriae facilitate surface attachment; flagella drive locomotion
C. Fimbriae transfer genetic material; flagella store nutrients
D. Fimbriae protect against phagocytosis; flagella synthesize ATP
Correct Answer: B. Fimbriae facilitate surface attachment; flagella drive locomotion
Rationale: Fimbriae are short hair-like protein appendages used for adhering to host
epithelial surfaces or biofilms. Flagella are long helical protein filaments that rotate to drive
bacterial chemotaxis and motility.
, 7. Penicillin selectively inhibits bacterial growth by targeting which specific enzyme
involved in peptidoglycan synthesis?
A. DNA gyrase
B. Transpeptidase (Penicillin-Binding Protein)
C. RNA polymerase
D. Topoisomerase IV
Correct Answer: B. Transpeptidase (Penicillin-Binding Protein)
Rationale: Beta-lactam antibiotics like penicillin structurally mimic D-Ala-D-Ala and
irreversibly bind to transpeptidase enzymes, preventing peptide cross-linking of NAM
subunits and causing cell lysis under osmotic pressure.
8. Which bacterial cell wall subunit molecule contains a pentapeptide chain attached to
its sugar backbone?
A. N-acetylglucosamine (NAG)
B. N-acetylmuramic acid (NAM)
C. Glycerol phosphate
D. Lipid A
Correct Answer: B. N-acetylmuramic acid (NAM)
Rationale: Peptidoglycan consists of alternating NAG and NAM glycan chains. The short
peptide chain (tetrapeptide/pentapeptide) is attached specifically to the lactic acid moiety of
NAM.
9. Bacteria that lack cell walls entirely and are membrane-stabilized by sterols belong to
which genus?
A. Chlamydia
B. Mycoplasma
C. Rickettsia
D. Treponema
Correct Answer: B. Mycoplasma
Rationale: Mycoplasma species (e.g., Mycoplasma pneumoniae) naturally lack a
peptidoglycan cell wall and incorporate host sterols into their cell membrane for structural
rigidity, making them intrinsically resistant to beta-lactam antibiotics.
