BIOL 3200 Exam 4 Actual Exam V1 | BIOL 3200 General Microbiology
(BIOL 3200 Exam 4) | Auburn University
1. Which of the following describes the mechanism of action for Penicillin?
A. Inhibition of protein synthesis at the 30S ribosome
B. Disruption of the cytoplasmic membrane integrity
C. Inhibition of peptidoglycan cross-linking via transpeptidase
D. Inhibition of DNA gyrase and topoisomerase
Answer: C
Rationale: Penicillin belongs to the beta-lactam class of antibiotics which target cell wall
synthesis. It specifically binds to penicillin-binding proteins to prevent the cross-linking of
glycan chains. This process results in a weakened cell wall that eventually leads to osmotic
lysis of the bacterium.
2. What is the primary difference between a disinfectant and an antiseptic?
A. Disinfectants target viruses, while antiseptics target bacteria exclusively
B. Disinfectants are always bactericidal, whereas antiseptics are only bacteriostatic
C. Antiseptics achieve sterilization, while disinfectants only reduce microbial load
D. Antiseptics are used on living tissue, while disinfectants are used on inanimate objects
Answer: D
Rationale: Antiseptics are formulated to be safe enough for application to skin or other
living tissues to prevent infection. Disinfectants are generally more toxic and are intended
for cleaning surfaces like countertops or medical instruments. Both agents aim to reduce
the number of microbes but do not necessarily guarantee sterilization.
3. A bacteria that can cause disease only when the host’s immune system is compromised is
known as a(n):
A. Opportunistic pathogen
B. Primary pathogen
C. Obligate pathogen
D. Commensal organism
Answer: A
,Rationale: Opportunistic pathogens are typically members of the normal flora or common
environmental microbes that do not harm healthy individuals. They take advantage of a
‘breach’ in host defenses, such as immunodeficiency or injury. An example is Pseudomonas
aeruginosa, which often infects burn victims or cystic fibrosis patients.
4. Which of the following parameters is standard for effective sterilization in an autoclave?
A. 121°C at 15 psi for 15-20 minutes
B. 100°C at 10 psi for 10 minutes
C. 160°C for 2 hours in dry heat
D. 72°C for 15 seconds (HTST)
Answer: A
Rationale: Autoclaving uses pressurized steam to achieve temperatures higher than the
boiling point of water. The standard setting of 121 degrees Celsius at 15 psi is sufficient to
kill even the most heat-resistant bacterial endospores. This method is considered the most
reliable form of moist heat sterilization in clinical and laboratory settings.
5. Endotoxins are associated with which component of the bacterial cell?
A. The Lipid A portion of Lipopolysaccharide (LPS)
B. The capsule of Gram-positive bacteria
C. Secreted proteins from the cytoplasm
D. The teichoic acids in the cell wall
Answer: A
Rationale: Endotoxins are integral parts of the outer membrane of Gram-negative bacteria,
specifically the Lipid A moiety of the LPS layer. They are released primarily when the
bacteria die and the cell wall breaks apart. The release of endotoxin into the bloodstream
can trigger a massive inflammatory response, potentially leading to septic shock.
6. Which antibiotic inhibits bacterial transcription by binding to RNA polymerase?
A. Ciprofloxacin
B. Rifampin
C. Tetracycline
D. Vancomycin
Answer: B
Rationale: Rifampin is a narrow-spectrum antibiotic often used to treat tuberculosis. It
functions by binding to the beta subunit of bacterial RNA polymerase, thereby blocking the
, initiation of RNA synthesis. Because eukaryotic RNA polymerase is structurally different,
the drug exhibits selective toxicity.
7. Resistance to which antibiotic is often mediated by the acquisition of the mecA gene in
Staphylococcus aureus?
A. Erythromycin
B. Linezolid
C. Gentamicin
D. Methicillin
Answer: D
Rationale: The mecA gene encodes an alternative penicillin-binding protein called PBP2a
which has a low affinity for beta-lactam antibiotics. This allows the bacteria to continue
synthesizing their cell wall even in the presence of methicillin or other related drugs.
Strains carrying this gene are referred to as MRSA, or Methicillin-resistant Staphylococcus
aureus.
8. The Minimum Inhibitory Concentration (MIC) is defined as:
A. The lowest concentration of a drug that prevents visible bacterial growth
B. The highest dose a patient can tolerate without side effects
C. The lowest concentration of a drug that kills 99.9% of bacteria
D. The concentration of a drug required to neutralize bacterial toxins
Answer: A
Rationale: The MIC test determines the susceptibility of a bacterial strain to a specific
antimicrobial agent. It is measured by observing the lowest dilution of the drug that results
in no visible turbidity in the culture medium. This value is critical for determining
appropriate clinical dosing to ensure the infection is controlled.
9. Which of the following is a classic example of an AB toxin?
A. Hemolysin
B. Superantigen
C. Lipid A
D. Cholera toxin
Answer: D
Rationale: AB toxins consist of two parts: the B subunit, which binds to a specific host cell
receptor, and the A subunit, which enters the cell and exerts a toxic effect. In the case of
(BIOL 3200 Exam 4) | Auburn University
1. Which of the following describes the mechanism of action for Penicillin?
A. Inhibition of protein synthesis at the 30S ribosome
B. Disruption of the cytoplasmic membrane integrity
C. Inhibition of peptidoglycan cross-linking via transpeptidase
D. Inhibition of DNA gyrase and topoisomerase
Answer: C
Rationale: Penicillin belongs to the beta-lactam class of antibiotics which target cell wall
synthesis. It specifically binds to penicillin-binding proteins to prevent the cross-linking of
glycan chains. This process results in a weakened cell wall that eventually leads to osmotic
lysis of the bacterium.
2. What is the primary difference between a disinfectant and an antiseptic?
A. Disinfectants target viruses, while antiseptics target bacteria exclusively
B. Disinfectants are always bactericidal, whereas antiseptics are only bacteriostatic
C. Antiseptics achieve sterilization, while disinfectants only reduce microbial load
D. Antiseptics are used on living tissue, while disinfectants are used on inanimate objects
Answer: D
Rationale: Antiseptics are formulated to be safe enough for application to skin or other
living tissues to prevent infection. Disinfectants are generally more toxic and are intended
for cleaning surfaces like countertops or medical instruments. Both agents aim to reduce
the number of microbes but do not necessarily guarantee sterilization.
3. A bacteria that can cause disease only when the host’s immune system is compromised is
known as a(n):
A. Opportunistic pathogen
B. Primary pathogen
C. Obligate pathogen
D. Commensal organism
Answer: A
,Rationale: Opportunistic pathogens are typically members of the normal flora or common
environmental microbes that do not harm healthy individuals. They take advantage of a
‘breach’ in host defenses, such as immunodeficiency or injury. An example is Pseudomonas
aeruginosa, which often infects burn victims or cystic fibrosis patients.
4. Which of the following parameters is standard for effective sterilization in an autoclave?
A. 121°C at 15 psi for 15-20 minutes
B. 100°C at 10 psi for 10 minutes
C. 160°C for 2 hours in dry heat
D. 72°C for 15 seconds (HTST)
Answer: A
Rationale: Autoclaving uses pressurized steam to achieve temperatures higher than the
boiling point of water. The standard setting of 121 degrees Celsius at 15 psi is sufficient to
kill even the most heat-resistant bacterial endospores. This method is considered the most
reliable form of moist heat sterilization in clinical and laboratory settings.
5. Endotoxins are associated with which component of the bacterial cell?
A. The Lipid A portion of Lipopolysaccharide (LPS)
B. The capsule of Gram-positive bacteria
C. Secreted proteins from the cytoplasm
D. The teichoic acids in the cell wall
Answer: A
Rationale: Endotoxins are integral parts of the outer membrane of Gram-negative bacteria,
specifically the Lipid A moiety of the LPS layer. They are released primarily when the
bacteria die and the cell wall breaks apart. The release of endotoxin into the bloodstream
can trigger a massive inflammatory response, potentially leading to septic shock.
6. Which antibiotic inhibits bacterial transcription by binding to RNA polymerase?
A. Ciprofloxacin
B. Rifampin
C. Tetracycline
D. Vancomycin
Answer: B
Rationale: Rifampin is a narrow-spectrum antibiotic often used to treat tuberculosis. It
functions by binding to the beta subunit of bacterial RNA polymerase, thereby blocking the
, initiation of RNA synthesis. Because eukaryotic RNA polymerase is structurally different,
the drug exhibits selective toxicity.
7. Resistance to which antibiotic is often mediated by the acquisition of the mecA gene in
Staphylococcus aureus?
A. Erythromycin
B. Linezolid
C. Gentamicin
D. Methicillin
Answer: D
Rationale: The mecA gene encodes an alternative penicillin-binding protein called PBP2a
which has a low affinity for beta-lactam antibiotics. This allows the bacteria to continue
synthesizing their cell wall even in the presence of methicillin or other related drugs.
Strains carrying this gene are referred to as MRSA, or Methicillin-resistant Staphylococcus
aureus.
8. The Minimum Inhibitory Concentration (MIC) is defined as:
A. The lowest concentration of a drug that prevents visible bacterial growth
B. The highest dose a patient can tolerate without side effects
C. The lowest concentration of a drug that kills 99.9% of bacteria
D. The concentration of a drug required to neutralize bacterial toxins
Answer: A
Rationale: The MIC test determines the susceptibility of a bacterial strain to a specific
antimicrobial agent. It is measured by observing the lowest dilution of the drug that results
in no visible turbidity in the culture medium. This value is critical for determining
appropriate clinical dosing to ensure the infection is controlled.
9. Which of the following is a classic example of an AB toxin?
A. Hemolysin
B. Superantigen
C. Lipid A
D. Cholera toxin
Answer: D
Rationale: AB toxins consist of two parts: the B subunit, which binds to a specific host cell
receptor, and the A subunit, which enters the cell and exerts a toxic effect. In the case of
