BIO 250 Final Exam V2 | BIO 250
Microbiology | Actual Q&A with Rationale
(BIO250 Final Exam) | StraighterLine
1. Which of the following structures are essential components found in all bacterial cells?
Select all that apply.
A. Cytoplasmic membrane
B. Ribosomes
C. Nucleoid
D. Cytoplasm
E. Capsule
F. Flagella
Answer: A, B, C, D
Rationale: All bacteria must possess a cytoplasmic membrane, ribosomes for protein
synthesis, a nucleoid for genetic material, and a cytoplasm to facilitate biochemical
reactions. Capsules and flagella are considered virulence factors or accessory structures
that are not present in every bacterial species. Identifying these core components is
fundamental to understanding prokaryotic cellular physiology.
2. In the Gram staining procedure, what is the specific role of Gram’s iodine?
A. Mordant
,B. Primary stain
C. Decolorizer
D. Counterstain
Answer: A
Rationale: Gram’s iodine acts as a mordant that forms a complex with crystal violet inside
the peptidoglycan layer. This complex is larger than the individual dye molecules, making it
harder to wash out of thick cell walls. Without this step, the primary stain would be easily
removed from both Gram-positive and Gram-negative cells during decolorization.
3. Which phase of the bacterial growth curve is characterized by a period where the rate of
cell division equals the rate of cell death?
A. Lag phase
B. Stationary phase
C. Log phase
D. Death phase
Answer: B
Rationale: The stationary phase occurs when nutrient depletion and waste accumulation
cause the growth rate to slow down until it matches the death rate. During this phase, the
total number of viable cells remains constant. This stage is often when secondary
metabolites, such as antibiotics, are produced by certain bacteria.
, 4. An organism that requires oxygen for metabolism but is inhibited by high concentrations of
oxygen is classified as a(n):
A. Obligate aerobe
B. Facultative anaerobe
C. Aerotolerant anaerobe
D. Microaerophile
Answer: D
Rationale: Microaerophiles require oxygen levels lower than atmospheric concentrations
(typically 2-10%) to grow optimally. They possess oxygen-sensitive enzymes or limited
antioxidant defenses that make high oxygen levels toxic. Examples of such organisms
include Campylobacter and Helicobacter species.
5. What is the standard temperature and pressure for sterilization in an autoclave?
A. 100°C at 10 psi for 30 minutes
B. 132°C at 20 psi for 5 minutes
C. 121°C at 15 psi for 15 minutes
D. 115°C at 10 psi for 20 minutes
Answer: C
Rationale: The standard autoclave cycle of 121°C at 15 psi for 15 minutes is designed to
destroy the most heat-resistant microbial forms, such as endospores. Increased pressure
Microbiology | Actual Q&A with Rationale
(BIO250 Final Exam) | StraighterLine
1. Which of the following structures are essential components found in all bacterial cells?
Select all that apply.
A. Cytoplasmic membrane
B. Ribosomes
C. Nucleoid
D. Cytoplasm
E. Capsule
F. Flagella
Answer: A, B, C, D
Rationale: All bacteria must possess a cytoplasmic membrane, ribosomes for protein
synthesis, a nucleoid for genetic material, and a cytoplasm to facilitate biochemical
reactions. Capsules and flagella are considered virulence factors or accessory structures
that are not present in every bacterial species. Identifying these core components is
fundamental to understanding prokaryotic cellular physiology.
2. In the Gram staining procedure, what is the specific role of Gram’s iodine?
A. Mordant
,B. Primary stain
C. Decolorizer
D. Counterstain
Answer: A
Rationale: Gram’s iodine acts as a mordant that forms a complex with crystal violet inside
the peptidoglycan layer. This complex is larger than the individual dye molecules, making it
harder to wash out of thick cell walls. Without this step, the primary stain would be easily
removed from both Gram-positive and Gram-negative cells during decolorization.
3. Which phase of the bacterial growth curve is characterized by a period where the rate of
cell division equals the rate of cell death?
A. Lag phase
B. Stationary phase
C. Log phase
D. Death phase
Answer: B
Rationale: The stationary phase occurs when nutrient depletion and waste accumulation
cause the growth rate to slow down until it matches the death rate. During this phase, the
total number of viable cells remains constant. This stage is often when secondary
metabolites, such as antibiotics, are produced by certain bacteria.
, 4. An organism that requires oxygen for metabolism but is inhibited by high concentrations of
oxygen is classified as a(n):
A. Obligate aerobe
B. Facultative anaerobe
C. Aerotolerant anaerobe
D. Microaerophile
Answer: D
Rationale: Microaerophiles require oxygen levels lower than atmospheric concentrations
(typically 2-10%) to grow optimally. They possess oxygen-sensitive enzymes or limited
antioxidant defenses that make high oxygen levels toxic. Examples of such organisms
include Campylobacter and Helicobacter species.
5. What is the standard temperature and pressure for sterilization in an autoclave?
A. 100°C at 10 psi for 30 minutes
B. 132°C at 20 psi for 5 minutes
C. 121°C at 15 psi for 15 minutes
D. 115°C at 10 psi for 20 minutes
Answer: C
Rationale: The standard autoclave cycle of 121°C at 15 psi for 15 minutes is designed to
destroy the most heat-resistant microbial forms, such as endospores. Increased pressure
