LA County Public Health Microbiologist
Written Exam Questions And Correct
Answers (Verified Answers) Plus
Rationales 2026-2027 Q&A | Instant
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1. Which of the following is the most effective method for sterilizing
heat-resistant laboratory glassware?
A. Boiling in water for 15 minutes
B. Autoclaving at 121°C for 15 minutes
C. Wiping with 70% ethanol
D. Exposing to UV light for 10 minutes
Autoclaving uses pressurized steam to destroy all microorganisms,
including spores, making it the most effective method for sterilization.
2. The Gram stain differentiates bacteria based on:
A. Motility
B. Cell wall structure
C. Shape
D. Spore formation
Gram-positive bacteria have a thick peptidoglycan layer, retaining
crystal violet, while Gram-negative bacteria do not.
, 3. Which medium is selective for Gram-negative enteric bacteria?
A. Blood agar
B. Mannitol salt agar
C. MacConkey agar
D. Chocolate agar
MacConkey agar contains bile salts and crystal violet, which inhibit
Gram-positive bacteria, allowing Gram-negative bacteria to grow.
4. A culture shows beta-hemolysis on blood agar. This indicates:
A. No hemolysis
B. Partial hemolysis
C. Complete hemolysis
D. Growth inhibition
Beta-hemolysis refers to complete lysis of red blood cells around
colonies, producing a clear zone.
5. The incubation temperature for most pathogenic bacteria in the
lab is:
A. 25°C
B. 30°C
C. 37°C
D. 42°C
Most human pathogens grow optimally at body temperature (37°C).
6. Which of the following is a common symptom of Salmonella
infection?
A. Jaundice
B. Diarrhea
, C. Skin rash
D. Hair loss
Salmonella primarily causes gastroenteritis, leading to diarrhea,
abdominal cramps, and fever.
7. Which method is used to quantify bacteria in a liquid culture?
A. Gram stain
B. Serial dilution and plate count
C. Catalase test
D. Coagulase test
Serial dilution followed by plating allows counting of colony-forming
units to estimate bacterial concentration.
8. The purpose of using a Bunsen burner flame in microbiology is to:
A. Sterilize growth media
B. Create an aseptic work area
C. Heat cultures
D. Kill spores
The flame produces an updraft that reduces contamination by airborne
microbes around the workspace.
9. Which of the following viruses is known for causing hemorrhagic
fever?
A. Influenza virus
B. Ebola virus
C. Rhinovirus
D. Adenovirus
Ebola virus infection leads to severe hemorrhagic fever and high
mortality.
, 10. PCR is used to:
A. Measure antibiotic susceptibility
B. Amplify DNA sequences
C. Detect RNA viruses only
D. Stain bacteria
Polymerase chain reaction (PCR) amplifies specific DNA sequences for
detection or research purposes.
11. Which of the following is considered a biosafety level 3 (BSL-
3) pathogen?
A. Escherichia coli K-12
B. Mycobacterium tuberculosis
C. Lactobacillus species
D. Saccharomyces cerevisiae
BSL-3 pathogens can cause serious or potentially lethal disease via
inhalation and require specialized containment.
12. In a Kirby-Bauer disk diffusion test, the size of the inhibition
zone indicates:
A. Growth rate of bacteria
B. Sensitivity to antibiotics
C. Gram stain reaction
D. Spore formation
Larger inhibition zones indicate greater susceptibility of bacteria to the
tested antibiotic.
13. A positive catalase test produces:
A. Bubbling when hydrogen peroxide is added
B. No reaction
Written Exam Questions And Correct
Answers (Verified Answers) Plus
Rationales 2026-2027 Q&A | Instant
Download Pdf
1. Which of the following is the most effective method for sterilizing
heat-resistant laboratory glassware?
A. Boiling in water for 15 minutes
B. Autoclaving at 121°C for 15 minutes
C. Wiping with 70% ethanol
D. Exposing to UV light for 10 minutes
Autoclaving uses pressurized steam to destroy all microorganisms,
including spores, making it the most effective method for sterilization.
2. The Gram stain differentiates bacteria based on:
A. Motility
B. Cell wall structure
C. Shape
D. Spore formation
Gram-positive bacteria have a thick peptidoglycan layer, retaining
crystal violet, while Gram-negative bacteria do not.
, 3. Which medium is selective for Gram-negative enteric bacteria?
A. Blood agar
B. Mannitol salt agar
C. MacConkey agar
D. Chocolate agar
MacConkey agar contains bile salts and crystal violet, which inhibit
Gram-positive bacteria, allowing Gram-negative bacteria to grow.
4. A culture shows beta-hemolysis on blood agar. This indicates:
A. No hemolysis
B. Partial hemolysis
C. Complete hemolysis
D. Growth inhibition
Beta-hemolysis refers to complete lysis of red blood cells around
colonies, producing a clear zone.
5. The incubation temperature for most pathogenic bacteria in the
lab is:
A. 25°C
B. 30°C
C. 37°C
D. 42°C
Most human pathogens grow optimally at body temperature (37°C).
6. Which of the following is a common symptom of Salmonella
infection?
A. Jaundice
B. Diarrhea
, C. Skin rash
D. Hair loss
Salmonella primarily causes gastroenteritis, leading to diarrhea,
abdominal cramps, and fever.
7. Which method is used to quantify bacteria in a liquid culture?
A. Gram stain
B. Serial dilution and plate count
C. Catalase test
D. Coagulase test
Serial dilution followed by plating allows counting of colony-forming
units to estimate bacterial concentration.
8. The purpose of using a Bunsen burner flame in microbiology is to:
A. Sterilize growth media
B. Create an aseptic work area
C. Heat cultures
D. Kill spores
The flame produces an updraft that reduces contamination by airborne
microbes around the workspace.
9. Which of the following viruses is known for causing hemorrhagic
fever?
A. Influenza virus
B. Ebola virus
C. Rhinovirus
D. Adenovirus
Ebola virus infection leads to severe hemorrhagic fever and high
mortality.
, 10. PCR is used to:
A. Measure antibiotic susceptibility
B. Amplify DNA sequences
C. Detect RNA viruses only
D. Stain bacteria
Polymerase chain reaction (PCR) amplifies specific DNA sequences for
detection or research purposes.
11. Which of the following is considered a biosafety level 3 (BSL-
3) pathogen?
A. Escherichia coli K-12
B. Mycobacterium tuberculosis
C. Lactobacillus species
D. Saccharomyces cerevisiae
BSL-3 pathogens can cause serious or potentially lethal disease via
inhalation and require specialized containment.
12. In a Kirby-Bauer disk diffusion test, the size of the inhibition
zone indicates:
A. Growth rate of bacteria
B. Sensitivity to antibiotics
C. Gram stain reaction
D. Spore formation
Larger inhibition zones indicate greater susceptibility of bacteria to the
tested antibiotic.
13. A positive catalase test produces:
A. Bubbling when hydrogen peroxide is added
B. No reaction
