CMLA Microbiology Basics 3 — Questions and Answers
Question 1: The catalase test differentiates which two groups of Gram-positive cocci?
- Staphylococcus vs. Streptococcus (Correct answer)
- Enterococcus vs. Streptococcus
- Micrococcus vs. Enterococcus
- Staphylococcus vs. Micrococcus
Correct answer: Staphylococcus vs. Streptococcus
Staphylococci are catalase-positive (produce bubbles with hydrogen peroxide) while Streptococci are catalase-negative, making this the first key differentiation test.
Question 2: Which of the following viruses requires a helper virus to replicate?
- Hepatitis B virus
- Hepatitis D virus (Correct answer)
- Hepatitis C virus
- Hepatitis A virus
Correct answer: Hepatitis D virus
Hepatitis D virus (delta agent) is a defective RNA virus that requires the surface antigen of Hepatitis B virus (HBsAg) for its own replication and packaging.
Question 3: A wet mount preparation showing motile, pear-shaped organisms with two nuclei is most consistent with:
- Entamoeba histolytica
- Giardia lamblia (Correct answer)
- Trichomonas vaginalis
- Cryptosporidium parvum
Correct answer: Giardia lamblia
Giardia lamblia trophozoites appear pear-shaped with bilateral symmetry, two nuclei, and characteristic tumbling motility on wet mount preparations.
Question 4: Which sterilization method is most appropriate for heat-sensitive items such as plastic tubing?
- Autoclaving at 121°C
- Dry heat oven
- Ethylene oxide gas (Correct answer)
- Boiling water
Correct answer: Ethylene oxide gas
Ethylene oxide (EO) gas sterilizes at low temperatures, making it ideal for heat-sensitive materials like plastics, rubber, and electronic devices.
Question 5: When performing a Gram stain, what is the correct sequence of reagents?
- Crystal violet → iodine → decolorizer → safranin (Correct answer)
- Safranin → iodine → decolorizer → crystal violet
- Crystal violet → decolorizer → iodine → safranin
- Methylene blue → iodine → decolorizer → safranin
Correct answer: Crystal violet → iodine → decolorizer → safranin
The correct Gram stain sequence is: crystal violet (primary stain), Gram's iodine (mordant), alcohol/acetone (decolorizer), then safranin (counterstain).
Question 6: Which laboratory safety level (BSL) is required for working with Mycobacterium tuberculosis?
- BSL-1
- BSL-2
- BSL-3 (Correct answer)
- BSL-4
Correct answer: BSL-3
Mycobacterium tuberculosis requires BSL-3 containment, which includes respiratory protection (N95 respirators), negative-pressure rooms, and HEPA filtration due to airborne transmission risk.
Question 7: What term describes bacteria that grow best at oxygen concentrations lower than atmospheric but not completely anaerobic conditions?
- Obligate aerobes
- Obligate anaerobes
- Microaerophiles (Correct answer)
- Facultative anaerobes
Correct answer: Microaerophiles
Microaerophiles require oxygen for growth but at concentrations (typically 2–10%) lower than atmospheric (21%), such as Campylobacter and Helicobacter pylori.
CMLA Microbiology Basics 3
The catalase test differentiates which two groups of Gram-positive cocci?