CMLA Microbiology Basics 2 — Questions and Answers
Question 1: Which staining technique differentiates bacteria based on cell wall peptidoglycan thickness?
- Acid-fast stain
- Gram stain (Correct answer)
- India ink stain
- Giemsa stain
Correct answer: Gram stain
The Gram stain exploits differences in peptidoglycan layer thickness: thick walls in Gram-positive bacteria retain crystal violet, while Gram-negative bacteria decolorize and take up safranin.
Question 2: A culture plate shows hemolysis where red blood cells are completely lysed, leaving a clear zone around colonies. This is classified as:
- Alpha hemolysis
- Beta hemolysis (Correct answer)
- Gamma hemolysis
- Delta hemolysis
Correct answer: Beta hemolysis
Beta hemolysis indicates complete lysis of red blood cells, producing a clear, transparent zone around the colony on blood agar.
Question 3: Which organism is responsible for causing tuberculosis?
- Mycobacterium leprae
- Mycobacterium tuberculosis (Correct answer)
- Nocardia asteroides
- Streptomyces species
Correct answer: Mycobacterium tuberculosis
Mycobacterium tuberculosis is the causative agent of tuberculosis and is identified by its acid-fast staining property due to mycolic acids in its cell wall.
Question 4: What is the purpose of a Mueller-Hinton agar plate in microbiology?
- Isolating fungi
- Performing antimicrobial susceptibility testing (Correct answer)
- Culturing anaerobic bacteria
- Detecting viral cytopathic effects
Correct answer: Performing antimicrobial susceptibility testing
Mueller-Hinton agar is the standard medium used for disk diffusion (Kirby-Bauer) antimicrobial susceptibility testing per CLSI guidelines.
Question 5: Which component of a bacterial cell is the primary target of penicillin?
- Cell membrane
- Ribosome
- Cell wall peptidoglycan synthesis (Correct answer)
- DNA gyrase
Correct answer: Cell wall peptidoglycan synthesis
Penicillin inhibits transpeptidase enzymes involved in cross-linking peptidoglycan strands during cell wall synthesis, causing cell lysis.
Question 6: A urine culture grows >100,000 CFU/mL of a single organism. This finding is most consistent with:
- Contamination from skin flora
- Significant bacteriuria (Correct answer)
- Normal urethral flora
- A fastidious organism
Correct answer: Significant bacteriuria
A colony count of ≥100,000 CFU/mL of a single organism in a clean-catch midstream urine sample is the threshold for significant bacteriuria indicating a urinary tract infection.
Question 7: Which media is selective and differential for isolation of Salmonella and Shigella from stool specimens?
- Blood agar
- Chocolate agar
- SS (Salmonella-Shigella) agar (Correct answer)
- MacConkey agar
Correct answer: SS (Salmonella-Shigella) agar
SS agar is both selective (inhibits most normal flora with bile salts and brilliant green) and differential (non-lactose fermenters appear colorless) for enteric pathogens Salmonella and Shigella.
CMLA Microbiology Basics 2
Which staining technique differentiates bacteria based on cell wall peptidoglycan thickness?