Laboratory Science Microbiology Techniques 1 — Questions and Answers
Question 1: What is the purpose of the Gram stain in clinical microbiology?
- Identify fungal infections
- Differentiate bacteria into gram-positive and gram-negative based on cell wall structure (Correct answer)
- Detect acid-fast organisms
- Measure bacterial colony counts
Correct answer: Differentiate bacteria into gram-positive and gram-negative based on cell wall structure
The Gram stain differentiates bacteria by their cell wall peptidoglycan thickness: gram-positive cells retain crystal violet (purple) while gram-negative cells stain red with safranin counterstain.
Question 2: Which growth medium is selective for gram-negative enteric bacteria while inhibiting gram-positive organisms?
- Blood agar
- Chocolate agar
- MacConkey agar (Correct answer)
- Sabouraud dextrose agar
Correct answer: MacConkey agar
MacConkey agar contains bile salts and crystal violet that inhibit gram-positive bacteria, while lactose and pH indicator differentiate lactose-fermenting from non-fermenting gram-negatives.
Question 3: What does a positive catalase test (bubbling with H₂O₂) indicate?
- The organism is gram-negative
- The organism produces catalase enzyme, suggesting Staphylococcus (not Streptococcus) (Correct answer)
- The organism is acid-fast
- The organism is oxidase-positive
Correct answer: The organism produces catalase enzyme, suggesting Staphylococcus (not Streptococcus)
Catalase-positive organisms (like Staphylococcus) produce catalase that converts H₂O₂ to O₂ and water; Streptococcus is catalase-negative, a key differentiator.
Question 4: Which temperature is standard for incubating most clinical bacterial cultures?
- 25°C
- 35–37°C (Correct answer)
- 42°C
- 55°C
Correct answer: 35–37°C
Most pathogenic bacteria grow optimally at 35–37°C (human body temperature), making this the standard incubation temperature for clinical cultures.
Question 5: What is the purpose of a streak plate technique in microbiology?
- Measure bacterial growth rate
- Isolate individual bacterial colonies from a mixed culture (Correct answer)
- Identify bacterial species by color
- Prepare specimens for staining
Correct answer: Isolate individual bacterial colonies from a mixed culture
The streak plate technique dilutes bacteria across successive sectors, producing isolated single colonies from a mixed specimen for pure culture isolation and identification.
Question 6: Which test is used to differentiate Staphylococcus aureus from other coagulase-negative staphylococci?
- Optochin sensitivity test
- Coagulase test (Correct answer)
- Bile solubility test
- CAMP test
Correct answer: Coagulase test
S. aureus produces coagulase enzyme that clots plasma, while coagulase-negative staphylococci (like S. epidermidis) do not, making the coagulase test the primary identifier.
Laboratory Science Microbiology Techniques 1
What is the purpose of the Gram stain in clinical microbiology?