Laboratory Science Microbiology Techniques

Question 1

What is the purpose of the Gram stain in clinical microbiology?

Accepted Answer

Differentiate bacteria into gram-positive and gram-negative based on cell wall structure

Answer

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?

Accepted Answer

MacConkey agar

Answer

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?

Accepted Answer

The organism produces catalase enzyme, suggesting Staphylococcus (not Streptococcus)

Answer

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?

Accepted Answer

35–37°C

Answer

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?

Accepted Answer

Isolate individual bacterial colonies from a mixed culture

Answer

The streak plate technique dilutes bacteria across successive sectors, producing isolated single colonies from a mixed specimen for pure culture isolation and identification.

Laboratory Science Practice Test

Laboratory Science Practice Test

Laboratory Science Microbiology Techniques