MLPAO Microbiology and Parasitology Fundamentals

Question 1

Which staining technique is used to classify bacteria as gram-positive or gram-negative?

Accepted Answer

Gram stain

Answer

The Gram stain is the most fundamental staining technique in bacteriology. It differentiates bacteria based on cell wall composition: gram-positive bacteria (thick peptidoglycan layer) retain crystal violet and appear purple, while gram-negative bacteria (thin peptidoglycan, outer membrane) lose crystal violet during decolorization and stain pink with safranin.

Question 2

Which selective and differential medium is used for the isolation of enteric gram-negative bacteria?

Accepted Answer

MacConkey agar

Answer

MacConkey agar is both selective (bile salts and crystal violet inhibit gram-positive bacteria) and differential (lactose and neutral red indicator distinguish lactose fermenters, which appear pink, from non-lactose fermenters, which appear colourless). It is essential for isolating enteric pathogens like Salmonella and Shigella.

Question 3

What is the catalase test used for in microbiology?

Accepted Answer

Differentiating Staphylococcus (positive) from Streptococcus (negative)

Answer

The catalase test differentiates staphylococci (catalase-positive, produce bubbles when exposed to hydrogen peroxide) from streptococci and enterococci (catalase-negative). Catalase breaks down hydrogen peroxide into water and oxygen gas, producing visible bubbles.

Question 4

Which test differentiates Staphylococcus aureus from coagulase-negative staphylococci?

Accepted Answer

Coagulase test

Answer

The coagulase test differentiates S. aureus (coagulase-positive) from coagulase-negative staphylococci (CoNS) like S. epidermidis and S. saprophyticus. Coagulase converts fibrinogen to fibrin, forming a clot. Both tube coagulase (free coagulase) and slide coagulase (bound coagulase/clumping factor) are used.

Question 5

What is the principle of the antimicrobial susceptibility test using the Kirby-Bauer disk diffusion method?

Accepted Answer

Measuring zones of growth inhibition around antibiotic-impregnated disks on agar

Answer

The Kirby-Bauer method measures the diameter of growth inhibition zones around standardized antibiotic disks placed on Mueller-Hinton agar inoculated with a standardized bacterial suspension. Zone sizes are compared to CLSI (or EUCAST) interpretive criteria to classify organisms as susceptible, intermediate, or resistant.

MLPAO - Medical Laboratory Professionals Association of Ontario Practice Test

MLPAO - Medical Laboratory Professionals Association of Ontario Practice Test

MLPAO Microbiology and Parasitology Fundamentals