MLPAO Microbiology and Parasitology Fundamentals Flashcards

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

Answer: Gram stain

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.

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

Answer: MacConkey agar

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.

What is the catalase test used for in microbiology?

Answer: Differentiating Staphylococcus (positive) from Streptococcus (negative)

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.

Which test differentiates Staphylococcus aureus from coagulase-negative staphylococci?

Answer: Coagulase test

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.

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

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

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.

Which organism is the most common cause of community-acquired urinary tract infections?

Answer: Escherichia coli

Escherichia coli is responsible for approximately 75-90% of uncomplicated community-acquired urinary tract infections. E. coli possesses virulence factors including type 1 pili and P fimbriae that facilitate adherence to uroepithelial cells. Other common uropathogens include Klebsiella, Proteus, and Staphylococcus saprophyticus.