MLPAO Microbiology and Parasitology Fundamentals 1 — Questions and Answers
Question 1: Which staining technique is used to classify bacteria as gram-positive or gram-negative?
- Acid-fast stain
- Gram stain (Correct answer)
- India ink stain
- Giemsa stain
Correct 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.
Question 2: Which selective and differential medium is used for the isolation of enteric gram-negative bacteria?
- Blood agar
- MacConkey agar (Correct answer)
- Chocolate agar
- Mannitol salt agar
Correct 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.
Question 3: What is the catalase test used for in microbiology?
- Differentiating Staphylococcus (positive) from Streptococcus (negative) (Correct answer)
- Identifying gram-negative rods
- Detecting spore-forming bacteria
- Differentiating aerobic from anaerobic bacteria
Correct 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.
Question 4: Which test differentiates Staphylococcus aureus from coagulase-negative staphylococci?
- Catalase test
- Coagulase test (Correct answer)
- Oxidase test
- Bile esculin test
Correct 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.
Question 5: What is the principle of the antimicrobial susceptibility test using the Kirby-Bauer disk diffusion method?
- Measuring the minimum concentration of antibiotic that inhibits growth
- Measuring zones of growth inhibition around antibiotic-impregnated disks on agar (Correct answer)
- Detecting antibiotic resistance genes by PCR
- Measuring bacterial growth curves with and without antibiotics
Correct 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.
Question 6: Which organism is the most common cause of community-acquired urinary tract infections?
- Staphylococcus aureus
- Escherichia coli (Correct answer)
- Pseudomonas aeruginosa
- Enterococcus faecalis
Correct 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.
MLPAO Microbiology and Parasitology Fundamentals 1
Which staining technique is used to classify bacteria as gram-positive or gram-negative?