What are the essential steps of the Kirby-Bauer disk diffusion method for antibiotic susceptibility testing?

What are the essential steps of the Kirby-Bauer disk diffusion method for antibiotic susceptibility testing?

• A. Inoculate a lawn, incubate, observe colony morphology.
• B. Inoculate a lawn, place antibiotic-impregnated disks, incubate, measure zones of inhibition and compare to standard breakpoints.
• C. Streak for isolation and perform Gram stain.
• D. Inoculate broth with antibiotic and measure turbidity.

Answer: (B)

Disk diffusion is a standardized way to assess antibiotic susceptibility by letting antibiotics diffuse from disks into a bacterial lawn and watching where growth is inhibited. Start with a uniform bacterial lawn on Mueller-Hinton agar so diffusion creates clear, comparable halos. Place antibiotic-impregnated disks on the surface, ensuring proper spacing. Incubate under standardized conditions, then measure the zones of inhibition around each disk. The measured diameters are compared to established interpretive breakpoints to categorize the organism as susceptible, intermediate, or resistant to each antibiotic. This approach differs from other options because merely noting colony morphology doesn’t test susceptibility, streaking for isolation with a Gram stain is for identifying the organism, and inoculating broth with antibiotic and measuring turbidity describes a broth dilution method rather than disk diffusion.

Disk diffusion is a standardized way to assess antibiotic susceptibility by letting antibiotics diffuse from disks into a bacterial lawn and watching where growth is inhibited. Start with a uniform bacterial lawn on Mueller-Hinton agar so diffusion creates clear, comparable halos. Place antibiotic-impregnated disks on the surface, ensuring proper spacing. Incubate under standardized conditions, then measure the zones of inhibition around each disk. The measured diameters are compared to established interpretive breakpoints to categorize the organism as susceptible, intermediate, or resistant to each antibiotic.

This approach differs from other options because merely noting colony morphology doesn’t test susceptibility, streaking for isolation with a Gram stain is for identifying the organism, and inoculating broth with antibiotic and measuring turbidity describes a broth dilution method rather than disk diffusion.