Which testing method uses paper discs impregnated with antibiotics on an agar plate to assess susceptibility?

Which testing method uses paper discs impregnated with antibiotics on an agar plate to assess susceptibility?

• A. Kirby-Bauer disc diffusion
• B. MIC
• C. E-test
• D. Automated broth microdilution

Answer: (A)

This item is about diffusion-based antimicrobial susceptibility testing on agar using antibiotic-impregnated paper discs. In the Kirby-Bauer disc diffusion method, you inoculate an agar plate with the bacterial isolate and place small paper discs soaked with specific antibiotics on the surface. The antibiotic diffuses into the surrounding agar, creating a concentration gradient. Where the drug concentration is high enough to stop growth, a clear zone forms around the disc. After incubation, you measure the diameter of these zones and compare them to standardized breakpoints to determine if the organism is susceptible, intermediate, or resistant to each antibiotic. This approach lets you test multiple drugs quickly and inexpensively and is widely used in routine clinical microbiology. The other methods provide different kinds of results: MIC-based approaches (like broth microdilution or agar dilution) yield exact inhibitory concentrations, and the E-test gives a MIC value along a gradient strip. Automated broth microdilution systems also report MICs with automation. These are not the disc diffusion method, which focuses on zone of inhibition around antibiotic discs.

This item is about diffusion-based antimicrobial susceptibility testing on agar using antibiotic-impregnated paper discs. In the Kirby-Bauer disc diffusion method, you inoculate an agar plate with the bacterial isolate and place small paper discs soaked with specific antibiotics on the surface. The antibiotic diffuses into the surrounding agar, creating a concentration gradient. Where the drug concentration is high enough to stop growth, a clear zone forms around the disc. After incubation, you measure the diameter of these zones and compare them to standardized breakpoints to determine if the organism is susceptible, intermediate, or resistant to each antibiotic. This approach lets you test multiple drugs quickly and inexpensively and is widely used in routine clinical microbiology.

The other methods provide different kinds of results: MIC-based approaches (like broth microdilution or agar dilution) yield exact inhibitory concentrations, and the E-test gives a MIC value along a gradient strip. Automated broth microdilution systems also report MICs with automation. These are not the disc diffusion method, which focuses on zone of inhibition around antibiotic discs.