Antibiotic susceptibility testing is a crucial laboratory procedure used to determine the effectiveness of antibiotics against bacterial pathogens. One of the most widely used methods is the Kirby-Bauer test, also known as the disk diffusion test. This simple yet effective technique helps clinicians select the most appropriate antibiotic for treating infections, ensuring both efficacy and minimizing the risk of resistance development.

Kirby-Bauer test/Disk diffusion test
Possible treatment options for Enterobacteriaceae
(e.g., E.coli, Klebsiella pneumoniae and Proteus mirabilis)

A     Antibiotic Disks: Seven antibiotic disks are placed on Mueller-Hinton agar plate:
(tested strain Escherichia coli)

• 1. Ampicillin
• 2. Amoxicillin/Clavulanate
• 3. Cefuroxime
• 4. Co-trimoxazole
• 5. Ciprofloxacin
• 6. Gentamicin
• 7. Nitrofurantoin

B     Inhibition Zones:

After overnight incubation, inhibition zones around the antibiotic disks are observed. There is no inhibition zone or the zone is very small, indicating resistance to ampicillin, amoxicillin/clavulanate, cefuroxime, co-trimoxazole, ciprofloxacin and gentamicin.

C     Breakpoints

In the disk diffusion method a breakpoint refers to a specific concentration of an antibiotic that is used to categorize the bacterial response as either susceptible, intermediate, or resistant.

D     Conclusion:

Based on the antibiotic susceptibility testing results, ampicillin, amoxicillin/clavulanate, cefuroxime, co-trimoxazole, ciprofloxacin and gentamicin should be avoided for the treatment of this E. coli infection. However, nitrofurantoin appear to be an effective treatment option, depending on the location and severity of the infection.

The Disk Diffusion Test and the Kirby-Bauer Method essentially refer to the same technique used in microbiology to test the effectiveness of antibiotics against bacterial pathogens, but there is a slight difference in their context and standardization.
Disk Diffusion Test is the general name for the method where antibiotic-impregnated paper discs are placed on an agar plate inoculated with a bacterial culture. The antibiotic diffuses out from the disc, creating a concentration gradient. If the antibiotic is effective, it will inhibit bacterial growth, creating a zone of inhibition (a clear area around the disc where bacteria cannot grow). The Kirby-Bauer method specifically refers to the standardized protocol developed by William Kirby and Alfred Bauer, which is commonly associated with CLSI (Clinical and Laboratory Standards Institute) guidelines.