Learning Objective

Explain the principle, procedure, advantages, and limitations of the Kirby-Bauer disk diffusion test for determining bacterial antibiotic susceptibility.

Principle

1. The Kirby-Bauer test assesses bacterial susceptibility to antibiotics by measuring zones of inhibition around antibiotic-impregnated disks on an agar plate.
2. Antibiotics diffuse radially into the agar, creating a concentration gradient.
3. Bacterial growth is inhibited where the drug concentration exceeds the minimum inhibitory concentration (MIC).

Procedure

1. Swab the patient’s bacterial isolate uniformly over the surface of a solid agar plate.
2. Place multiple antibiotic disks on the agar.
3. Incubate to allow bacterial growth and diffusion of antibiotics.
4. Measure the diameter of the zones of inhibition around each disk.
5. Compare measured zones to standardized charts to classify bacteria as:
   1. Resistant (R)
   2. Intermediate (I)
   3. Susceptible (S)

Advantages

• Inexpensive and simple.
• Multiple antibiotics can be tested on a single plate.
• Provides clinically correlated data to guide therapy.

Disadvantages

• Qualitative, not quantitative.
• Does not provide MIC values.

Activity