Antimicrobial Susceptibility Testing: Kirby-Bauer Disk Diffusion Method

Antimicrobial Susceptibility Testing

Kirby-Bauer Disk Diffusion Method

The Kirby-Bauer disk diffusion method is a standardized technique used to evaluate the effectiveness of antibiotics against specific bacteria. This method helps determine the susceptibility or resistance of bacteria to various antimicrobial agents, which is essential for guiding clinical treatment decisions.

• Purpose: To assess the sensitivity of bacteria to antibiotics by measuring zones of inhibition around antibiotic-impregnated disks.

• Applications: Used in clinical microbiology laboratories to guide antibiotic therapy and monitor resistance trends.

Materials Required

• Mueller-Hinton (MH) agar plate (or TSA plate labeled as MH for simulation)

• Antibiotic disks: Penicillin, Tetracycline

• Control disk (no antibiotic)

• Forceps (tweezers)

• Cotton swabs

• Sterile saline solution (0.85%)

• Pipettes (20 µl and 200 µl) and disposable tips

• Disposable pipettes

• Empty petri dish (for waste)

• Fresh bacterial cultures: Escherichia coli or Staphylococcus epidermidis

• McFarland Standard (for turbidity comparison)

Procedure Overview

1. Plate Preparation: Obtain a Mueller-Hinton agar plate. Label the agar side with your name, date, bacterial species, and media type.

2. Quadrant Labeling: Draw three quadrants on the agar side and label them for Penicillin (1), Tetracycline (2), and Control (3).

3. Bacterial Suspension Preparation: Dilute the bacterial culture in sterile saline until the turbidity matches the McFarland Standard. Note: Do not open the McFarland Standard; use it only for visual comparison.

4. Inoculation: Pipette 200 µl of the diluted bacterial suspension onto the center of the plate. Use a sterile cotton swab to evenly spread the bacteria across the entire surface.

5. Disk Application: Using flame-sterilized tweezers, place the Penicillin disk in quadrant 1, Tetracycline disk in quadrant 2, and the control disk in quadrant 3. Sterilize tweezers between each disk application.

6. Incubation: Tape the plate closed and incubate at 37°C.

7. Cleanup: Disinfect the work area and pipettes with bleach, dispose of used pipettes and tips properly, and wash hands.

Key Concepts

• Zone of Inhibition: The clear area around an antibiotic disk where bacterial growth is prevented. The diameter of this zone is measured to determine susceptibility.

• Mueller-Hinton Agar: A standardized medium that supports the growth of most non-fastidious bacteria and allows for reproducible diffusion of antibiotics.

• McFarland Standard: A turbidity standard used to ensure a consistent bacterial inoculum density.

• Antibiotic Disks: Small, sterile paper disks impregnated with a specific concentration of antibiotic.

Example: Testing E. coli and S. epidermidis

• Each plate is inoculated with either Escherichia coli or Staphylococcus epidermidis.

• Zones of inhibition are measured after incubation to determine if the bacteria are susceptible, intermediate, or resistant to Penicillin and Tetracycline.

Table: Disk Placement and Quadrant Labeling

Quadrant	Disk Type	Purpose
1	Penicillin	Test susceptibility to Penicillin
2	Tetracycline	Test susceptibility to Tetracycline
3	Control	Negative control (no antibiotic)

Additional info:

• After incubation, the diameter of the zone of inhibition (in millimeters) is compared to standardized charts to interpret susceptibility.

• Results guide clinical decisions for antibiotic therapy and help monitor emerging resistance.

• Proper aseptic technique and accurate labeling are critical for reliable results.