Overview
This lecture explains the Kirby-Bauer disk diffusion method, a key microbiology lab technique used to assess the effectiveness of antibiotics against bacteria.
Kirby-Bauer Disk Diffusion Method
- The Kirby-Bauer method tests how well antibiotics inhibit bacterial growth on an agar plate.
- A confluent lawn is created by covering the entire plate with bacteria using a cotton-tipped applicator.
- Paper discs containing known amounts of antibiotics are placed on the plate.
- After incubation, zones of inhibition—clear areas with no bacterial growth—form around effective antibiotic discs.
Standardization and Interpretation
- Mueller-Hinton agar plates, with controlled thickness, are used for standardization.
- The diameter of the zone of inhibition is measured and compared to reference tables.
- Sensitivity, intermediate response, or resistance are determined by matching zone size with chart criteria for each antibiotic-organism pair.
- Largest zone size does not always mean most effective antibiotic due to various influencing factors.
Limitations and Considerations
- The method does not distinguish between bactericidal (kills bacteria) and bacteriostatic (inhibits growth) effects.
- Simply observing a zone does not fully indicate clinical effectiveness.
- Method is useful for comparing antibiotics but must be interpreted with care.
Common Bacteria Used
- Typical test organisms include Staphylococcus aureus, Escherichia coli, Pseudomonas, and Proteus vulgaris.
- Antibiotic effectiveness can vary greatly between different bacterial species.
Key Terms & Definitions
- Kirby-Bauer method — a lab technique to evaluate antibiotic effectiveness using antibiotic-impregnated discs on a bacterial lawn.
- Confluent lawn — a uniform layer of bacterial growth covering the entire agar plate.
- Zone of inhibition — clear region around an antibiotic disc where bacteria failed to grow.
- Mueller-Hinton agar — standardized medium used for antibiotic susceptibility testing.
- Bactericidal — capable of killing bacteria.
- Bacteriostatic — inhibits the growth of bacteria without killing them.
Action Items / Next Steps
- Practice preparing a confluent lawn and performing the Kirby-Bauer disk diffusion test in lab.
- Review zone of inhibition interpretation charts for different bacteria and antibiotics.