Experiences with the Kirby-Bauer Method of Antibiotic Susceptibility Testing

The wide variety of bacterial susceptibility test methods has become a major concern of clinical laboratories. In response to this situation, the World Health Organization and the National Communicable Disease Center have set up an expert committee on antibiotics to assess the possibility of establishing internationally acceptable standards for susceptibility testing. One of the technics under examination is the Kirby-Bauer method. It was the purpose of our study to evaluate this method as a standard technic. Seven organisms were selected and tested against 23 high-potency antimicrobial agents. Susceptibility studies of these organisms were made according to the Kirby-Bauer method, using two media-Mueller-Hinton and the Oxoid sensitivity test medium. Other variables also were examined to determine whether they would significantly alter the interpretation of the zone diameter readings. Our experiments showed that either medium provides consistently reproducible results. However, it appears best to preincubate the test organisms in trypticase soy broth. Refrigerated cultures did not yield altered zone diameters, but we do not recommend refrigerating the plates prior to overnight incubation. In general, we found the Kirby-Bauer method to be simple, accurate, and reproducible, and recommend it as a standard testing method for interpretation of susceptibility to chemotherapeutic agents.