Antimicrobic Sensitivity Testing: The Kirby-Bauer Method

Once the causative organism of a specific disease in a patient has been isolated, it is up to the attending physician to administer a chemotherapeutic agent that will inhibit or kill the pathogen without causing serious harm to the individual. The method must be relatively simple to use, be very reliable, and yield results in as short a time as possible. The Kirby-Bauer method of sensitivity testing is such a method. It is used for testing both antibiotics and drugs. Antibiotics are chemotherapeutic agents of low molecular weight produced by microorganisms that inhibit or kill other microorganisms. Drugs, on the other hand, are antimicrobic agents that are man-made. Both types of agents will be tested in this laboratory session according to the procedure shown in next figure. The effectiveness of an antimicrobic in sensitivity testing is based on the size of the zone of inhibition that surrounds a disk that has been impregnated with a specific concentration of the agent. The zone of inhibition, however, varies with the infusibility of the agent, the size of the inoculum, the type of medium, and many other factors. Only by taking all these variables into consideration can a reliable method be worked out. The Kirby-Bauer method is a standardized system that takes all variables into consideration. It is sanctioned by the U.S. FDA and the Subcommittee on Antimicrobial Susceptibility Testing of the National Committee for Clinical Laboratory Standards.