27 Jan

Antimicrobial Sensitivity Testing

As soon as the causative agent of an infection has been determined it is up to the physician to presribe the most effective antimicrobial agent that will kill the pathogen without harming the patient.  Antibiotics vary in effectiveness against different bacteria.  Some antibiotics are more effective against gram negative bacteria while some are referred to as broad spectrum and are effective against both gram negative and gram positive bacteria.  Antibiotics must be tested against specific organisms to determine how effective they are.  The Kirby-Bauer method of sensitivity testing is one of the most popular methods for testing antibiotics.  This test is performed by uniformly streaking a medium with the test organism.  Paper disks containing specific concentrations of an antibiotic are deposited on the agar surface.  A chemical in the disk diffuses out and forms a concentration gradient.  If the agent inhibits the growth of the bacteria there will be a zone around the disk where no growth occurs called the zone of inhibition.