Standardizing antimicrobial disk sensitivity testing

Before antimicrobial sensitivity testing, any pathogens must be isolated by streaking the clinical specimen on a plate of nutritive medium. Selecting a drug based on tests of a mixed culture from which a pathogen has not been isolated encourages unnecessary treatment. A plate of Mueller-Hinton agar is swabbed with an inoculum from a standardized broth culture. Drug-impregnated paper disks are applied to the plate at least 3 cm apart and at least 2 cm from the plate's edge. The zone of inhibition varies with each drug and is measured, after overnight incubation at 35-37 C, with the aid of over-the-shoulder lighting and a dark background.