Influence of medium and diluent pH and diffusion time on antibiotic bioassay

The effects of pH assay medium and diluents and of diffusion time on antibiotic bioassays were investigated to determine optimal assay conditions. Assays of penicillins, cephalosporins and macrolide derivatives were found to be little affected by the pH of the diluent, whereas the pH of the diluent exerted a noticeable influence on aminoglycoside assays, in which, therefore, it is essential to use a diluent at approximately p H 8.0. The data obtained have also demonstrated that the assays of penicillins, cephalosporins, macrolide derivatives and aminoglycosides were influenced by the pH of the assay medium. It is important to carry out penicillin or cephalosporin assays on plates of medium adjusted to approximately pH 6.0 and macrolide derived or aminoglycoside assays on a medium which is adjusted to approximately pH 8.0. The optimal duration of preculturing of assay plates for diffusion at 4 degree C after addition of the sample was determined to be 2 hours for the penicillins and cephalosporins. Diffusion of more than 2 hours was inadequate for assays of these antimicrobial agents. Diffusion times of less than 2 hours. For the macrolide derivatives and aminoglycosides, preculturing of 2 hours for diffusion at 4 degrees C was noted to be optimal, while preculturing for less than 2 hours proved inadequate. Diffusion times exceeding 2 hours had little effect on the assay of these antibiotics.