Effect of agar percentage, agar thickness, and medium constituents on antibiotics assay by disc diffusion method.

The factors affecting the sensitivity of nafcillin, vancomycin, oleandomycin, chloramphenicol, methacycline and novobiocin assay, using the paper disc diffusion method, were studied. Agar concentration and pour volume were found to influence the assay sensitivity of antibiotics. The type of assay medium was effective e.g. brain heart infusion agar was the best medium for nafcillin and methacycline; synthetic amino acid medium for oleandomycin; Muller Hinton agar for vancomycin and chloramphenicol; and G & R medium for novobiovicin. All additives, including inorganic ions, sugar, organic acids, complexing agents, surface active agents and blood, decreased the activities of the studied antibiotics. Higher activities of vancomycin, methacycline were observed at pH = 5, of nafcillin, chloramphenicol, and novobiocin at pH = 6, and of oleandomycin at pH = 7.