Studies on bactericidal activities of beta-lactam antibiotics on agar plates: the correlation with the antibacterial activities determined by the conventional methods.

A simplified method to determine minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) of beta-lactam antibiotics on agar plates is described. MIC values were determined on agar plates for benzylpenicillin, methicillin and cephalothin using Staphylococcus aureus and Klebsiella pneumoniae. A beta-lactamase solution was then sprayed onto the plates to inactivate the drug(s). After further incubation at 37 degrees C overnight, the minimal concentration at which no test bacteria were visible on the plates was defined as MBC. Both MIC and MBC values decreased with decreased inoculum size. The two values were almost coincidental when high dilutions were used as the inocula. These values were compared with those obtained by the conventional broth dilution method. In this study, MIC as well as MBC values determined by the simplified method were generally smaller than the values determined by the broth dilution technique.