Comparison of four methods for the determination of MIC and MBC of penicillin for viridans streptococci and the implications for penicillin tolerance.

Four methods of MIC and MBC determination were examined to compare their reproducibility and ability to detect penicillin tolerance. The MIC and MBC of penicillin were determined for 28 strains of viridans streptococci by a microdilution method, a macrodilution method, a membrane method, and a gradient-plate method using a spiral-plating device. The macrodilution, membrane and gradient plate methods were found to be acceptable for MIC determination with a reproducibility to within one doubling dilution, but only the gradient method gave acceptable reproducibility for MBC determinations. The consequence of these findings with respect to penicillin tolerance is discussed.