Reverse inoculum effect in bactericidal activity and other variables affecting killing of group B streptococci by penicillin.

Variables of the effect of penicillin G on the numbers of viable group B streptococci in broth cultures were studied. One-fourth of the MIC was the lowest concentration that reduced the viable count compared with antibiotic-free controls. The rate of killing increased with the concentration of penicillin up to 4 X MIC, but no further. During the first 2 or 3 h, the bactericidal activity was more rapid than later on. The MIC and supraoptimal concentrations of penicillin killed an inoculum of 10(6) organisms more rapidly than an inoculum of 10(4) organisms. The MIC was not inoculum dependent. The reverse inoculum effect was revealed by the killing curves but not by the MBC. There were reproducible differences among strains as to the rate of killing by penicillin; these did not correlate with the rate of multiplication, which also varied among strains. Among the 11 strains tested, there were no tolerant ones.