Group G Streptococci: susceptibility patterns and the effect of the inoculum size and growth phase on the bactericidal activity of penicillin

Twenty-seven independent group G streptococcal isolates were studied by in vitro susceptibility testing against 22 different antimicrobial agents. Penicillin with a MIC90 of 0.03 micrograms/ml and ampicillin with a MIC90 of less than or equal to 0.015 micrograms/ml remain the agents of first choice for treatment of group G streptococcal infections. Tolerance was not demonstrated using the macrobroth dilution method in four media, Todd-Hewitt, Mueller-Hinton, Mueller-Hinton (cation-supplemented), and Tryptose Phosphate broths. Multiple regression analyses of time-kill curves of group G streptococci showed that the rate of cell death with penicillin at 0.04 micrograms/ml (five times greater than each organism's MIC) for both logarithmic- and stationary-phase cells with low-inocula were the same, but were five to six times greater in rate of death compared to the high-inocula cultures. Increasing the concentration to 1 microgram/ml of penicillin (125 times greater than each organism's MIC) did not significantly affect the rate of cell death for low-inocula cultures of either phase. Therefore, the size of the inoculum was found to be more significant than the phase of bacterial growth. These findings may explain the therapeutic discrepancy of relapses or prolonged group G streptococcal infections despite the organism being susceptible to the given antibiotic.