Synergism, killing kinetics, and antimicrobial susceptibility of group A and B streptococci

The susceptibility of 110 group A and 179 group B streptococci to 25 antimicrobics was tested by broth microdilution and agar disk diffusion tests. Representative strains were used in killing kinetics, penicillin-gentamicin synergy, and minimal bactericidal concentration tests. Group A streptococci were more susceptible than group B streptococci to 17 of the 25 antimicrobics tested. Group A and B streptococci were killed at the same rate if the amount of penicillin used was equivalent to their respective penicillin minimal inhibitory concentrations. Synergism was demonstrated for both group A and B streptococci when penicillin was used at concentrations equal to each respective minimal inhibitory concentration and subinhibitory concentration of gentamicin. This synergy could be demonstrated best using minimal bactericidal concentrations obtained by culturing 3- and 6-h cultures from the microdilution checkerboard tests rather than from 24-h subcultures. A greater synergistic effect was achieved by adding penicillin first and then adding gentamicin rather than in the reverse order, or simultaneously.