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

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Abstract
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 [MIC]. Synergism was demonstrated for both group A and B streptococci when penicillin was used at concentrations equal to each respective MIC 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.