Penicillin tolerance of human isolates of group C streptococci.

Seventeen clinical isolates of group C streptococci were tested for penicillin tolerance. Sixteen of the strains showed penicillin tolerance with a 32-fold or greater difference between the minimal inhibitory concentration and the minimal bactericidal concentration. Synergism was demonstrated with a combination of penicillin and gentamicin for all 17 strains tested. The rate of antibiotic killing was measured for five of the streptococcal strains by using the combination of penicillin and gentamicin. All isolates were killed within 5 h with the combination, but viable organisms were recovered after 48 h when either drug was used alone. Our study suggests that penicillin tolerance with group C streptococci may occur frequently and may account for the poor outcome of serious group C streptococcal infections tested with penicillin alone.