Susceptibility of group A beta-hemolytic Streptococcus isolates to penicillin and erythromycin.

We have reevaluated the antibiotic susceptibilities of group A beta-hemolytic streptococci in view of recent reports of a high prevalence of erythromycin resistance in Japan and of an increase in penicillin treatment failures in the United States. A total of 474 isolates recovered during a 2- to 3-month period in 1980 were tested. All were susceptible by microtiter broth dilution to a penicillin concentration of less than or equal to 0.03 micrograms/ml (minimal inhibitory concentration), and 473 were killed by less than or equal to 0.06 micrograms/ml (minimal inhibitory concentration). Erythromycin minimal inhibitory concentrations showed a bimodal distribution: 95% were less than or equal to 0.06 micrograms/ml, and 5% were greater than or equal to 1 microgram/ml. Of the minimal bactericidal concentrations, 21% were greater than or equal to 1 microgram/ml and 3% were greater than or equal to 16 micrograms/ml. Group A beta-hemolytic streptococci remain susceptible to the inhibitory and bactericidal actions of penicillin, thus providing no in vitro explanation for the bacteriological relapses reported in some clinical studies. Unlike the Japanese experience, only 5% of our isolates were resistant to erythromycin (minimal inhibitory concentration, greater than or equal to 1 microgram/ml); however, 22% were tolerant (ratio of minimal inhibitory/bactericidal concentrations, greater than or equal to 32).