Efficacy of beta-lactamase-resistant penicillin and influence of penicillin tolerance in eradicating streptococci from the pharynx after failure of penicillin therapy for group A streptococcal pharyngitis.

We studied the ability of dicloxacillin, a beta-lactamase-resistant penicillin, to eradicate persistent group A streptococci from the upper respiratory tract of children previously given penicillin V, and concomitantly assessed the role of antibiotic (penicillin and dicloxacillin) tolerance as a contributing factor. During a group A streptococcal pharyngitis outbreak, 66% of 324 children were found to be culture positive for group A streptococci. Ninety percent of these isolates were serotype M-1, T-agglutination pattern T-1. The treatment failure rate after initial orally administered penicillin treatment was 21% (42 of 204). These 42 children then received either a second course of penicillin V orally or a course of dicloxacillin. The resulting rates of failure to eradicate the homologous streptococcal serotype were 83% (20 of 24) and 50% (9 of 18), respectively (P less than 0.02). Of 189 streptococcal isolates tested by the gradient replicate plate method for penicillin tolerance, 18 (10%) were tolerant. In this study, penicillin-tolerant strains of group A streptococci were no more frequently isolated from children in whom initial penicillin treatment failed than from those who were successfully treated. Dicloxacillin tolerance was not a factor in failure of dicloxacillin therapy. These data, when evaluated with data from previous studies, suggest that reasons for failure to eradicate group A streptococci from the upper respiratory tract are complex, but dicloxacillin may be beneficial in some patients who fail to respond to orally administered penicillin therapy.