Treatment failure in streptococcal pharyngotonsillitis. An attempt to identify penicillin tolerant Streptococcus pyogenes.

Penicillin tolerance in group A streptococci has been suggested to cause treatment failures in pharyngotonsillitis. In the present study, group A streptococci from patients with pharyngotonsillitis, who healed (n = 33) or failed (n = 25) on phenoxymethylpenicillin therapy for 10 days, as well as isolates obtained following the first (n = 25) and second (n = 7) failure were tested for penicillin tolerance by a plate-screening method. For most strains, the survival rate after a 6-h exposure of log-phase bacteria (10(4) CFU) to a phenoxymethylpenicillin concentration of 4 times the minimum inhibitory concentration (MIC) was below 0.1%. Five strains from cases of failure, exhibiting survival rates of 0.2-0.5%, were subjected to time killing kinetic tests with phenoxymethylpenicillin at 12 times the MIC. At 6 h each of the strains from failures showed survival rates below 0.03%. One single group A strain, previously selected in our laboratory, showed a survival rate of 0.4-1.2%, which was close to tolerance as defined. Four streptococcal strains, earlier reported as tolerant, showed survival rates of > 1% but were found to be group G. Penicillin tolerance does not significantly contribute to failures in penicillin therapy of group A streptococcal pharyngotonsillitis, but seems to be a common property of group C and G streptococci.