Detection of penicillin tolerance in Streptococcus pyogenes

Three traditional assays were used to determine the minimal bactericidal concentration (MBC) and minimal inhibitory concentration (MIC) for Streptococcus pyogenes (group A streptococci) in two phases of growth and the time taken to kill the organisms. Three other methods were used for the determination of penicillin tolerance: a cell-lysis assay, the beta-lactamase disk method and the replication method. Twenty strains, comprising penicillin-tolerant clinical isolates and two laboratory mutants, were used to evaluate the six tests. Results indicated that two groups of S. pyogenes can be distinguished--four highly tolerant and three moderately tolerant strains. The moderately tolerant strains were not recognised when rapidly growing instead of stationary cultures were used for the MBC and MIC determinations. The MBC/MIC ratio for tolerant strains was > 100. Tolerance percentage ranged from 0.30 to 1.07 and 0.29 to 3.96 for cultures in the mid-logarithmic and stationary phases of growth, respectively. The cell-lysis assay, the beta-lactamase disk method and the replication method may be used to screen for tolerance. Detection of high or moderate tolerance by determining the MBC/MIC ratio for mid-logarithmic or stationary cultures is recommended.