Guidelines for detection of penicillin tolerance in Streptococcus pyogenes by MIC-MBC method

Disagreement on a uniform and accepted method of testing penicillin tolerance in Streptococcus pyogenes prevents a meaningful comparison of results of various studies on tolerance and an assessment of the contribution of this phenomenon to treatment failure. Therefore guidelines are needed.

Susceptibility of Streptococcus pyogenes to azithromycin, clarithromycin, erythromycin and roxithromycin in vitro

The susceptibility of 180 clinical isolates of Streptococcus pyrogenes from six regions of The Netherlands to the macrolide antibiotics azithromycin, clarithromycin, erythromycin and roxithromycin was analysed. The results of a microbroth MIC method, the E-test method and a disk diffusion assay were compared, and the MBC determined. In addition, the susceptibility to erythromycin of 436 clinical isolates of S. pyogenes from the Leiden region was determined. The microbroth MIC90s of azithromycin, clarithromycin, erythromycin and roxithromycin for group A streptococci were < or = 0.5 mg/L. Erythromycin had the lowest MIC90 (0.09 mg/L). The MIC data obtained with the E-test method suggested that clarithromycin and erythromycin had slightly higher anti-streptococcal activity than azithromycin and roxithromycin in vitro. MICs obtained with the E-test were lower than those found with the microbroth method. Only minor discrepancies were observed among the three methods. The MBC50 for both clarithromycin and erythromycin was 0.75 mg/L and 5.0 mg/L for azithromycin and roxithromycin. None of the 180 strains and two of the collection of 436 strains (0.5%) were resistant to erythromycin and the other macrolides tested; MICs ranged from 1 to 16 mg/L. The erythromycin-resistant strains showed an inducible type of macrolide-lincosamide-streptogramin B (MLS) resistance.

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.