Gradient plate method to induce Streptococcus pyogenes resistance

In recent years, increasing numbers of Streptococcus pyogenes (GAS) strains displaying resistance to macrolides have been reported in Finland, Japan, Asia and Spain. Antibiotic use has been shown to be a risk factor for infec tion with and carriage of drug-resistant streptococci. The aim of this stud y was to compare in-vitro development of resistance of streptococci to beta -lactams (penicillin, amoxycillin, cefotiam and cefuroxime) and erythromyci n by serial passages in subinhibitory concentrations of antibiotics (subMIC s) by gradient plate method. Three clinical strains of GAS were tested. Two were susceptible to erythromycin (MIC = 0.015 mg/L and 0.013 mg/L) and one resistant. Serial passages were performed daily by gradient plate method u ntil a four-fold increase of the MIC was achieved. GAS variants obtained af ter serial passages in beta-lactams had MICs increased at least four-fold. They remained susceptible to these antibiotics. With erythromycin, final MI Cs reached intermediate and resistant level. Results obtained in this study with erythromycin are in good correlation with clinical studies showing th at prior exposure to macrolides may help to facilitate the emergence of dru g-resistant strains of streptococci.