Increased activity of 16-membered lactone ring macrolides against erythromycin-resistant Streptococcus pyogenes and Streptococcus pneumoniae: characterization of South African isolates

The susceptibility of 40 erythromycin-resistant isolates of Streptococcus p yogenes and 40 multiply-resistant isolates of Streptococcus pneumoniae to s ix macrolide antibiotics, representing 14-, 15- and 16-membered lactone rin g structures, was tested. The genetic basis for macrolide resistance in the strains was also determined. Both erm and mef determinants were encountere d in the 36 S. pneumoniae Isolates tested, but only mef in the five S. pyog enes isolates tested. All isolates showed cross-resistance among the 14-mem bered macrolides erythromycin, clarithromycin and roxithromycin and the 15- membered macrolide, azithromycin. However, the erythromycin-resistant S pyo genes isolates retained full susceptibility to spiramycin and josamycin (16 -membered agents). These latter two antibiotics were also more active than the other macrolides against erythromycin-resistant S. pneumoniae isolates, especially josamycin which was 8-64 times more active than erythromycin; s piramycin was only two to eight times more active than erythromycin.