In vitro activities of the novel ketolide telithromycin (HMR 3647) againsterythromycin-resistant Streptococcus species

The in vitro susceptibilities of 184 erythromycin-resistant streptococci to a novel ketolide, telithromycin (HMR 3647), were tested. These clinical is olates included 111 Streptococcus pyogenes, 18 group C streptococcus, 18 gr oup G streptococcus, acid 37 Streptococcus pneumoniae strains. The MICs for all but eight S. pyogenes strains were less than or equal to0.5 mug/ml, in dicating that telithromycin is active in vitro against erythromycin-resista nt Streptococcus strains. All strains for which MICs were greater than or e qual to1 mug/ml had an erm(B) resistance gene and six strains for which MIC s were greater than or equal to4 mug/ml had a constitutive erm(B) gene (MIC range, 4 to 64 mug/ml). Interestingly, for S. pneumoniae strains with a co nstitutive erm(B) gene, MICs were less than or equal to0.25 mug/ml (MIC ran ge, less than or equal to0.008 to 0.25 mug/ml). Our in vitro data show that for S. pyogenes strains which constitutively express the erm(B) methylase gene, MICs are so high that the strains might be clinically resistant to te lithromycin.