High prevalence of erythromycin-resistant, clindamycin/miocamycin-susceptible (M phenotype) Streptococcus pyogenes: results of a Spanish multicentre study in 1998

Using the standard agar dilution method we studied the prevalence of suscep tibility to 14-, 15- and 16-membered ring macrolides and clindamycin in Str eptococcus pyogenes isolated in 1998 from 21 laboratories in Spain. The num ber of strains admitted to the study was proportional to the numbers of inh abitants in each geographical area. We also determined the susceptibility p henotypes and the genetic basis for the antibiotic resistance. A total of 4 86 unduplicated isolates of S. pyogenes were used. Throat swab samples prov ided 359 (73.9%) isolates, and the remaining 127 isolates were from other s ources. One hundred and fourteen (23.5%) isolates were resistant to erythro mycin, a 14-membered ring macrolide, and azithromycin, a 15-membered macrol ide, whereas only 1% of isolates were resistant to miocamycin, a 16-membere d macrolide and 0.8% were resistant to clindamycin. Of the 114 erythromycin -resistant strains, 109 (95.6%) were susceptible to clindamycin and miocamy cin. Since induction with erythromycin did not modify susceptibility to the latter antibiotics, these 109 strains were considered to have the M phenot ype. Twenty strains with the M phenotype, one per laboratory, were assayed by PCR and showed the presence of the mef gene, which is responsible for an tibiotic resistance by an efflux system. Among comparable studies covering entire countries, ours demonstrates one of the highest rates of S. pyogenes erythromycin resistance and clindamycin and miocamycin susceptibility in t he world. Strains with the M phenotype account for the great majority of th ese isolates.