The emergence of Streptococcus pneumoniae resistant to macrolide antimicrobial agents: A 6-year population-based assessment

From 1994 through 1999, the available isolates (4148 isolates) from active population-based surveillance of invasive pneumococcal disease in metropoli tan Atlanta were serotyped and were tested for antimicrobial susceptibility . Macrolide-resistant isolates were studied for the presence of ermAM (a ri bosomal methylase gene), mefE (a macrolide efflux gene), and tetM (the clas s M tetracycline resistance gene). Macrolide resistance increased from 16% of all invasive isolates in 1994 to 32% in 1999. Of the macrolide-resistant pneumococcal isolates studied, 99% contained genomic copies of mefE or erm AM Isolates with ermAM were mainly serotypes 6B, 23F, 14, or 19F and contai ned tetM; mefE-associated isolates were predominantly serotypes 14, 6A, or 19F, and most did not contain tetM. The frequency of the ermAM-mediated phe notype in invasive Streptococcus pneumoniae remained stable over the 6-year surveillance. However, the mefE-mediated phenotype increased from 9% in 19 94 to 26% of all isolates in 1999 and was noted in new serotypes, By 1999, 93% of the mefE-containing strains had minimum inhibitory concentrations gr eater than or equal to8 mug/mL. Dissemination of the mefE determinant accou nted for the rapid increase in the rate of macrolide resistance in our S. p neumoniae population.