Multicentre surveillance of the prevalence and molecular epidemiology of macrolide resistance among pharyngeal isolates of group A streptococci in the USA

Objectives: Rates of macrolide resistance in group A streptococci (GAS) were reported to be low in the US in the 1990s. However, we documented an unexpectedly high rate of macrolide resistance among GAS in Pittsburgh, PA, in 2001 and 2002. In an effort to define the current prevalence of macrolide-resistant GAS in the US, a multicentre surveillance project was initiated. Methods: Between October 2002 and May 2003, 50 pharyngeal GAS isolates per month were requested from each of the nine participating sites representing a wide geographical distribution. Standard susceptibility testing was performed and the macrolide resistance phenotype was assessed using double-disc diffusion testing. Monthly and annual rates of macrolide resistance were calculated for each site. An adjusted overall rate of macrolide resistance was determined to account for differences in the numbers of GAS isolates sent from each centre. Results: Overall, 171 of the 2797 collected isolates of GAS (6.1%) were resistant to erythromycin. The adjusted overall resistance rate was 5.2%. Rates of macrolide resistance varied by site (range 3.0-8.7%) and also by month (< 2% to > 10%). The M phenotype of macrolide resistance accounted for > 60% of all macrolide-resistant isolates recovered in this study. Conclusions: These data suggest an increasing prevalence and broad geographical distribution of macrolide-resistant GAS in the US, indicating the need for ongoing local and national longitudinal surveillance to define the extent of this problem.