Characterization and prevalence of MefA, MefE, and the associated msr(D) gene in Streptococcus pneumoniae clinical isolates

Recent work has shown that the efflux genes in Streptococcus pneumoniae that are responsible for acquired macrolide resistance can be distinguished as either mef(E) or mef(A). The genetic elements on which mef(A) and mef(E) are found also carry an open reading frame (ORF) that is 56% homologous to msr(A) in Staphylococcus. The prevalence of mef(A/E) and of the msr-like ORF [msr(D)] was evaluated in 153 mef(+) S. pneumoniae clinical isolates collected in North America, Europe, Africa, and Asia from 1997 to 2002. Clinical isolates were screened with PCR primers specific for either mef(A) or mef(E) and for msr(D). mef(A), mef(E), and msr(D) were cloned from mef(+) strains and transformed into a susceptible, competent strain of S. pneumoniae. The transformants were tested for antimicrobial susceptibilities and efflux pump induction. The results of this work demonstrated that mef(A) is more often isolated in parts of Europe, with some incidence in Canada, and that the msr-like gene alone can confer the efflux phenotype.