Distribution of macrolide resistance genes erm(B) and mef(A) among 160 penicillin-intermediate clinical isolates of Streptococcus pneumoniae isolated in southern France.

Two prevalent mechanisms of macrolide resistance are currently described in pneumococci: production of rRNA methylase that modify 23S ribosomal RNA resulting in MLSB phenotype, and an active efflux system resulting in M-phenotype. These two mechanisms are mediated by erm(B) and mef(A) genes respectively. Several studies reported a predominance of mef(A) gene in United-States and Canada. In European countries, erm(B) determinant is prevalent and mef(A)-mediated erythromycin resistance was recently reported in about 10% of strains in Belgium and Italy. In order to evaluate implication of mef(A) gene in pneumococci erythromycin resistance, 160 clinical isolates of S. pneumoniae with low-level of penicillin resistance and resistance to macrolides recovered between April 1999 and April 2000 were collected. These isolates were tested for their macrolide susceptibility by disc diffusion method, 155 showed the MLSB phenotype and 5 the M phenotype. Genotypic analysis was performed by erm(B) and mef(A) specific-mediated PCR: erm(B) gene was detected in 154 isolates, mef(A) gene in 5 isolates, and both genes in one strain. The phenotype seems to be well correlated to the genotyping result except for strain harboring both resistance determinants. Molecular typing of isolates harboring mef(A) gene performed by pulsed-field gel electrophoresis (PFGE) after restriction by Smal shows these strains to be epidemiologically unrelated. Our results show the predominance of the erm(B) gene in erythromycin resistant S. pneumoniae isolates. mef(A)-mediated resistance is effective in Southern France (3.7%) but this rate is the lowest published from European countries.