Emergence of mefA and mefE genes in beta-haemolytic streptococci and pneumococci in France.

Abstract

Sir, Since the early 1990s, most developed countries have experienced a rapid and substantial increase in erythromycin resistance among isolates of Streptococcus pyogenes and Streptococcus pneumoniae, probably related to the increased prescribing of this antibiotic or its novel semisynthetic derivatives. Until recently, the unique mechanism of macrolide resistance recognized in streptococci was target modification by 23S rRNA methylases encoded by erm genes, which convey cross-resistance to macrolides, lincosamides and the streptogramin B compounds (MLSB phenotype). With the increase of erythromycin resistance, a new phenotype designated M, consisting of resistance to 14and 15-membered macrolides, but susceptibility to 16-membered macrolides, lincosamides and type-B streptogramins, has emerged. The mechanism of this resistance is a proton-dependent efflux system encoded by mef genes: mefA in S. pyogenes, and mefE in S. pneumoniae. S. pyo genes and S. pneumoniae with an M phenotype and/or mef genes have been reported in a number of countries, where their prevalence among erythromycin-resistant isolates has reached very high rates; for S. pyogenes, mainly in Europe (38–97%) and for S. pneumoniae, principally in North America (41–85%). Subsequently, mef genes have been detected in Group C streptococci in Finland, where they represented 95% of the erythromycin-resistant isolates. In France, such an increase in erythromycin resistance has not been observed, at least in our geographical area. The current rate of erythromycin resistance in the South West of France appears to be stabilized around 10% of S. pyogenes and Group C streptococci, 15% of Streptococcus agalactiae and Group G streptococci, and 45% of S. pneu moniae, with high local and temporal variations. Between 1993 and 1998, we collected 18 isolates of S. agalactiae ( 1%) with an M phenotype, which have been demonstrated to possess either an mefA or an mefE gene. Over the same period, we have recovered only two S. pyogenes isolates, one Group C streptococcus and two pneumococci with an M phenotype ( 0.1%). By the disc diffusion method, these five isolates exhibited a reduced zone of inhibition (diameter 10–20 mm) around the erythromycin disc (15 IU), with neither blunting zones around the discs of spiramycin (100 g), lincomyin (15 g) or pristinamycin (15 g) placed nearby, nor zonal lincomycin resistance, even after prolonged incubation (72 h). MICs of 12 MLS drugs, determined by a agar dilution method in Mueller– H i n t o n medium supplemented with 5% horse blood, confirmed that these isolates were low-level resistant to erythromycin (2–8 mg/L) and other 14and 15-membered macrolides, although the intrinsically more active new ketolide HMR3647 retained a significant activity; in contrast, they remained fully susceptible to 16-membered macrolides, lincosamides, and A (pristinamycin II) and B (pristinamycin I) streptogramin components (Table). The DNA of the five isolates was amplified using primers specific to the mefA or mefE gene as previously described. All isolates except for one (S. pneumoniae No. 4) yielded a PCR product of the expected size (1.2 kb), whereas no amplification was obtained with the DNA of negative control isolates (sensitive or MLSB phenotype isolates). The amplicons were analysed by enzymatic restriction using four endonucleases designed to differentiate mefA and mefE: the ClaI and HindIII enzymes cleave only the mefA gene in two fragments (0.701 and 0.517 kbp for ClaI, and 0.89 and 0.328 kbp for H i ndIII), whereas A c cI and H h aI enzymes cleave only the mefE gene in two fragments (0.651 and 0.567 kbp for AccI, and 0.8 and 0.418 kbp for HhaI). The results showed that four isolates carried an mefE gene, whereas the remaining one (S. pyogenes No. 2) possessed an mefA gene (Table). Thus, there is no species-specific distribution of the mefA or mefE gene, and mefE appears to be predominant among streptococci with an M phenotype as previously observed for S. agalactiae and pneumococci. The S. pneumoniae No. 4 isolate carried neither an mef gene nor an erm gene, using PCR with primers specific for ermA (erm1, 5 -TCTAAAAAGCATGTAAAAGAA-3 ; erm2, 5 -CTTCGATAGTTTATTAATATTAGT-3 ), ermB (erm3, 5 -GAAAAGRTACTCAACCAAATA-3 ; erm4, 5 -AGTAACGGTACTTAAATTGTTTAC-3 ), ermC (erm5, 5 -TCAAAACATAATATAGATAAA-3 ; erm6, 5 -GCTAATATTGTTTAAATCGTCAAT-3 ) and