Macrolide resistance genes in Enterococcus spp.

Seventy-eight isolates of different Enterococcus species (E. faecalis, n = 27; E. faecium, n = 23; E, durans, n 8; E. avium, n = 6; E. hirae, n = 9; E . gallinarum, n = 3; and E. casseliflavus, n = 2) with a variety of erythro mycin resistance phenotypes were examined for the presence of macrolide res istance genes (ermA, ermB, ermC, ermTR, mefA/E, and msrA), Positive PCR amp lifications of ermB were obtained for 39 of 40 highly erythromycin-resistan t Enterococcus isolates (MICs, >128 mu g/ml) of different species; the rema ining highly resistant E. faecium isolate was positive for PCR amplificatio n of ermA but was negative for PCR amplification of the ermB and ermC genes . For all enterococcal strains for which erythromycin MICs were less than o r equal to 32 mu g/ml PCRs were negative for erm methylase genes. For all E . faecium isolates PCR amplified products of the expected size of 400 bp we re obtained when msrA primers were used, with the results being independent of the erythromycin resistance phenotype, All the other enterococcal speci es gave negative results by msrA PCRs, Sequencing of the msrA PCR products from either erythromycin-susceptible, low-level-resistant, or highly resist ant E. faecium strains showed that the amplicons did not correspond to the msrA gene described for Staphylococcus epidermidis but corresponded to a ne w putative efflux determinant, which showed 62% identity with the msrA gene at the DNA level and 72% similarity at the amino acid level. This new gene was named msrC.