Heterogeneity of macrolide-lincosamide-streptogramin B resistance phenotypes in enterococci

We determined the macrolide resistance phenotypes of 241 clinical isolates of erythromycin-resistant enterococci (MICs, greater than or equal to1 mug/ml), including 147 Enterococcus faecalis strains and 94 Enterococcus faecium strains, collected from a hospital in Seoul, Korea, between 1999 and 2000. By the erythromycin (40 mug) josamycin (100 mug) double-disk test, 93 strains were assigned to the constitutive macrolide, lincosamide, and streptogramin B (MLSB) resistance (cMLS(B)) phenotype, and the remaining 148 strains were assigned to the inducible MLSB resistance (iMLS(B)) phenotype. Of the strains with the iMLS(B) phenotype, 36 exhibited a reversibly inducible MLSB (riMLS(B)) phenotype, i.e., blunting of the erythromycin zone of inhibition, which indicates that the 16-membered-ring macrolide josamycin is a more effective inducer than the 14-membered-ring macrolide erythromycin. Sequence analysis of the regulatory regions of the erm (B) genes from all of the strains exhibiting the riMLS(B) phenotype revealed not only erm(Bv) [where v represents variant; previously erm(AMR)] (n=13), as reported previously, but also three kinds of erm(B) variants, which were designated erm(Bv1) (n=17), erm(Bv2) (n=3), and erm(Bv3) (n=3), respectively. In lacZ reporter gene assays of these variants, the 16-membered-ring macrolide tylosin had stronger inducibility than erythromycin at greater than or equal to0.1 mug/ml. These findings highlight the versatility of erm(B) in induction specificity.