Heterogeneity of Macrolide-Lincosamide-Streptogramin B Resistance Phenotypes in Enterococci

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Abstract
We determined the macrolide resistance phenotypes of 241 clinical isolates of erythromycin-resistant enterococci (MICs, ≥1 μg/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 μg)-josamycin (100 μg) double-disk test, 93 strains were assigned to the constitutive macrolide, lincosamide, and streptogramin B (MLS B ) resistance (cMLS B ) phenotype, and the remaining 148 strains were assigned to the inducible MLS B resistance (iMLS B ) phenotype. Of the strains with the iMLS B phenotype, 36 exhibited a reversibly inducible MLS B (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 ≥0.1 μg/ml. These findings highlight the versatility of erm (B) in induction specificity.