A composite transposon associated with erythromycin and clindamycin resistance in group B Streptococcus

Group B Streptococcus (GBS) resistant to erythromycin and clindamycin has been isolated with increasing frequency since the mid-1990s. This work studied GBS isolates from three US cities to determine the genetic basis of the macrolide resistance phenotype. ermB genes were amplified from five isolates collected in Boston, Pittsburgh and Seattle from infant and adult sources. Gene-walking methods were used to determine the chromosomal location of ermB and to identify associated genes. Southern mapping and random amplified polymorphic DNA (RAPD) analyses were used to distinguish the isolates. The ermB gene was present on the chromosome within a composite Tn917/Tn916-like transposon similar to one identified in Streptococcus pneumoniae. Four strains from Boston and Pittsburgh were serotype V and identical by Southern hybridization and RAPD analysis. The Seattle isolate was serotype Ib, with different patterns on RAPD analysis and Southern mapping. The composite transposon was integrated at an identical chromosomal site in all five isolates. The presence of this composite transposon in both GBS and pneumococci suggests that ermB-mediated macrolide resistance in streptococci may be due to the horizontal transfer of a mobile transposable element, and raises concern for further dissemination of high-grade erythromycin and clindamycin resistance among streptococcal species.