Identification and characterization of plasmid-borne erm(T) macrolide resistance in group B and group A Streptococcus

One hundred and seven group B Streptococcus (GBS) isolates and 344 group A Streptococcus (GAS) isolates were collected between 2005 and 2009 from 2 area hospitals and studied for resistance to erythromycin (ERY) and clindamycin (CLI) and the presence of the erm(T) macrolide resistance gene. The erm(T) gene was found in 5 (8%) of 61 erythromycin nonsusceptible GBS isolates and in 22 (55%) of 40 erythromycin nonsusceptible GAS isolates. The erm(T) gene in all 27 GBS/GAS erm(T) gene-positive isolates was located on a plasmid. Three erm(T) gene-positive plasmids were DNA sequenced. Two plasmids (1 each from GBS and GAS isolates) were both 4967 bp in size, contained the erm(T) gene, and differed by only 2 base pairs, suggesting interspecies horizontal transfer of the erm(T) gene containing plasmid. The third (GBS) plasmid was 6825 bp in size and contained GBSi1, a group II bacterial intron, as well as the erm(T) gene. Pulsed-field gel electrophoresis of all 27 erm(T) gene containing isolates and a selection of erm(T) gene-negative isolates indicated possible clonal expansion among erm(T) gene containing GAS isolates, but not among the 5 erm(T) gene-positive GBS isolates.