Polyclonal spread of erythromycin-resistant Streptococcus agalactiae in southern Taiwan

Resistance to erythromycin is common among Streptococcus agalactiae in Taiwan, however the genetic relatedness of erythromycin-resistant isolates has not yet been reported. From 1991 to 2001, 629 clinical isolates of S. agalactiae were collected in a medical center at Tainan in southern Taiwan, of which 189 (30.0%) were resistant to erythromycin. The isolation rate of erythromycin-resistant group B streptococcus (GBS) was stable, irrespective of the clinical sources or study period. Among them, 145 (76.7%) isolates showed the macrolide-lincosamide-streptogramin B (MLS)-resistant phenotype, and 44 (23.3%) had the macrolide (M)resistant phenotype. Of the isolates with MLS phenotype, 141 (97.2%) isolates harbored the ermB gene alone and only three (2.1%) the ermTR gene, whereas 41 (93.2%) of 44 isolates with M phenotype harbored the mefA/E gene. Of 177 typeable isolates, there were 26 unrelated pulsed-field gel electrophoresis (PFGE) patterns. PFGE type 1 accounted for 17.8% (24/135) of MLS phenotype isolates with the ermB gene and 48.7% (18/37) of M phenotype isolates with the mefA/E gene. During the study period, the proportion of PFGE type 6 decreased significantly, whereas that of type 8 increased. Our results suggest that erythromycin resistance is not uncommon among clinical isolates of S. agalactiae and is, at least, partially related to polyclonal spread in southern Taiwan.