Isolation of Erythromycin (EM)-Resistant Enterococci Carrying Erm(B) From Aquaculture Water and Investigation of the Potential Conjugative Transfer of EM Resistance to Emerging Fish Pathogens, Lactococcus formosensis and Lactococcus garvieae Serotype III.

Abstract

Erythromycin (EM) resistance in the emerging and prevalent fish pathogen Lactococcus formosensis is mediated by the erythromycin ribosomal methylase B gene [erm(B)], which is located on a transferable R-plasmid. The R-plasmid structure in EM-resistant L. formosensis was highly homologous to that of the drug resistance plasmid pAMβ1 found in Enterococcus faecalis. Thus, we speculate that the source of the erm(B) gene in fish-pathogenic L. formosensis may have been environmental bacteria in aquaculture areas, particularly enterococci, such as E. faecalis and Enterococcus faecium. However, no direct evidence has been obtained for the genetic exchange of the EM-resistant trait between EM-resistant enterococci and fish-pathogenic lactococci in aquaculture environments. This study aimed to isolate enterococci resistant to EM carrying the erm(B) gene from seawater in aquaculture areas. The detection levels of EM-resistant enterococci ranged from below the detection limit to 130/100 mL as determined using the most probable number method. Among the 17 erm(B)-positive isolates, five EM-resistant enterococcal strains transferred EM resistance to the recipient fish pathogens (L. formosensis or L. garvieae serotype III). These erm(B)- carrying EM-resistant enterococci (n = 5) exhibited high minimal inhibitory concentrations (MIC) for EM, lincomycin (LCM), and oxytetracycline (OTC). Recipient strains that acquired erm(B) displayed elevated MIC values against EM and LCM, but not against OTC. These results suggest that EM-resistant enterococci carrying erm(B) are present in aquaculture environments, and that the erm(B) gene can potentially be transferred to fish pathogens.