Dissemination and characteristics of high-level erythromycin-resistant Enterococcus faecalis from bulk tank milk of dairy companies in Korea

Enterococci are environmental pathogens that can cause bovine mastitis, which is treated with macrolides, one of which is erythromycin (ERY). The aim of this study was to compare the characteristics of high-level erythromycin-resistant (HLER) Enterococcus faecalis (E. faecalis) isolates from bulk tank milk of 4 dairy companies, identified as A to D, in order to assess the threat to public health. Although isolates from company D showed the highest prevalence of E. faecalis, the prevalence of HLER E. faecalis in isolates from company A showed a significant difference. A total of 149 of the 301 HLER E. faecalis isolates showed the highest rate of resistance to tetracycline. In the distribution of antimicrobial resistance genes, 147 isolates carried the ermB gene alone and 2 isolates carried both ermA and ermB genes. Also, 72 and 60 isolates carried both tetM and tetL genes and the tetM gene alone, respectively, and 38 isolates carried the optrA gene. The prevalence of both aac(6ʹ)Ie-aph(20)-la and ant(6ʹ)-Ia genes was the highest and 104 isolates harbored the Int-Tn gene carrying the Tn916/1545-like transposon. Although the distribution of the ermB gene showed no significant difference among dairy companies, the prevalence of other resistance genes and transposons showed significant differences among dairy companies. Virulence genes were highly conserved in the HLER E. faecalis isolates. Our results indicated that there were significant differences in phenotypic and genotypic characteristics of HLER E. faecalis isolates in milk from 4 different dairy companies. A structured management protocol by companies and constant monitoring are therefore necessary to minimize public health hazards.