Dissemination and Characteristics of High-Level Erythromycin-Resistant Enterococcus Faecalis From Bulk Tank Milk of Dairy Companies in Korea

Background: Enterococci are environmental pathogens that can cause bovine mastitis and macrolides are widely used for the treatment of bovine mastitis caused by staphylococci and streptococci/enterococci. The aim of this study was performed to compare the phenotypic and genotypic characteristics of high-level erythromycin-resistant (HLER) Enterococcus faecalis ( E. faecalis ) collected from bulk tank milk of four dairy companies (A, B, C, and D) in Korea. Results: : Although isolates from company D showed the highest prevalence of E. faecalis , the prevalence of HLER E. faecalis in company A (73.1%) and C (57.0%) was significantly higher than company D (33.9%) ( P < 0.05). A total of 149 HLER E. faecalis isolates showed high rates of resistance to tetracycline (93.3%), followed by doxycycline (70.0%), and chloramphenicol (48.3%). In the distribution of macrolides resistance genes, 147 (98.7%) isolates carried ermB gene alone, and two isolates carried both ermA and ermB genes. No isolates carried ermC , msrA , msrC , or mef genes. In the distribution of other resistance genes, 72 (48.3%) and 60 (40.3%) isolates carried both tetM and tetL genes, and tetM gene alone, respectively, and 38 (25.5%) isolates carried optrA gene. For aminoglycosides resistance genes, the prevalence of both aac(6′)Ie-aph(2″)-la and ant(6′)-Ia genes (43.0%) was the highest. Moreover, 104 (70.0%) isolates harbored Int-Tn gene carrying the Tn916/1545 -like transposon. Although the distribution of ermB gene showed no significant difference between the dairy companies, the prevalence of other resistance genes and transposons showed a significant difference between the dairy companies ( P < 0.05). Virulence genes, such as ace (99.3%), cad1 and efaA (each 98.7%), and gelE (83.9%), were also highly conserved in the 149 HLER E. faecalis isolates . Conclusions: : Our results indicated that HLER E. faecalis isolates from bulk tank milk showed significant differences in phenotypic and genotypic characteristics between the dairy companies. In addition, the prevalence of resistance genes and virulence factors was also high in HLER E. faecalis isolates.