Molecular Characteristics of Enterococcus faecalis and Enterococcus faecium from Bulk Tank Milk in Korea

SIMPLE SUMMARY: Enterococci can be an opportunistic pathogen in milk, which can easily disseminate antimicrobial resistance and virulence genes. The purpose of this study was to characterize and compare the enterococci isolates from samples of bulk tank milk obtained from four dairy companies in Korea to prevent the spread of pathogenic and antimicrobial-resistant enterococci in dairy companies. The results demonstrated various degrees of antimicrobial resistance and virulence-factor distribution in enterococci from bulk tank milk in Korea and support the assessment that pathogens from bulk tank milk can also become a reservoir for dissemination of antimicrobial resistance and virulence factors through cross-contamination processes. ABSTRACT: Enterococci are considered to be environmental mastitis-causing pathogens that can easily spread antimicrobial resistance or virulence genes via horizontal transfer. In this study, the molecular characteristics of enterococci from bulk tank milk were investigated to assess the importance of dairy herd management. A total of 338 enterococci (305 Enterococcus faecalis and 33 Enterococcus faecium) were isolated from 1584 batches of bulk tank milk samples from 396 farms affiliated with four dairy companies in Korea, and significant differences (40.6–79.7%) (p < 0.05) in the prevalence of enterococci were observed in the samples from different companies. Enterococci showed the highest resistance to tetracycline (TET) (73.4%), followed by doxycycline (DOX) (49.7%) and erythromycin (ERY) (46.2%), while two enterococci isolates showed resistance to vancomycin (VAN). Among 146 tetracycline (TET) and ERY-resistant enterococci, each 50 (19.4%) enterococci carried combination-resistance and transposon gene types erm(B) + tet(M) + IntTn and erm(B) + tet(L) + tet(M) + IntTn, respectively. The virulence genes such as ace (99.0%), efaA (97.7%), cad1 (95.7%), and gelE (85.9%) were highly conserved in E. faecalis and significantly predominated over E. faecium (p < 0.001). Our results indicate that pathogens from bulk tank milk can also become a reservoir for the dissemination of antimicrobial resistance and virulence factors through cross-contamination processes.