Virulence Genes, Biofilm Formation, and Antimicrobial Resistance of <i>Enterococcus Faecalis</i> and <i>Enterococcus Faecium</i> Isolated from Bovine Mastitis Milk Samples from 2015 to 2020

Abstract

Enterococcus spp. are pathogens that cause environmental mastitis and are difficult to eliminate owing to their resistance to disinfectants and antibiotics. Matrix-Assisted Laser Desorption Ionization – Time of Flight Mass Spectrometry was used to identify 122 Enterococcus spp. isolated from bovine mastitis milk and normal raw milk collected from 119 dairy farms in South Korea between 2015 and 2020 and the majority of species were identified as Enterococcus faecalis (73.8%) followed by Enterococcus faecium (26.2%). Among these 70 bovine mastitis milk isolates, 85.7%, 71.4%, 54.3%, and 30.0% of the E. faecalis isolates harbored the gelE, asa1, esp, and cylA genes, respectively, and were significantly more abundant in bovine mastitis milk than in normal raw milk, except for asa1 (p < 0.0001). Interestingly, all isolates formed biofilms, but strong biofilm formation was predominately observed for bovine mastitis milk isolates, and this was significantly related to the presence of the esp, and gelE genes (p < 0.05). E. faecalis and E. faecium isolates from bovine mastitis milk demonstrated higher resistance to tetracycline (59.3%) followed by chloramphenicol (21.0%), rifampin (18.5%), doxycycline (4.9%), ciprofloxacin (1.2%), and nitrofurantoin (1.2%) compared with those from normal raw milk. Our results emphasize the importance of monitoring and preventing outbreaks of bovine mastitis caused by Enterococcus spp., which may act as a reservoir for the transmission of virulence or antibiotic resistance genes to humans.