Enterococcus is a potential reservoir associated with antibiotic resistance in the "environment-animal-human" chain of dairy farms: Evidence from ecological linkage.

Employing the "One Health" framework, this study investigated the diversity of recovered isolates, antibiotic resistance profiles, and the distribution of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) in culturable bacterial isolates from dairy farms to evaluate the co-occurrence and ecological linkage of antibiotic resistance across the environment-animal-human chain. A total of 94 air, cow feces (CF), and worker feces (PF) samples were collected, yielding 228 bacterial isolates. All isolates were tested against 41 antibiotics and screened for 118 ARGs, MGEs, and virulence genes; 61 Enterococcus isolates were further typed by MLST. Enterococcus (26.75%, 61/228) and Enterobacter (23.68%, 54/228) were the most frequently recovered genera among culturable bacterial isolates. Over 50% of isolates were resistant to multiple major antibiotic classes, and the overall prevalence of multidrug-resistance (MDR) was 79.17%. A total of 79 ARGs/MGEs were identified, with 33 forming a shared resistome. Statistical analyses revealed that the recovery proportion of Enterococcus and MDR prevalence were comparable between PF and CF samples (χ2 = 4.38, P = 0.036; χ2 = 0.006, P = 0.941). The total proportion of ARGs/MGEs was significantly higher in CF than in PF and air samples, and higher in PF than in air samples (all P < 0.05). Enterococcus was the major ARG reservoir (56 types, 47.45%) and carried high prevalence of MDR (85.25%) and virulence genes (e.g., cpd, 75.41%). MLST identified hospital-associated E. faecalis clones across sample types. In conclusion, Enterococcus serves as a putative core taxon and potential reservoir of ARGs and high-risk clones in dairy farms, with ecological linkage to the distribution of these resistance-related traits across the environment-animal-human continuum, highlighting a potential public health concern that warrants continued surveillance.