Farm management factors associated with antimicrobial resistance in dairy herds: Insights from a bulk tank milk study

Antimicrobial resistance (AMR) in dairy production represents an important One Health concern, particularly in countries with predominantly small-scale farms and limited AMR surveillance in livestock. This cross-sectional study investigated the herd-level prevalence of selected antibiotic-resistant bacteria in bulk tank milk (BTM) from 100 conventional dairy farms across seven districts in Serbia and evaluated associations with farm management practices. Samples were screened for methicillin-resistant Staphylococcus aureus (MRSA), extended-spectrum beta-lactamase (ESBL)- and AmpC beta-lactamase-producing Escherichia coli, carbapenemase-producing E. coli (CPE), colistin-resistant E. coli (ColR-E), and vancomycin-resistant Enterococcus spp. (VRE). Isolates from selective media were identified and characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), antimicrobial susceptibility testing, and PCR-based confirmation of antimicrobial resistance genes (ARGs). Farm management data were collected through structured questionnaires and analysed using logistic and Firth's penalised regression models. MRSA was confirmed on five farms and ESBL E. coli on four farms, while VRE, CPE, and ColR-E were not detected. All confirmed MRSA and ESBL E. coli isolates carried the corresponding ARGs, and most isolates were multidrug-resistant (MDR). While most farm management variables showed no significant association with antibiotic-resistant bacteria, higher average daily milk production per cow was significantly linked to ESBL E. coli prevalence. Although the overall AMR prevalence was low, the detection of MDR strains highlights the importance of continued surveillance. BTM analysis proved to be a practical tool for farm-level AMR monitoring and may inform targeted antimicrobial stewardship strategies across countries, including the dairy sector in Serbia.