Antimicrobial Resistance in Indicator Microorganisms Escherichia coli and Enterococcus spp. from Healthy Dairy Cattle...

Antimicrobial resistance (AMR) in food-producing animals is a growing One Health concern. However, data on AMR in indicator microorganisms from clinically healthy dairy cattle in Latvia remain limited. This study aimed to characterize the AMR profiles of Escherichia coli and Enterococcus spp. isolated from rectal swabs and bulk-tank milk collected from 18 dairy farms between February and May 2025. Bacterial identification was performed using conventional culturing and MALDI-TOF mass spectrometry, and antimicrobial susceptibility was determined using the disk diffusion (Kirby-Bauer) method, interpreted according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines. Resistance levels were further quantified using resistance scores (R-scores) and the Multiple Antibiotic Resistance Index (MARI). In total, 582 E. coli and 428 Enterococcus spp. isolates were recovered from rectal swabs, with E. coli showing the highest resistance to ampicillin (12.5%) and amoxicillin-clavulanic acid (6.7%), whereas resistance to tetracycline was rare (0.3%). Enterobacteriaceae from milk exhibited higher resistance to ampicillin (45.8%) and amoxicillin-clavulanic acid (20.8%). Among Enterococcus spp., resistance was highest to an antibiotic not used in dairy cows in Latvia quinupristin-dalfopristin (69.2%), while resistance to vancomycin and linezolid remained low (0.5% each). Milk-derived enterococci showed a comparable pattern, with additional resistance to streptomycin (25%). Overall, resistance levels and multidrug resistance were low. However, the presence of sporadic resistant isolates and elevated MARI values, particularly in large-scale farms and milk-derived bacteria, highlights the importance of continued AMR surveillance and prudent antimicrobial use in the Latvian dairy sector.