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

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 <i>Escherichia coli</i> and <i>Enterococcus</i> 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 <i>E. coli</i> and 428 <i>Enterococcus</i> spp. isolates were recovered from rectal swabs, with <i>E. coli</i> showing the highest resistance to ampicillin (12.5%) and amoxicillin-clavulanic acid (6.7%), whereas resistance to tetracycline was rare (0.3%). <i>Enterobacteriaceae</i> from milk exhibited higher resistance to ampicillin (45.8%) and amoxicillin-clavulanic acid (20.8%). Among <i>Enterococcus</i> 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.