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

## TL;DR

This study examines antibiotic resistance in gut bacteria from healthy Latvian dairy cows, finding low resistance overall but noting higher risks on larger farms.

## Contribution

The study provides new data on antimicrobial resistance in indicator bacteria from clinically healthy Latvian dairy cattle, emphasizing farm size as a potential risk factor.

## Key findings

- Resistance levels in Escherichia coli and Enterococcus spp. from healthy Latvian dairy cows were generally low.
- Multidrug resistance was more common in isolates from larger farms and milk samples.
- Enterococcus spp. showed high resistance to quinupristin–dalfopristin, an antibiotic not used in Latvian dairy farming.

## Abstract

Antimicrobial resistance (AMR) is a growing global concern because bacteria that no longer respond to antibiotics can spread between animals, the environment, and humans. Within the One Health framework, dairy production systems play an important role in this process, as bacteria from healthy cows can be disseminated through milk, manure, and the farm environment. In this study, bacteria isolated from clinically healthy dairy cows in Latvia were examined to determine their resistance to antibiotics that are critically important in human medicine. Rectal swab samples were collected from cows to isolate two common gut bacteria as indicators of cumulative antimicrobial pressure, and milk samples were also taken from bulk tanks to assess AMR relevant to food safety. Overall, resistance levels were low, and most bacteria remained susceptible to critically important antibiotics. However, some bacteria showed resistance to multiple drugs, particularly those isolated from larger farms, suggesting that farm conditions may increase the risk of more resistant bacteria developing and spreading. These findings highlight the importance of responsible and prudent antibiotic use on farms and the need for continued monitoring of antibiotic resistance. Understanding how resistance develops and spreads helps protect animal health, food safety, and public health.

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.