# Analysis of Antimicrobial Residues and Resistance Profiles of Escherichia coli and Enterococcus spp. in Lagoon Water from California Dairies

## TL;DR

This study examines antibiotic residues and resistance in bacteria from dairy lagoon water, highlighting the need for better monitoring and responsible antibiotic use.

## Contribution

The study identifies specific antibiotic residues and their associations with bacterial resistance in dairy lagoon water.

## Key findings

- Florfenicol and tilmicosin residues were detected in over 90% of lagoon water samples.
- Tilmicosin residues were linked to reduced resistance in E. coli to certain antibiotics.
- Sulfamethoxazole residues were associated with increased tetracycline resistance in Enterococcus spp.

## Abstract

Antibiotics play a vital role in treating sick animals on dairy farms, but they also contribute to antibiotic resistance, which impacts human, animal, and environmental health. In our study of lagoon water samples from nine California dairy farms, we frequently found residues of antibiotics, including florfenicol, tilmicosin, and tetracycline, while penicillin was much less common. The high florfenicol levels may be attributed to its use in young animals, as it is not approved for adults. We also tested Escherichia coli and Enterococcus spp./Streptococcus spp. (ES) bacteria for antibiotic resistance and observed seasonal, regional, and sampling variability in levels of resistance, though results were not statistically significant. Notably, specific antibiotics showed some associations with bacterial resistance; for instance, tilmicosin residues corresponded with lower E. coli resistance to certain antibiotics such as danofloxacin and enrofloxacin, while sulfamethoxazole was linked to increased tetracycline resistance in ES. Overall, our findings highlight the complexity of antibiotic resistance on dairy farms and underscore the need for improved monitoring and prudent antibiotic use to mitigate these risks.

The widespread use of antimicrobial drugs (AMDs) in livestock production contributes to antimicrobial resistance (AMR), a global One Health concern affecting humans, animals, and the environment. This study analyzed AMD residues and the AMR profiles in Escherichia coli and Enterococcus spp./Streptococcus spp. (ES) isolated from lagoon water samples collected from nine California dairies. Antimicrobial susceptibility testing was performed using the microbroth dilution method, and enzyme-linked immunosorbent assay (ELISA) kits were used to detect AMD residues in lagoon water. Overall, residues of florfenicol and tilmicosin were detected in more than 90% of the samples, while tetracycline was detected in 74.2 ± 4.6% of the samples. In contrast, penicillin and sulfamethazone residues were low, observed in only 3.4 ± 1.9% and 32.3 ± 5.0% of samples, respectively. The very low prevalence of penicillin was likely due to limited use in dairy cattle, given its prolonged withdrawal period. Prevalence estimates for AMR in the lagoon samples showed 100% resistance of E. coli to tiamulin, tilmicosin or tylosin and high prevalence against florfenicol (96.0% ± 2.0) or gamithromycin (92.0% ± 1.9). However, low AMR estimates (less than 10%) were observed against other AMDs tested. Similarly, the prevalence estimates for AMR of ES isolates in the studied lagoon were high against florfenicol (95.1% ± 2.0), tildipirosin (97.6% ± 1.7), or tilmicosin (98.8% ± 1.2), but low against ampicillin (4.9% ± 1.9) and penicillin (8.5% ± 2.4). Despite numerical differences in AMR prevalence by season, region, and sampling point, these variations were not statistically significant. Logistic regression models were applied to explore associations between AMD residues and AMR phenotypes where appropriate. Tilmicosin residues were significantly associated with reduced resistance to danofloxacin, enrofloxacin, and tildipirosin in E. coli isolates, while sulfamethoxazole residues were linked to increased tetracycline resistance in Enterococcus spp. The presence of florfenicol residues, potentially originating from treated calves and heifers, helps explain the high prevalence of resistance to this drug in both bacterial species. However, not all AMD residues were associated with AMR, underscoring the complex ecological and genetic factors involved in the development and maintenance of resistance in dairy environments. These findings underscore the importance of integrating AMR surveillance and prudent AMD use practices across all segments of dairy production systems.