Frequency of Antimicrobial-Resistant Fecal <em>Escherichia coli</em> Among Small, Medium, and Large Beef Cow-Calf Operations in Florida

Antimicrobial resistance (AMR) is a growing threat to both animal and public health. This study describes the occurrence of AMR in Escherichia coli isolated from fecal samples from beef cow-calf operations and compares the frequency of antimicrobial-resistant fecal E. coli among small, medium, and large beef cow-calf operations in Florida, United States. This study was conducted using nine beef cow-calf operations. During farm visits conducted between December 2023 and April 2024, 743 fecal samples from cows (n=429) and calves (n=314) were collected either directly from the rectum or from fresh defecated feces. A total of 3,475 E. coli isolates (five isolates/animal) obtained from fecal samples of 695 cows and calves were selected for antimicrobial susceptibility testing. The antimicrobial susceptibility profile of each E. coli strain was determined by the Kirby-Bauer disk diffusion method. A panel of eight antibiotics was used to assess AMR in the fecal E. coli isolates. Irrespective of farm size, cows and calves showed higher resistance to streptomycin (47% and 30%), oxytetracycline (46% and 34%), sulfadimethoxine (42% and 27%), ampicillin (41% and 22%), and florfenicol (18% and 13%) at animal and isolate levels, respectively. In contrast, lower resistance frequencies were observed for gentamicin, ceftiofur, and trimethoprim/sulfamethoxazole, with values below 5% at the animal level, and approximately 1% at the isolate level. E. coli isolated from cows and calves showed higher resistance to one or more antibiotics in large beef cow-calf operations (66%) as compared to small (41%) and medium (53%) beef cow-calf operations (p &amp;lt; 0.0001). This study provides new data on AMR in fecal bacteria from beef cow-calf operations in Florida, which can help researchers, veterinarians, and producers develop strategies for monitoring and reducing AMR in these operations.