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

Antimicrobial resistance (AMR) is a growing threat to both animal and human health. This study investigated the occurrence of AMR in Escherichia coli isolated from fecal samples collected from beef cow-calf operations and compared the frequency of antimicrobial-resistant fecal E. coli among small, medium, and large beef cow-calf operations in Florida, United States. The study included nine beef cow-calf operations. Between December 2023 and April 2024, a total of 743 fecal samples were collected from cows (n = 429) and calves (n = 314), either directly from the rectum or from fresh voided feces. From these samples, 3475 E. coli isolates (five isolates/animal) were subjected to antibiotic susceptibility testing using the Kirby-Bauer disk diffusion method. A panel of eight antibiotics was used to assess AMR profiles. Irrespective of farm size, cows and calves showed higher resistance to streptomycin (47% or 330/695), oxytetracycline (46% or 319/695), sulfadimethoxine (42% or 291/695), ampicillin (41% or 283/695), and florfenicol (18% or 126/695). In contrast, lower resistance frequencies were observed for gentamicin (4% or 27/695), ceftiofur (3% or 19/695), and trimethoprim-sulfamethoxazole (3% or 18/695). Animals from medium (OR = 1.67) and large operations (OR = 2.70) had greater odds of harboring E. coli resistant to ≥1 antibiotic than those from small operations. Streptomycin (medium OR = 1.92; large OR = 4.17) and sulfadimethoxine resistance (medium OR = 1.64; large OR = 3.45) were also more frequent in medium and large operations, respectively. Additionally, E. coli resistance to florfenicol was higher in calves (OR = 3.57) than in cows, after controlling for farm size. This study provides new insights into AMR patterns in fecal bacteria from beef cow-calf operations in Florida and can help producers and veterinarians develop informed strategies for monitoring and mitigating AMR.