Purpose: The purpose of this study was to determine the AMR patterns of generic Escherichia coli isolated from feedlot cattle fed stem-flaked, corn-based diets supplemented with either DFM (L. salivarious L28) or tylosin.
Methods: A newly isolated DFM, L. salivarious L28, was used in this study. Three treatments based on conventional high concentrate diets were fed to finish cattle for harvest: base (no DFM, tylosin, or monensin), MonPro (DFM with monensin, but no tylosin), and a control (tylosin and monensin). A total of 36 composite fecal samples from beef cattle feedlot pens, housing 3 animals per pen, were collected after 56 days of feeding each treatment. Samples were weighed, enriched, and plated onto MacConkey agar, and three isolates were randomly selected and streaked onto blood agar plates for antimicrobial resistance analysis using Sensititre™ susceptibility MIC plates, following the National Antimicrobial Resistance Monitoring System (NARMS) protocol.
Results: Generic E. coli was isolated from 100% (n=36) of fecal samples collected. After 56 days of feeding, 61% (n=22) of the control group isolates were resistant to at least one antibiotic and 11% (n=4) were multi-drug resistant (MDR) to three or more antibiotics. Isolates from the base treatment group, on day 56, exhibited 56% (n=20) resistance to one drug and 19% (n=7) were MDR. Thirty three percent of isolates in the MonPro group showed resistance on day 56, where only 6% (n=2) of isolates were MDR.
Significance: With the recent restrictions of antibiotic use (tylosin) in cattle feeding operations, this study suggests that using the DFM, L. salivarious L28, may help reduce the rates of emergence of AMR to E. coli in cattle.