Effects of a Novel Direct-fed Microbial on Occurrences of Antimicrobial Resistance in Salmonella enterica, Escherichia coli, and Enterococcus spp. Measured Longitudinally From Feedlot Arrival to Harvest in Finishing Beef Steers

Before implementation of the Veterinary Feed Directive in 2017, medically important antimicrobials, like tylosin, were approved for both therapeutic and subtherapeutic use. Nevertheless, subtherapeutic practices are now considered injudicious because their use increases antimicrobial resistance risk. Therefore, heightened consumer concerns have increased the interest in antimicrobial alternatives like direct-fed microbials. Two-hundred forty Angus beef steers (mean initial BW = 263 kg ± 18.0 kg) were assigned randomly to one of three dietary treatments; negative control, dietary supplement contained no tylosin (NCON); positive control, dietary supplement contained tylosin (PCON); or novel direct-fed microbial fed at 1 g mixture/steer with 1 × 10¹¹ CFU/g (DFM). Fecal samples were collected on days 0, 59, 128, and at study end. Pen and hide swabs were collected two days before harvest, and subiliac lymph nodes were collected on the day of harvest. All targeted bacterial populations differed across time (p ≤ 0.05), except 128ERY^REscherichia coli. Fecal Salmonella concentration and prevalence differed among dietary treatments (p = 0.02) with NCON having greater fecal Salmonella concentrations than PCON and DFM. No differences in Salmonella prevalence among pen swabs, hide swabs, or subiliac lymph nodes were detected (p ≥ 0.40). Salmonella resistant to tetracycline or cefotaxime were not detected in feces. The effect of treatment differed by day for total and 128ERY^REnterococcus spp. concentrations. Total Enterococcus spp. concentrations were greatest for the DFM treatment on day 128 and at study end (p ≤ 0.01). At study end, 128ERY^REnterococcus spp. concentrations were greatest for PCON (p ≤ 0.01). Total, TET^R, COT^R, and CTX^RE. coli concentrations increased from d 0 to study end among treatments (p ≤ 0.01). These data suggest that the in-feed inclusion of a novel direct-fed microbial is not directly implicated in the antimicrobial resistance of feedlot beef cattle.