Effects of Tylosin, a Direct-Fed Microbial and Feedlot Pen Environment on Phenotypic Resistance among Enterococci Isolated from Beef Cattle Feces.

In two sequential replicates ( n = 90 and n = 96 feedlot finisher cattle, respectively) we measured the impact of an Enterococcus faecium -based probiotic (DFM) and an altered feedlot pen environment on antimicrobial resistance among fecal enterococci in cattle fed (or, not fed) the macrolide tylosin. Diluted fecal samples were spiral-plated on plain and antibiotic-supplemented m- Enterococcus agar. In the first replicate, tylosin significantly ( p < 0.05) increased the relative quantity of erythromycin-resistant enterococci. This effect was diminished in cattle fed the DFM in conjunction with tylosin, indicating a macrolide susceptible probiotic may help mitigate resistance. A similar observed effect was not statistically significant ( p > 0.05) in the second replicate. Isolates were speciated and resistance phenotypes were obtained for E. faecium and E. hirae . Susceptible strains of bacteria fed as DFM may prove useful for mitigating the selective effects of antibiotic use; however, the longer-term sustainability of such an approach remains unclear.