| dc.description.abstract | Antimicrobial resistance (AMR) represents a growing crisis in both human and veterinary medicine. We evaluated the use of two antibiotic alternatives – heavy metals and essential oils – in beef cattle and their effects on gram-negative and gram-positive bacteria. Further, we explored whether direct-fed microbials and a tylosin voluntary withdrawal period would decrease macrolide resistance among enterococci.
In the first randomized and controlled field trial, we measured the impact of supplemental zinc and menthol on antimicrobial resistance among commensal enteric bacteria of feeder cattle. Fecal suspensions were plated onto plain- and antibiotic-supplemented MacConkey and m-Enterococcus agar for quantification of total and antimicrobial-resistant Escherichia coli and Enterococcus spp., respectively. Temporal effects on overall E. coli growth were significant (p<0.05); however, there were no significant effects on antibiotic-supplemented agar. Zinc was associated with significant increases in growth on erythromycin-supplemented m-Enterococcus agar. Cattle fed zinc had significantly higher macrolide resistance among fecal enterococci isolates.
In the second field trial, within two sequential replicates (n=90 and n=96 finisher cattle, respectively) we measured the impact of an Enterococcus faecium-based probiotic (DFM) and an altered pen environment on antimicrobial resistance among fecal enterococci in cattle fed 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. This 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. E. faecium isolates were sequenced, which yielded sequence types (ST), resistance genes and phylogeny. Samples of the DFM were sequenced and found to contain E. faecium ST296, which was not present on Day 0 of either replicate. This DFM sequence type was found in fecal samples after Day 0, the majority of which were isolated from cattle in one of the DFMfed pens. Increased prevalence of ST296 occurred with a concomitant decrease in ST240; of note, the latter typically harbored both ermB and tet(M).
We also explored the effects of the same trial factors on the fecal microbiome using 16S rRNA metagenomics. Bacterial taxonomic analyses based on alpha- (Shannon index) and beta-diversity (Bray-Curtis dissimilarity index), were not different (P>0.05) across treatment groups, suggesting there was no effect of the DFM or tylosin on the microbiome. Period effects, independent of trial factors, differed at P < 0.05.
In conclusion, zinc and menthol were not associated with significant changes in antibiotic resistance among Escherichia coli; however, excess dietary zinc was associated with increased macrolide resistance among enterococci. Tylosin exhibited a significant (P<0.05) – though inconsistent – effect on macrolide resistance. An E. faecium-based probiotic mitigated macrolide resistance, with an increase in the relative prevalence of ST296 (i.e., probiotic strain) accompanying a decrease in multi-drug resistant ST240 strains. Of importance, the addition of tylosin, a probiotic, or both did not affect the more abundant components of the fecal microbiome of finisher steers. | en |
