Mitigation of antimicrobial drug resistance and changes in the microbiome of feedlot cattle supplemented with Lactobacillus salivarius L28 as an alternative to sub-therapeutic antibiotics
English, Andrea R.
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The objectives of this study were to evaluate the efficacy of utilizing L. salivarius L28 as a direct-fed microbial in feedlot cattle to 1) reduce the frequency of pathogen shedding, 2) determine the AMR in commensal and pathogenic bacteria and investigate the microbiome and cytokine changes. A total of three dietary treatments (control, base and monpro) based on conventional high concentrate diets were fed to finish cattle (n=144) for harvest. Fecal samples were collected every 28 days until harvest at 140 days. Traditional culture methods were used to isolate bacteria. Antibiotic susceptibility testing was done using the micro-broth dilution (Sensititre™) susceptibility minimum inhibitory concentration plates, following the National Antimicrobial Resistance Monitoring System protocol. For microbiome analysis DNA was extracted from fecal samples and 16s-metageomics was performed on an Illumina Miseq. Blood samples were collected for cytokine and chemokine analysis on day 140 from 1 animal per pen. The base treatment group shed significantly less (10.4%; P = 0.04) E. coli O157 compared the control (19.4%) and monpro (12.0%) groups. No significant differences were detected across treatment for the presence of Salmonella over time. At slaughter, one carcass tested positive for E. coli O145 at pre-evisceration. Overall resistance for generic E. coli to at least one antimicrobial for each treatment group was 39.8%, 44.2% and 18.3% for treatments base, control and monpro, respectively. Enterococcus resistance was much higher overall with base, control and monpro having 87.8%, 93%, and 83.3% of isolates with resistance, respectively. Enterococcus isolates were most commonly resistant to lincomycin, tetracycline, tylosin tartrate, daptomycin and erythromycin. Relative abundance of bacteria in the phyla Bacteroidetes increase over time with the overall reduction in relative abundance of all other bacteria. There were no changes based on treatment following application of either control or monpro regiments. For the base treatment group there was the greatest changes over time. No differences across treatments were detected for cytokine and chemokine quantification. Cattle supplemented with sub-therapeutic antimicrobials had a higher rate of E. coli O157 present in the feces. While feeding L28 or tylosin had no effect on the presence of Salmonella. Supplementation of cattle with L28 was the most effective at mitigating the AMR of generic E. coli. However, marginal effects on the AMR of Enterococcus isolates collected. Supplementation had no apparent negative effects on the microbiome or cytokine levels of cattle during the feeding trial.