Quantitative dynamics of <i>Salmonella</i> and <i>E</i>. <i>coli</i> in feces of feedlot cattle treated with ceftiofur and chlortetracycline

Published on 2019-12-02T23:08:37Z (GMT) by
<div><p>Antibiotic use in beef cattle is a risk factor for the expansion of antimicrobial-resistant <i>Salmonella</i> populations. However, actual changes in the quantity of <i>Salmonella</i> in cattle feces following antibiotic use have not been investigated. Previously, we observed an overall reduction in <i>Salmonella</i> prevalence in cattle feces associated with both ceftiofur crystalline-free acid (CCFA) and chlortetracycline (CTC) use; however, during the same time frame the prevalence of multidrug-resistant <i>Salmonella</i> increased. The purpose of this analysis was to quantify the dynamics of <i>Salmonella</i> using colony counting (via a spiral-plating method) and hydrolysis probe-based qPCR (TaqMan<sup>®</sup> qPCR). Additionally, we quantified antibiotic-resistant <i>Salmonella</i> by plating to agar containing antibiotics at Clinical & Laboratory Standards Institute breakpoint concentrations. Cattle were randomly assigned to 4 treatment groups across 16 pens in 2 replicates consisting of 88 cattle each. Fecal samples from Days 0, 4, 8, 14, 20, and 26 were subjected to quantification assays. Duplicate qPCR assays targeting the <i>Salmonella invA</i> gene were performed on total community DNA for 1,040 samples. Diluted fecal samples were spiral plated on plain Brilliant Green Agar (BGA) and BGA with ceftriaxone (4 μg/ml) or tetracycline (16 μg/ml). For comparison purposes, indicator non-type-specific (NTS) <i>E</i>. <i>coli</i> were also quantified by direct spiral plating. Quantity of NTS <i>E</i>. <i>coli</i> and <i>Salmonella</i> significantly decreased immediately following CCFA treatment. CTC treatment further decreased the quantity of <i>Salmonella</i> but not NTS <i>E</i>. <i>coli</i>. Effects of antibiotics on the imputed log<sub>10</sub> quantity of <i>Salmonella</i> were analyzed via a multi-level mixed linear regression model. The <i>invA</i> gene copies decreased with CCFA treatment by approximately 2 log<sub>10</sub> gene copies/g feces and remained low following additional CTC treatment. The quantities of tetracycline or ceftriaxone-resistant <i>Salmonella</i> were approximately 4 log<sub>10</sub> CFU/g feces; however, most of the samples were under the quantification limit. The results of this study demonstrate that antibiotic use decreases the overall quantity of <i>Salmonella</i> in cattle feces in the short term; however, the overall quantities of antimicrobial-resistant NTS <i>E</i>. <i>coli</i> and <i>Salmonella</i> tend to remain at a constant level throughout.</p></div>

Cite this collection

Ohta, Naomi; Norby, Bo; Loneragan, Guy H.; Vinasco, Javier; Bakker, Henk C. den; Lawhon, Sara D.; et al. (2019): Quantitative dynamics of Salmonella and E. coli in feces of feedlot cattle treated with ceftiofur and chlortetracycline. PLOS ONE. Collection. https://doi.org/10.1371/journal.pone.0225697