Effect of prophylactic and therapeutic antibiotic administration on fecal excretion of antibiotic resistance genes by dairy cows
The objective of this study was to determine the effect of prophylactic and therapeutic antibiotic administration on fecal excretion of antibiotic resistance genes (ARG) in dairy cows. Twelve primiparous lactating Holstein cows were used in a completely randomized design. Four treatments included the administration of cephapirin (intramammary), pirlimycin (intramammary), or ceftiofur (subcutaneous), as well as a control group where no antibiotic was given. Fecal samples were collected from all cows prior to antibiotic administration on d 0 and then on d 1, 3, 5, 7, 14, 21, and 28 following treatment. Fecal samples were freeze-dried and DNA was extracted using the FastDNA® Spin Kit for Soil. Extracted DNA was analyzed using quantitative polymerase chain reaction for genes encoding for resistance to tetracyclines (tetO and tetW) and beta-lactams (ampC) as well as a housekeeping gene (16S rRNA) to monitor bacterial abundance. Statistical analysis was performed using Proc Glimmix in SAS with a model including treatment, day, and their interaction with pre-treatment (d 0) abundance as a covariate. Absolute (log10 copies/g feces) and relative (gene copies/16S rRNA) abundance of tetO, tetW, and ampC were not influenced by antibiotic treatment. Absolute abundance of 16S rRNA was lower (P < 0.04) in feces of pirlimycin-treated cows than in feces of cephapirin-treated cows and also tended to be lower (P < 0.06) than in feces of control cows. This suggests effects of pirlimycin on total fecal bacterial numbers. There was an effect of day on relative abundance of tetW (P < 0.04) and ampC (P < 0.04) in all cows, with fecal excretion highest on d 3 post-treatment. Absolute abundance of tetW in feces was influenced by day (P= 0.01), with excretion highest 28 d post-treatment. In this study prophylactic and therapeutic antibiotic administration did not increase excretion of the three target ARG by dairy cows. The observed temporal pattern in ARG excretion will help identify the most useful time frame for possible interventions to reduce dissemination of ARG from dairy farms. These results will inform further analysis of the entire complement of known ARG using shotgun metagenomics.
Keywords: antibiotic resistance genes, dairy cow, feces