Long-term effects of systemic ceftiofur and ampicillin on the abundance and duration of shedding of resistant Gram-negative bacteria in the feces of healthy dairy cows: a randomized clinical trial

Introduction We aimed to evaluate the long-term effects of systemic ceftiofur (CEF) and ampicillin (AMP) on the abundance and duration of fecal shedding of resistant Gram-negative (GN) bacteria from healthy dairy cows, as compared to cows that did not receive antibiotic treatment. Methods Healthy lactating Holstein cows were randomly assigned to receive once daily treatments for 5 days with CEF (2.2 mg/kg subcutaneously, n = 8), AMP (11 mg/kg intramuscular, n = 8) or isotonic sodium chloride solution (25 cc, subcutaneously, n = 8). Fecal grab samples were collected before the initial treatment (day 0) and weekly for 11 weeks, beginning 2 days after the final treatment. Briefly, samples were diluted and inoculated in duplicate using a spiral plater on MacConkey agar or MacConkey agar supplemented with AMP or CEF. The phenotypic abundance of GN bacteria resistant to AMP or CEF was enumerated using a logarithmic scale and as a proportion of the total number of GN bacteria. Generalized linear mixed models for repeated measurements were used to compare the abundance of resistant bacteria among treatments over a 12-week clinical trial period. Results The abundance of GN bacteria did not vary within or among treatments over the weeks of the trial. The abundance of GN bacteria resistant to AMP and CEF did not differ among treatment groups within a given week. After treatment, systemic treatment with AMP or CEF did not increase the proportion of resistant GN bacteria in feces of treated cows as compared to a control group that did not receive antibiotic treatment. Our preliminary molecular results suggest that the overall bacterial composition remained similar across groups. The relative abundance of antimicrobial resistance genes (ARG) revealed that the macrolide-lincosamide-streptogramin B (MLS), aminoglycoside, tetracycline, and elfamycin classes were the most predominant, whereas beta-lactams were moderate, ranging from 4 to 7%. Conclusion In summary, a long-term effect of systemic treatment with AMP or CEF in healthy dairy cows produced minimal changes in the abundance of resistant GN bacteria, which indicates that systemically administered treatments of AMP and CEF had similar effects on the abundance and duration of resistant bacteria shedding in feces.