Selection of antibiotic-resistant bacterial populations in the dairy cow gut following intramuscular ceftiofur treatment for metritis.

Third-generation cephalosporins such as ceftiofur are critically important antibiotics because human pathogens with resistance to these drugs contribute to high mortality rates. These antibiotics are also frequently given to dairy cattle for treating infections, emphasizing the critical role they play in both human and veterinary medicine. To investigate the effect of intramuscular ceftiofur treatment on the concentration of resistant bacteria in the gut, we focused on cows with metritis, a common bacterial infection that frequently requires antibiotic intervention. Twelve cows with metritis (cases) were enrolled and treated with intramuscular ceftiofur for 5 d along with 12 matched healthy cows that were not given ceftiofur (controls). Fecal samples were collected weekly from cows in both the case and control groups for 4 wk, starting before the treatment of the case group. Five fecal samples per cow were used for analysis (n = 120 samples). The abundance of gram-negative bacteria was quantified per sample after plating on MacConkey agar, which was also used to quantify the abundance of gram-negative bacteria with resistance to ceftiofur, ampicillin, and tetracycline. Interestingly, the case cows with metritis had a greater abundance of gram-negative bacteria than the control cows just before treatment, but no difference in abundance was observed between groups at wk 1-4. The abundance of ceftiofur-resistant gram-negative bacteria was also similar between the case and control cows immediately before treatment of the cases. However, a significant increase in abundance of ceftiofur-resistant gram-negative bacteria was observed in the case cows 1-wk after treatment that persisted through wk 3. Although the recovery of ampicillin- and tetracycline-resistant bacteria was similar between the 2 groups post-treatment, cases had significantly higher levels of ampicillin-resistant bacteria before treatment. Collectively, these findings demonstrate that metritis and intramuscular ceftiofur treatment can affect the abundance of cultivable gram-negative bacteria and ceftiofur-resistant populations that can persist for up to 3 wk. Judicious use practices are needed to ensure that ceftiofur and other critically important antibiotics are administered only when necessary to minimize the spread of resistance and safeguard public and animal health.