Antibiotic effects on the selection of enteric bacteria in the gastrointestinal tract of weaned piglets. Enterobacteriaceae isolates from swine

Pork is the most consumed animal protein source in the world. To achieve high levels of performance and productive efficiency, antibiotics are typically included in the piglet diets. However, the use of antibiotics as growth promoters in swine has been questioned due to the potential selection of multidrug-resistance enterobacteria. The present study aimed to evaluate the susceptibility profile of enterobacteria isolated from fecal samples of weaned piglets (21 to 35 day) fed or not with growth promoters (colistin and tylosin). Six-hundred and eighteen phenotypically distinct bacterial cultures were isolated from fecal samples of weaned piglets fed a control (CON; n=384) or control diet with antibiotics (ATB; n=234). All isolates were tested for their susceptibility to 12 different antibiotics belonging to six distinct antibiotic classes. Approximately 90% (n=553), 85% (n=525) and 81% (n=498) of the isolates were resistant to ampicillin, amoxicillin and tetracycline, respectively. There was a significant increase (P < 0.05) in the levels of resistance to cephalexin, kanamycin, doxycycline and colistin in the isolates from treatment ATB when compared to treatment CON. Our findings demonstrate that colistin and tylosin, two antibiotics commonly used as feed additives for pig production, contributes to select antibiotic resistant enterobacteria in weaned piglets (up to 35 days of life). Our data also revealed a high frequency of antibiotic resistance in bacteria isolated from the feces of pigs that did not receive antibiotics as growth promoters in their diets, suggesting that the microbiota containing multidrug resistant bacteria present in the gastrointestinal tract these animals may have been acquired from its mother and its surrounding environment, representing a potential reservoir of multidrug resistant enterobacteria.