Effects of antibiotic regimens on bacterial resistance

In 2 replicated trials, 144 weaned pigs were used to test the effects of antibiotic dosing schemes on resistance in bacteria. Pigs were inoculated with the foodbome pathogen, Salmonella enterica var Typhimurium, prior to being treated with feed- and water- based antibiotics. Treatments included maximum label use, rotation of similar and non-similar antibiotics, increasing gradient doses, and pulse dosing of antibiotics for a period of 2 weeks following pathogen challenge. Fecal samples were obtained prior to initiation of treatments, and on various days during the treatment and throughout the grow-finish phase. The challenge organism and non- pathogenic E. coli were recovered from fecal samples and tested against all antibiotics used in the study to determined effects on resistance patterns. Antibiotic resistance was affected to a greater extent in non-pathogenic E. coli compared to Salmonella Typhimurium. Greater (P < .0001) resistance occurred when similar antibiotics (apramycin, gentamicin, neomycin) were used in rotation compared to the other treatments. Significant (P < .05) Time by Treatment interactions also occurred during or just following rotational treatment with similar antibiotics compared to other groups. These data indicate that dosing regimens affect antibiotic resistance patterns in bacteria associated with swine.