Growth performance and the development of antibiotic resistant bacteria in swine fed growth-promoting antimicrobials

Three experiments were conducted to study the impact of growth-promoting levels of various antimicrobials on performance and the development of antibiotic-resistant commensal bacteria in swine. In experiment one, feeding subtherapeutic levels of antibiotics, or high levels of zinc and copper did not improve growth performance of nursery pigs not previously exposed to antibiotics. In experiment two, feeding nursery pigs high levels of zinc resulted in increased ADG and ADFI compared to pigs fed a diet containing no antimicrobials or high levels of copper. In experiment three, subtherapeutic levels of chiortetracycl ine improved growth performance only in the first week after weaning. High levels of zinc may be as effective as subtherapeutic antibiotics in improving nursery pig performance. Although pigs in experiment one were not previously exposed to antibiotics, commensal bacteria isolated from these pigs exhibited resistance to multiple antibiotics. E. coli resistant to neomycin were isolated only from pigs fed diets containing antibiotics. In experiment two, feeding pigs diets containing growth-promoting levels of antibiotics, zinc, or copper resulted in an increased percentage of E. coli and Enterococcus that were resistant to various antimicrobials. In experiment three, percentage of E. coli and Enterococcus isolated from pigs that were resistant to chlortetracycline and virginiamycin fluctuated over 19 weeks, however no differences were observed between pigs fed diets containing antibiotics or no antibiotics at the conclusion of the trial. In the fourth experiment, manure from pigs receiving growth-promoting antibiotics was spread onto soil boxes and subjected to a simulated rainfall event to measure antibiotic resistance in water runoff. A greater amount of antibiotic resistant bacteria were recovered from water runoff at the initial sampling period compared to sampling 30 min after runoff began. Percent of antibiotic resistant bacteria isolated from swine can fluctuate over time. Development of antibiotic-resistant bacteria may be increased by the use of antimicrobial growth-promoters, however, resistant bacteria are present in swine facilities regardless of their use. Antibiotic resistance will not be easily reversed by removal of antibiotics from livestock feeds.