Anti-zinc Intestinal Bacteria And The Genes Of Resistance To Zinc In Pig Manure, Antibiotic Resistance Genes And Virulence Gene Research

While inclusion of high amounts of zinc salts in feed of weaned pigs has shown to decrease the incidence of post-weaning scours, to stimulate growth and to increase feed conversion efficiency, such growth-promoting action of dietary zinc leads to selection for resistance to zinc and resistance genes in enteric bacteria isolated from piggeries. The spread of such resistance isolates into the environment may lead to transferring resistance genes to human, which is seen as posing potential threat to human health. It has been reported that there was no significant reduction in the incidence of antibiotic resistant bacteria on pig farms in foreign countries, even given the ban on use of some antibiotics as a growth promoter. This indicates that use of copper sulfate and zinc oxide as a growth promoter may contribute to selection and maintenance of bacterial resistance to antibiotics. In China, copper and zinc compounds are still added to pig feed as growth promoters, however, there appeared to be no published report of the proportions of zinc-resistant coliforms isolated from piggeries. Whether these isolates carry antibiotic resistance genes and virulence genes are still unknown.Samples of pig feces from different ages were taken from nine commercial piggeries around Kunming, where different levels of metal salts were added to pig feeds. Isolation and identification of zinc-resistant enteric bacteria were carried out. The detections of the zinc resistant determinant, antibiotic resistant determinants and virulence genes harbored by these isolates were performed. 301 zinc-resistant coliforms were isolated from pig feces samples, including 246 E. coli isolates, 53 Proteus isolates and 2 Shigella isolates. The percentage of zinc-resistant E. coli from the piggeries where the feed additive premix was fully used was at least 48.9%, the percentage of zinc-resistant E. coli from the piggeries where the 50% feed additive premix was used was at least 25.6%, The percentage of zinc-resistant E. coli from the piggeries where no feed additive premix was used was under 3.33%. The prevalence of zinc-resistant E. coli from weaner feces was between 48.3% and 72.0%, the prevalence of zinc-resistant E. coli from grower feces was between 25.0% and 47.5%, and the prevalence of zinc-resistant E. coli from finisher feces was between 8.3% and 27.8%. The zntA gene was detected in both E. coli and non- E. coli (included Proteus and Shigella). Antibiotic resistance genes, such as strA, strB and tet(B) and virulence genes, including fimH and traT were found in some isolates which harbor the zntA gene. The results of this work indicate that zinc-resistant coliforms are not uncommon in piggeries where the high level of copper and zinc salts is added to pig feeds. The proportions of zinc-resistant E. coli isolates in samples from different piggeries are associated with the levels of zinc oxide added in pig diets. The percentage of zinc-resistant E. coli isolates in samples from different ages is related to the levels of zinc salts in pig feeds. The results of this thesis also appear to support the view that the use of copper and zinc salts in pig feeds may lead not only to selection for zinc resistance, but also to co-selection and maintenance of antibiotic resistance and virulence amongst enteric bacteria isolated from piggeries.