| The high amounts of heavy metal, e.g. CuSO4 and ZnO2 wereadded to pig feed because inclusion of antibiotics and heavy metal has been shown to decrease the incidenceand severity of post-weaning scours, to increase growth rate and to increase feedconversion efficienvy. Weighed against benefits, there were two concerns to be considered: one was residue levels in edible animal tissues, another was environmental issues associated with the disposal of effluent. Antibiotic-resistant bacteria have been found in the food animal farms although antibiotics were baned to be added to feed except for the treatment purpose. Heavy metal resistsant genes and antibiotic resistant genes have been detected in the plasmids isolated from such resistant bacteria. The results suggested that inclusion of heavy metal in feed has selected and maintained antibiotic resistant genes in food animal farms after ban of antibiotics as growth promoters. The amounts of copper and zinc added to pig feed have been reduced to nutritional requirments in EU. In China, the high amounts of CuSO4 and ZnO2 have been added to pig feed as growth promoters. However, the prevalence of copper resistant bacteria in pig farms and the occurrence of the copper resistant genes, antibiotic resistant genes and virulence genes carried by such resistant bacteria are still unknown.To investigate the prevalence of resistant bacteria in pig farm, samples of pig feces from different ages were taken from 9 commercial piggeries around Kunming, where different amounts of metal salts were added into the pig diets. Soil samples around piggeries were also taken. Isolation and Identification of copper-resistant enteric bacteria were performed. 340 copper-resistant coliforms were isolated from pig feces samples, including 329 E. coli isolates, 9 Proteus isolates and 2 Shigella isolates. The prevalence of copper-resistant E. coli from the piggeries where the feed additive premix was fully used was at least 45.8%, the prevalence of copper-resistant E. coli from the piggeries where the 50% feed additive premix was used was at least 28.3%, The prevalence of copper-resistant E. coli from the piggeries where no feed additive premix was used was under 3.3%. The percentage of copper-resistant E. coli from weaner feces was between 82.2% and 47.5%, the percentage of copper-resistant E. coli from grower feces wasbetween 60.0% and 26.3%, the percentage of copper-resistant E. coli from finisher feces was between 33.3% and 13.8%. The prevalence of copper-resistant coliforms from the soil around pig farms was 15%. The results indicate that copper-resistant coliforms are not uncommon in the piggery where the high levels of metal salts are added to pig feeds. The proportions of copper-resistant E. coli isolates in samples from different piggeries are associated with the levels of copper sulfate added in pig diets. The percentage of copper-resistant E. coli isolates in samples from different ages is related to the levels of copper sulfate in pig diets. The results salso suggest that resistant bacteria in pig farms start to spread.To detect the occurrence of the copper resistant determinant, antibiotic resistant determinants and virulence genes in these copper-resistant isolates, the method of PCR were employed. The copper-resistant genes pcoA, pcoC and pcoD were detected in both E. coli isolates and non-E. coli (Proteus and Shigella) isolates. 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 pco genes. 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 copper resistance, but also to co-selection and maintenance of antibiotic resistance and virulence amongst coliforms isolated from piggeries. |
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