Spread of multidrug-resistant Escherichia coli harboring integron via swine farm waste water treatment plant

Wastewater treatment plants (WWTPs) that release treated wastewater into the environment have emerged as a major threat to public health. In this study, we investigated Escherichia coli load and antibiotic-resistance profiles across different treatment processes at a swine farm WWTP. The frequency of the detection of class 1 and 2 integrons, and their association with antibiotic resistance, were also analyzed. Samples were obtained at each of five sampling sites that represented each processing step within the WWTP. The largest decrease in E. coil load was observed during the anaerobic digestion step (from 4.86 to 2.89 log CFU/mL). Isolates resistant to-lactam antibiotics were efficiently removed after a series of treatment steps, whereas the proportions of isolates resistant to non-lactam antibiotics and multidrug-resistant strains were maintained across treatments. The occurrence of integron-positive strains was not significantly different at the various sampling sites (43.4-70%; p > 0.05). Of the class 1 integron-positive isolates, 17.9% harbored the integron-associated gene cassettes aadA2, aadAl2, aadA22, and dfrA15. To the best of our knowledge, this is the first description of a class 1 integron containing the aadAl2 gene cassette from a swine farm and the presence of a class 1 integron containing dfrA15 in E. coll.. This suggests that novel antibiotic-resistance gene cassette arrays could be generated in swine farm WWTPs. Moreover, 75% of integron-positive strains were categorized as multidrug resistant, whereas only 15.4% of integron-negative strains were multidrug resistant (p < 0.05), indicating that integrons may be responsible for mediating resistance in WWTPs. With regard to the occurrence of multidrug-resistant, integron-positive E. coil recovered from the final effluent, our results highlighted the potential risks associated with wastewater discharge from swine farm WWTPs in terms of the spread of antibiotic-resistant bacteria to the aquatic environment.