Dynamics of class 1 integrons in aerobic bio film reactors spiked with antibiotics

Class 1 integrons are strongly associated with the dissemination of antibiotic resistance in bacteria. However, little is known about whether the presence of antibiotics a ffects the abundance of integrons and antibiotic resistance genes during biological wastewater treatment. To explore the roles of class 1 integrons in spreading antibiotic resistance genes in environmental compartments, the dynamics of integrons were followed in bio film reactors treating synthetic wastewater respectively spiked with streptomycin (STM) and oxytetracycline (OTC). The relative abundance of the integron-integrase gene (intI1 ) increased 12 or 29-fold respectively when treated with STM or OTC, under incrementally increasing dosage regimes from 0 to 50 mg L-1 . Signi ficant increases in intI1 abundance initially occurred at an antibiotic dose of 0.1 mg L-1 . At the beginning of the experiment, 51% to 64% of integrons carried no gene cassettes. In STM and OTC spiked systems, there was a signi ficant increase in the proportion of integrons that contained resistance gene cassettes, particularly at intermediate and higher antibiotic concentrations. Gene cassettes encoding resistance to aminoglycosides, trimethoprim, beta-lactam, erythromycin, and quaternary ammonium compounds were all detected in the treated systems. Three tetra- cycline resistance genes (tetA, tetC, tetG ) were signi ficantly correlated with the abundance of intI1 (p < 0.01), despite no tet resistance being present as a gene cassette. Genome sequencing of isolates showed synteny be- tween the tet resistance genes and intI1 , mediated through linkage to transposable elements including Tn3 , IS26 and IS CR3 . Class 1 integrons appeared to be under positive selection in the presence of antibiotics, and might have actively acquired new gene cassettes during the experiment.