Enhanced solar photodegradation of a plasmid-encoded extracellular antibiotic resistance gene in the presence of free chlorine

The rapid spread of antibiotic resistance in aquatic environments is closely related to the prevalence of antibiotic resistance genes (ARGs). Environmental ARGs undergo some degree of solar photolysis and chlorine oxidation. This report is the first study to comprehensively investigate the solar photodegradation mechanism of a plasmid-encoded extracellular antibiotic resistance gene (pUC19) via the solar/free chlorine process. The pUC19 degradation rate via the solar/free chlorine process was significantly enhanced (1.15 ± 0.05 min−1) compared to chlorination (0.04 min−1) and direct solar photolysis (0.05 min−1) alone. Increasing the pH values from 6 to 8 resulted in a higher yield of ozone and accelerated pUC19 photodegradation. Within 20 min of the reaction time, ozone and hydroxyl radicals (•OH) were responsible for 37.5% and 55.1% of the pUC19 degradation, respectively, and the contribution of reactive chlorine species (RCS) was negligible. Degradation rates, determined by different amplicon lengths in the same segment, showed a linear correlation with amplicon length. The attack by solar/free chlorine may be nonselective because similar amplicon lengths located in different pUC19 segments (ampR and ori) showed similar degradation rates. The pUC19 lesions caused by the solar/free chlorine process were the most effective treatment for breaking DNA double bonds. An enhanced solar/free chlorine photolysis phenomenon was also observed in real swimming pool waters, despite interference and inhibition by the water matrix components, which provides valuable insight into the control of ARGs in chlorine-receiving aquatic environments. [Display omitted] ••OH and O3 accounted for over 90% pUC19 degradation, while RCS played a minor role.•pUC19 degradation increased at higher pH values.•The degradation rates of pUC19 shows a liner correlation with amplicon sizes.•Fragmentation was detected by agarose gel electrophoresis.•An enhanced solar/free chlorine phenomenon was observed in real pool waters.