Reaction Kinetics of Singlet Oxygen with an Extracellular Antibiotic Resistance Gene (e-ARG) and Its Implications for the e-ARG Photodegradation Pathway in Sunlit Surface Water

Extracellular antibiotic resistance genes (e-ARGs) are emerging water contaminants that are degraded in sunlit surface waters via UV photolysis and reactions with reactive species, such as singlet oxygen (O-1(2)). However, the relative contribution of these pathways to e-ARG degradation in sunlit surface waters is poorly understood. This study examined the degradation and deactivation kinetics of an ampicillin resistance gene (amp(R)) in plasmid pUC19 with O-1(2) using a Rose Bengal photosensitizer. Degradation rates of the amp(R) measured via quantitative polymerase chain reaction increased with the number of guanine bases (#G) in the target amplicons (97-420). The calculated second-order rate constant for pUC19 degradation (1360) by O-1(2) was 8.7 x 10(6) M-1 s(-1) at pH 7.0, assuming G as the main damage site. Loss of pUC19's transforming activity was slower than the degradation of pUC19, suggesting repair of O-1(2)-induced DNA damage by the recipient Escherichia coli DH5 alpha. Simulations using the sunlight photolysis model incorporating the experimental kinetic data showed that O-1(2) plays a minor role in e-ARG degradation, while (OH)-O-center dot and direct UV photolysis were more significant. Further investigation is still needed to confirm the contribution of O-1(2) to e-ARG photodegradation using dissolved organic matter as the photosensitizer, considering the microheterogeneous nature of O-1(2) near dissolved organic matter.