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 (1O2). 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 (ampR) in plasmid pUC19 with 1O2 using a Rose Bengal photosensitizer. Degradation rates of the ampR 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 1O2 was 8.7 × 106 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 1O2-induced DNA damage by the recipient Escherichia coli DH5α. Simulations using the sunlight photolysis model incorporating the experimental kinetic data showed that 1O2 plays a minor role in e-ARG degradation, while •OH and direct UV photolysis were more significant. Further investigation is still needed to confirm the contribution of 1O2 to e-ARG photodegradation using dissolved organic matter as the photosensitizer, considering the microheterogeneous nature of 1O2 near dissolved organic matter.