Sodium hypochlorite residual in the environment facilitated the spread of antibiotic resistance genes: through microplastics as a medium

Most pollutants in the environment exist in complex forms, and exploring the impact of a single pollutant lacks wide applicability. The co-exposure of microplastics (MPs) and NaClO in wastewater treatment plants (WWTPs) is a widespread occurrence. At present, relevant reports on the impact of individual NaClO or MPs on antibiotic resistance genes (ARGs) have been established. Herein, this study investigated the fate of antibiotic-resistant bacteria (ARB) and ARGs after exposure to MPs with or without NaClO stress. In this study, the total ARG abundance increased by 11.83% under MPs stress, and further increases by 17.89% under NaClO stress with MPs co-exposure. The mechanism was that the presence of NaClO promoted the selective enrichment of potential ARB and ARGs on the MPs-biofilm. The surface morphology of the MPs was changed and the attached biofilm became thicker, which provided a suitable environment for the proliferation of ARB and the spread of ARGs. Vertical gene transfer (VGT) and horizontal gene transfer (HGT) of ARGs were facilitated by MPs under NaClO stress. Specifically, the VGT of ARGs was facilitated via enhanced bacterial cell proliferation (by 132.66%), and relevant functional genes are also increased. HGT of ARGs is promoted by the increasing relative abundance of mobile genetic elements (MGEs). ARG-carrying plasmids are also demonstrated that MPs promoted HGT of ARGs in the presence of NaClO. The increase in oxidative stress, cell membrane permeability, and Type IV secretion system (T4SS) collaboration facilitated the HGT of ARGs. In summary, co-exposure to NaClO and MPs promote VGT and HGT of ARGs through the variation in MPs structure and the enhancement of MPs-biofilms. Furthermore, the presence of MPs restrained the disinfection effect of NaClO, with an inhibition rate higher than 50%.