Assessment of bioavailability of microplastic-sorbed tetracycline to bacteria for activation of antibiotic resistance genes in water environments

The excessive use of antibiotics by humans has led to their widespread dissemination in the environment, thereby promoting the emergence and proliferation of antibiotic resistance genes (ARGs) in water environments. Microplastics exhibit strong adsorption capacities for various antibiotics and are considered capable of immobilizing them, potentially mitigating their impact on aquatic bacterial communities. However, little is known about whether antibiotics adsorbed by microplastics retain bioavailability and whether they can exert selective pressure on aquatic bacteria. This study investigated the adsorption of tetracycline by microplastics and microplastic biofilms in the landscape water of a university campus, as well as the bioavailability of the adsorbed tetracycline. Results revealed that bacteria inhabiting microplastic biofilms could still utilize tetracycline adsorbed by either microplastics or microplastic biofilms, indicating that the adsorbed tetracycline remains bioavailable. Compared with microplastic-sorbed tetracycline, microplastic biofilm-sorbed tetracycline exhibits a stronger capacity to promote ARG expression. This is because the formation of biofilms enhances the adsorption capacity of microplastics for tetracycline, and bacteria can more easily access tetracycline adsorbed onto biofilms than that embedded in the microplastic surface. Furthermore, tetracycline adsorbed onto microplastics and microplastic biofilms promoted the vertical transfer of ARGs, increased the relative abundance of antibiotic-resistant bacteria (ARB) within biofilms, and enhanced the tetracycline resistance of microplastic biofilm bacteria. These findings suggest that antibiotics adsorbed by microplastics retain bioavailability and pose a potential risk of facilitating the dissemination of ARGs.