Antagonistic effects of microplastic biofilms on antibiotic resistance gene horizontal transfer in water environments

Emerging pollutants, microplastics, found in water environments, accumulate microorganisms on their surfaces, forming biofilms that concentrate antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs). Horizontal gene transfer (HGT) of ARGs is one of the primary ways bacteria acquire antibiotic resistance. Most studies reported that biofilm formation promoted the HGT of ARGs. However, this study found that microplastic biofilms might inhibit ARG conjugation. Previous research focused on the impact of environmental factors on ARG conjugation among suspended bacteria, but studies on microplastic biofilms were lacking. Therefore, this study selected environmental factors that have been extensively investigated and are recognized as significant facilitators and inhibitors of ARG conjugation, namely nano-alumina and free nitrous acid (FNA), to compare their effects on ARG conjugation in suspended bacteria and microplastic biofilms. The results showed that when the concentration of nano-alumina was 5.0 mmol/L, the ARG conjugation frequency in microplastic biofilms was significantly lower than in suspended bacteria. Nano-alumina could enhance cell membrane permeability and increase the bacteria's ability to transfer DNA, thereby promoting ARG conjugation. However, microplastic biofilms could reduce the promoting effect of nano-alumina, thereby inhibiting ARG conjugation. FNA could inhibit ARG conjugation among suspended bacteria. Microplastic biofilms could reduce the inhibitory effect of FNA, ultimately leading to a higher frequency of conjugation in microplastic biofilms compared to suspended bacteria. This study reveals the mechanisms by which microplastic biofilms promote or inhibit ARG conjugation, providing new insights for dialectically studying the effects of microplastic biofilms on ARG transmission.