Hitchhikers on traveling microplastics: Three necessary steps for bacteria becoming dangerous invaders

Microplastics (MPs) have received great concern in recent years, and whether the bacteria carried by traveling MPs would cause ecological risks is a hot topic for debate. The colonized bacteria (i.e., hitchhikers) on traveling MPs which become invasive species in the new environment, need to be completed in three steps: arriving, falling, and growing. However, most previous studies only focused on the first step, which we think is insufficient to discuss species invasion. Thus, in this frontier review, we reviewed the progress of the current research on the uniqueness of bacterial communities on MPs, and we summarized that the uniqueness of the plastisphere was not as high as previously thought. Moreover, we explained why the three steps were necessary to complete the bacterial species invasion. Furthermore, we analyzed the technical difficulties hindering discussing MPs as invasive species carriers, as well as the perspectives in future research. Therefore, this frontier review presents new insights into the role of MPs as bacterial carriers, and suggests study directions for future research. • Detection of unique bacteria is inadequate to judge MPs causing species invasion. • Ecological risks of MPs causing bacterial invasion might be overestimated previously. • There are three necessary steps causing invasion: arriving, falling, and growing. • Technical difficulties hindering the process of the subsequent study are analyzed. • Semipermeable membranes show potential to provide close enough in-situ environments.