Shotgun metagenomic profiling reveals a high diversity of taxa and genes within biofilms formed on microplastics incubated in urbanised aquatic ecosystems

Microplastics (MPs) are ubiquitous in aquatic ecosystems, where they are colonized by microbial communities, called the plastisphere. Of great concern is the detection of potential pathogens and antimicrobial resistance genes (ARG) in the plastisphere, which might be transported across ecosystems through MPs drifting. We used shotgun metagenomic profiling to assess taxa diversity, ARG and virulence genes (VG), within biofilm formed on polypropylene (PP) particles incubated in situ in five locations, following an anthropic gradient around Ostend (Belgium). Our results demonstrated significant variability of the plastisphere across incubation sites, but not between PP and control glass beads. Potential pathogenic bacteria (PPB) represented about 7% of bacterial reads within biofilms and VG were mainly involved in nutrition and adherence. Using dqPCR results to normalize metagenomic reads, we demonstrated a selective enrichment of ARG and VG in biofilms, while these were less abundant but more diverse in surrounding water. These findings highlight the presence of PPB, ARG and VG across all sites, likely driven by anthropogenic pressures. Although no substrate-specific effect was detected, the ability of PP particles to act as microbial reservoirs, coupled with their high mobility, reinforces concerns about their potential role in the transport and dissemination of microbial hazards.