Microplastics are increasingly recognised not only as environmental pollutants but also as vectors for disease-causing and antimicrobial-resistant (AMR) bacteria. A new study in Environment International by Dr Emily Stevenson and colleagues from the University of Exeter and Plymouth Marine Laboratory shows how particles from hospital wastewater can carry resistant pathogens into rivers and coastal environments.
The research, titled “Sewers to Seas: exploring pathogens and antimicrobial resistance on microplastics from hospital wastewater to marine environments,” mapped a pollution gradient from hospital tanks to the open ocean. It revealed that plastic particles, including bio-beads, nurdles and polystyrene fragments, support dense microbial biofilms containing over 100 unique antibiotic-resistance genes. These findings confirm that microplastics can act as tiny mobile habitats for harmful bacteria, linking wastewater management failures directly to marine ecosystem health.
“Microplastics aren’t just waste,” Stevenson said in the paper. “They’re vehicles for superbugs.”
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How the team tracked resistant bacteria from source to sea
To understand how these microbial communities develop, the team designed an in situ incubation system — five different materials suspended along a waterway downstream of a hospital wastewater treatment plant. Over two months, bacterial biofilms grew on each substrate, from heavily contaminated hospital water to marine surface sites 3.5 miles away.
Using metagenomic sequencing, the researchers found that polystyrene and high-density polyethylene (HDPE) nurdles were hotspots for antibiotic-resistance genes. The biofilms contained AMR determinants for tetracyclines, macrolides, aminoglycosides and oxazolidinones, antibiotics critical to human medicine.
The paper shows how the relative abundance of bacterial taxa shifted downstream. Hospital effluent hosted the greatest bacterial density, but even marine sites showed high persistence of resistance genes.
Surprisingly, some pathogenic groups, including Flavobacteriia and Sphingobacteriia, increased in prevalence in downstream and marine sites. These classes contain species capable of infecting fish and humans, raising concerns for aquaculture and recreational waters.
Why microplastics act as mobile biofilm incubators
Unlike free-living bacteria, biofilms on microplastic surfaces are enclosed in sticky extracellular polymeric substances that promote horizontal gene transfer, the exchange of resistance genes between bacterial species.
Because microplastics persist for decades, they provide long-term platforms for the evolution and spread of AMR genes. Polystyrene and HDPE nurdles, in particular, can adsorb antibiotic residues, creating “micro-hotspots” of selective pressure where resistant strains thrive.
“This makes microplastics far more than passive debris,” said co-author Prof. Pennie Lindeque. “Each particle becomes a transport vessel for resistance, from wastewater to beaches and shellfish beds.”
A warning for wastewater and aquaculture sectors
For the global water industry, the study underscores the link between waste management, antimicrobial resistance and microplastic control. Wastewater treatment plants (WWTPs) are known hotspots for microbial gene exchange, yet they may also serve as launch points for resistant microbes when microplastic effluent escapes into waterways.
The authors suggest monitoring microplastic effluents from WWTPs and landfill leachate, alongside antibiotics and metals that drive selection pressure. They also recommend protective measures for field researchers and volunteers, gloves during beach cleanups and proper hygiene after handling suspected plastic debris.
According to Dr Aimee Murray, co-author and Senior Lecturer in Microbiology at the University of Exeter, “Microplastics aren’t just an environmental issue, they’re a public health one. We need cross-sectoral strategies that integrate waste management, pollution control and AMR surveillance.”
The hidden biosecurity risk beneath the waves
While most wastewater regulations focus on chemical contamination, this study adds a new dimension: microbial and genetic pollution. The paper showed over 100 antibiotic-resistance gene (ARG) sequences unique to plastic biofilms, compared with fewer than 30 on natural substrates like wood or glass.
These microplastic-bound communities are particularly concerning near aquaculture operations, where filter-feeding species can ingest colonised particles. Once incorporated into marine food webs, resistant bacteria could bypass standard wastewater barriers and re-enter human systems through seafood consumption.
The authors conclude that the “sewers to seas” pathway represents a critical and underestimated conduit for AMR transmission, one that will require both infrastructure upgrades and behavioural change to address.