Wastewater outfall study finds microbes on some plastics kept changing through day 60

A study published on 24 September in npj Biofilms and Microbiomes found that microbial communities on some plastics at a wastewater treatment plant outfall continued to change through day 60, even after initially stabilising. The finding matters for researchers assessing whether shorter experiments capture how microbes develop on plastic particles in flowing water.

The researchers placed five types of plastic and two natural materials at the outfall, then examined the communities growing on them after 7, 15, 30 and 60 days. Their results describe which microbes were present and how the communities changed. They do not show that the studied particles caused harm downstream or led to human infection.

Why the 60-day sampling period mattered

The tested plastics were PVC, polyethylene (PE), polypropylene (PP), PET and PLA. Sawdust and gravel served as natural comparison materials. The team used quantitative 16S rRNA sequencing, biofilm characterisation and ecological analyses to study microbes attached to the different surfaces over time.

According to the study, the communities appeared to stabilise by day 15. That was not the end of the observed changes: communities on PE, PP and PET continued to differentiate through day 60. The authors therefore suggest that an experiment ending at 30 days may miss later changes relevant to assessing microbes associated with plastics in flowing water.

Microbial communities on the plastics more closely resembled those on gravel than those on sawdust. The authors associated the sawdust’s distinct, sustained enrichment of microbes with its greater bioavailability. Together, the comparisons show why a natural surface can be a useful reference when interpreting what is distinctive about a plastic-associated community.

What other waterways show about plastic-associated microbes

A separate study in Connecticut examined polypropylene pellets and similarly sized stones at an impaired Quinnipiac River site near a wastewater treatment plant and at an unimpaired tributary. Its researchers collected samples after 30, 60 and 90 days during a May-to-August 2023 deployment. It offers a comparison in a river setting, rather than confirmation that the new outfall findings will recur elsewhere.

The Connecticut researchers found higher total coliform colony counts on plastic at the impaired site than in their other site-and-material comparisons. Yet the overall microbial communities on plastic and stone differed relatively little. The authors cautioned that summer-only sampling could not establish seasonal effects, while their coliform method could not determine what fraction of the organisms were pathogens.

Another independent study, published in 2025, took a different approach in South Africa. Researchers put plastic pieces in experimental containers containing seawater mixed with wastewater effluent for 30 days, then isolated bacteria from the resulting biofilms. Some isolates resisted multiple drugs when tested against 16 antibiotics. Those findings give researchers a reason to investigate plastic-associated microbes, but the experiment did not measure infections or ecosystem damage, and it does not establish the risks posed by particles in the new outfall study.

What remains unknown beyond the outfall

The new study reports microbial community measurements and predicted functional responses. A prediction about what a community may do is different from observing an environmental effect. The reported results do not establish what happens to these communities after the particles enter receiving waters, or whether their presence changes the health of an ecosystem.

The experiment covered one outfall and stopped at 60 days. It therefore cannot establish whether the same sequence of changes occurs at other treatment plants, in other seasons or over longer periods. Its clearest finding is about the observation window: an apparent early plateau did not mean the communities on every tested plastic had stopped changing.