Wastewater Treatment Fails To Remove Pathogens Stuck on Microplastics

Biofilms on microplastics in wastewater treatment systems can act as a refuge for harmful pathogens, according to a new study from the Norwegian University of Life Sciences. This includes bacteria such as Escherichia coli (E. coli) and viruses such as norovirus – posing risks to human health and ecosystems.

The study, published in PLOS One, highlights that even treated wastewater can carry pathogen-laden microplastics.

Microplastics can harbor pathogens

Microplastics are found in virtually every corner of the Earth, from deep ocean trenches to remote mountaintops, and are recognized as a significant environmental pollutant. These tiny plastic particles, less than 5 millimeters in size, are a byproduct of plastic waste breaking down in the environment and can originate from a range of sources.

Emerging research suggests that microplastics pose several risks to human health. In aquatic environments, they act like tiny rafts, providing surfaces for microorganisms to attach to and grow. This biofilm-covered microplastic – also called a "plastisphere" – can harbor harmful pathogens. Studies have shown that microplastics can absorb toxic chemicals and heavy metals, transporting these pollutants through ecosystems and potentially into the human food chain. When ingested by fish or other aquatic organisms that are later consumed by humans, these particles can introduce both chemical toxins and harmful bacteria into our bodies.

Due to their tiny size, microplastics can evade the filtration systems in wastewater treatment plants, allowing them to flow into natural water bodies, where they may linger for decades. In wastewater treatment, a combination of physical, biological and chemical processes are used to remove solids, organic matter and pathogens before discharge. However, the resilience of microplastics enables them to bypass these treatments, carrying with them various contaminants.

Studies have detected potentially pathogenic bacteria, such as Klebsiella pneumoniae, Acinetobacter baumannii and Enterobacteriaceae, as well as viruses like norovirus and adenovirus, in the effluent from treated wastewater. Understanding the impact of plastic surfaces as a source of potential pathogens in wastewater is vital for preventing negative impacts on human health and the environment.

Plastispheres contain bacterial and viral pathogens

The team submerged 3 types of plastic – Polypropylene, Polyvinyl chloride and High-density polyethylene (PP, PVC and HDPE) – in both raw and treated wastewater at a Norwegian wastewater treatment plant to allow biofilm communities, or plastispheres, to form over 14 and 30 days.

After retrieval, biofilms were swabbed, and pathogens were isolated on selective media and identified via mass spectrometry. DNA and RNA were extracted from the pooled biofilms to detect bacterial pathogens and viruses, and microbial communities were further characterized by 16S rRNA sequencing.

Plastispheres from both raw and treated wastewater harbored foodborne pathogens, including E. coli, Listeria monocytogenes (L. monocytogenes) and Salmonella. The researchers also identified opportunistic pathogens, such as Klebsiella pneumoniae and Acinetobacter. These are types of bacteria that do not typically cause disease in healthy individuals but can become pathogenic when the host's immune system is weakened.

Viral pathogens, like norovirus and adenovirus, were also found in plastispheres from both wastewater types.

While qPCR detected foodborne pathogens like E. coli and L. monocytogenes, traditional cultivation techniques did not, indicating that pathogens may be present in nonviable forms or below detection limits without enrichment.

Biofilms on microplastics offer protection to pathogens

The presence of pathogenic bacteria within the plastisphere biofilms in both raw and treated wastewater, despite the typically harsh conditions of wastewater treatment, indicates that the biofilm may act as a protective barrier.

Evidence from field samples showed that potential pathogens could be isolated and cultivated from both raw and treated wastewater plastispheres, but similar pathogens were not as easily found or viable in wastewater without biofilms.

Despite current treatment processes, significant amounts of microplastics remained in the treated effluent with potential pathogens.

The authors emphasized that improved management and innovation in wastewater treatment are recommended to reduce the release of plastic and pathogen-laden particles: “This study highlights the potential for plastispheres to contribute to the spread of pathogens from treated wastewater, posing challenges for environmental health and water reuse efforts,” they said.

Reference: Witsø IL, Basson A, Aspholm M, Wasteson Y, Myrmel M. Wastewater-associated plastispheres: A hidden habitat for microbial pathogens? Nicolai E, ed. PLoS ONE. 2024;19(11):e0312157. doi: 10.1371/journal.pone.0312157

This article is a rework of a press release issued by PLOS. Material has been edited for length and content.