Foodborne and opportunistic pathogens can survive wastewater treatment when they hitch a ride on microplastics in the water and quickly form a supportive and protective microbial biofilm, posing a potential threat to human and environmental health when the treated water is reused for things like drinking and crop irrigation, suggest researchers from the Norwegian University of Life Sciences.
Tiny plastic fragments have proliferated
The investigators identified foodborne pathogens that had formed plastispheres (colonies of pathogens in a sticky microbial biofilm) in three common types of plastic (polypropylene, polyvinyl chloride, and high-density polyethylene propylene) in raw and treated wastewater at a single plant in Norway in August and September 2021.
The plastispheres were allowed to grow for 14 and 30 days in raw and treated wastewater, respectively. The team cultured the pathogens under aerobic or anaerobic conditions and genetically analyzed the pathogens, the results of which were published last week in PLOS One.
Microplastics, or plastic fragments and particles less than 5 millimeters (0.20 inch) in size, have proliferated in the environment as plastic manufacturing has increased and are recognized as significant food contaminants.
"Municipal WWTPs [wastewater treatment plants] are recipients of plastic particles of different sizes originating from household activities such as washing synthetic clothes, personal care, and rinsing cosmetic products directly down household drains," the study authors wrote. "In addition, tire wear particles represent a major source of microplastics entering the environment and are often transported into WWTPs through storms and meltwater."
Viable Klebsiella pneumoniae, Acinetobacter
On reverse-transcription qualitative polymerase chain reaction (RT-qPCR), microplastics yielded evidence of harmful bacteria and viruses such as Listeria monocytogenes, Escherichia coli, norovirus, and adenovirus. The pathogens' abundance was less than 1%, but they were detectable for 14 and 30 days in raw or treated wastewater.
These findings underscore the potential of plastispheres to harbor and disseminate pathogenic species, posing challenges to water reuse initiatives.
Cultures grew Klebsiella pneumoniae and Acinetobacter species from raw and treated wastewater, indicating that the plastisphere biofilms likely help them withstand wastewater treatment.
The taxonomic diversity and composition of the plastispheres were significantly influenced by the environment and the length of time the plastic was in the wastewater. The type of plastic material didn't affect the bacterial composition but did influence the bacterial diversity.
"These findings underscore the potential of plastispheres to harbor and disseminate pathogenic species, posing challenges to water reuse initiatives," the researchers wrote. "Without efficient wastewater treatment and proper plastic waste management, wastewater could act as a source of transferring plastic-associated pathogens into the food chain and possibly pose a threat to human health."
"Continued research and innovation are necessary to develop alternative sustainable technologies or supplement conventional treatment processes to ensure more effective removal of microplastics and to degrade potential pathogenic microorganisms," they concluded.
