An electron micrograph of a cluster of Escherichia coli bacteria.
| Photo Credit: Public domain
Nanoplastics are bad news even though each one is only as small as a smoke particle. Research has shown that micro- and nano-plastics are present at the top of the tallest mountains, at the bottom of the deepest trenches, and even in our bloodstream, tissues, and in newborns as well. Their build-up has many toxic effects, including damage to cells and the genetic material inside.
Now, a new study from researchers at the University of Illinois, Urbana-Champaign, has revealed that nanoplastics aren’t just risky on their own: they also increase the risk from pathogens. The researchers found that nanoplastics with positively charged surfaces could make Escherichia coli, a foodborne pathogen, more virulent.
The findings were published in the Journal of Nanobiotechnology.
How E.coli bacteria work
E. coli bacteria have a negative charge on their outer membranes. This can attract positively charged nanoplastics and increase the stress on the bacterium, making it produce more Shiga-like toxins. These are the proteins E. coli secrete and which cause disease.
In older studies, scientists have looked at the impact of charged surfaces on non-pathogenic bacteria. The new study examined how charged surfaces affected a particular strain of E. coli. Similarly, while previous studies focused primarily on free-floating cells, the new one studied both free-floating cells and biofilms — a community of bacteria sticking to each other over a surface. Biofilms are hotspots where bacteria exchange genetic material with their neighbours.
The research process
The team members used a pathogenic E. coli strain that was resistant to the antibiotic rifampicin. They cultured the strain on agar plates and a nutrient-rich liquid medium called LB broth. Then they exposed the bacteria to polystyrene-based nanoplastics with three kinds of charges: positive, negative, and neutral.
The team also cultured a second set of E. coli cells in LB broth for use as a control. This group wasn’t exposed to nanoparticles.
Finally, the researchers observed the growth of the free-floating cells and the biofilm at seven- and 15-day intervals, and used different tests to quantify the growth.
For the study’s purposes, the researchers used an environmental scanning electron microscope (ESEM) to track interactions between nanoparticles and free-floating cells. Unlike microscopy techniques that require samples to be prepared in protracted processes, ESEM can image ‘wet’ samples without too much preparation. It also operates in a low-vacuum or controlled gas atmosphere that prevents natural fibres and cellulose from becoming charged.
The scientists conducted two tests to measure the concentration of carbohydrates and proteins in the biofilm. They performed a catalase assay to determine the cells’ response to oxidative stress. E. coli produce the catalase enzyme to protect themselves against oxidative stress. Higher stress would mean more catalase activity. The team also extracted RNA from the bacteria for study, because gene transfers take place across biofilms. Changes in RNA may indicate such transfers have happened.
What the study found
The investigations revealed that nanoplastics with surface charges did add to the toxicity of bacterial cells. At first, charged nanoplastics kept cells from growing. But some cells eventually overcame the stress and began to grow. The development and viability of cells exposed to charged nanoplastics were different from those of cells that weren’t. Changes in the bacterial RNA suggested genes were changing to ensure the E. coli survived better and became more able to cause disease. The study said both positively and negatively charged nanoplastics caused these changes.
Beena D.B., assistant professor of biology at Azim Premji University’s School of Arts and Sciences, has conducted preliminary work on the effect of microplastics on soil microbes and said her team had observed similar results. She cautioned that microplastic-induced biofilms are a significant health hazard and that increasing horizontal gene transfer between microbes raised the risk of them developing antibiotic resistance. It also causes excessive production of proteins that contribute to more severe disease, she added.
(Manaswini Vijayakumar is interning with The Hindu.)
Published – June 05, 2025 04:32 pm IST