Could microplastics in soil lead to antibiotic-resistant bacteria in our food?
It's a real risk, according to scientists at the University of Illinois Urbana-Champaign, as modern farming relies heavily on plastics, from plastic mulch lining vegetable beds to polyethylene covering tunnels.
The researchers warn that microplastics are now widely dispersed in agricultural soils and could lead to antibiotic resistant bacteria having a route into our food supply.
Microplastics in soil can concentrate bacteria and change their genes, leading to antibiotic resistance.
Study author Jayashree Nath, postdoctoral researcher in the Department of Food Science and Human Nutrition (FSHN) in the College of Agricultural, Consumer and Environmental Sciences (ACES), said: "Plastic itself may not be very toxic, but it can act as a vector for transmitting pathogenic and antimicrobial resistant bacteria into the food chain.
"This phenomenon is not very well known to people, so we wanted to raise awareness."
The danger comes from the fact that plastics are an excellent adsorbent. That means chemical substances and microscopic organisms love to stick to plastic.
Chemicals that would ordinarily move through soil quickly – things like pesticides and heavy metals – instead stick around and are concentrated when they encounter plastics.
Similarly, bacteria and other microorganisms that occur naturally in soil congregate on the stable surfaces of microplastics, forming what are known as biofilms.
When bacteria encounter unusual chemical substances in their new home base, they activate stress response genes that incidentally help them resist other chemicals too, including, sometimes, antibiotics.
As groups of bacteria attach to the same surface, they have a habit of sharing these genes through a process called horizontal gene transfer.
Nanoplastics, which can enter bacterial cells, present a different kind of stress, but that stress can have the same outcome.
Co-author Pratik Banerjee, associate professor in FSHN, said: "Bacteria have been evolving genetic mechanisms to cope with stress for millions of years. Plastic is a new material bacteria have never seen in nature, so they are now evoking these genetic tool sets to deal with that stress.
"We have also shown bacteria may become more virulent in the presence of plastics, in addition to becoming more resistant to antimicrobials."
Gene transfer between bacteria on microplastics has been documented in other environments, particularly water.
So far, the phenomenon is only hypothetical in agricultural soil, but that doesn't mean it's not happening.
Banerjee said: "Soil is an under-researched area in this field. We have an obligation to understand what's going on in soil, because what we suspect and what we fear is that the situation in soil could be even worse than in water.
"One of the technical problems is that soil is a very difficult medium to handle when it comes to fishing out microplastics. Water is so easy, because you can simply filter the microplastic out."
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