Microplastics Could Be Creating Dangerous Antibiotic-Resistant Bacteria

Microplastics Could Be Turning Bacteria into Drug-Resistant Superbugs

By Marta Zaraska edited by Lauren J. Young

For bacteria, microplastics are the perfect meetup spot—tiny, intimate surfaces where microbes can cling, huddle close and swap genes. And these crowded bacterial breeding grounds may pose a threat to human health. A growing body of new research shows that microplastics may fuel antimicrobial resistance—the phenomenon in which pathogens adapt to withstand drugs, making it challenging to treat infections. The growing antimicrobial resistance crisis claimed about five million lives in 2019, a number projected to double by 2050. In an August research review, scientists called attention to the “silent tsunami” of plastics-driven antibiotic resistance. Several other recent papers suggest microplastics serve as better homes for pathogens than some natural substances do, although the mechanisms are not fully understood.

“We’ve just really scratched the surface,” says Timothy Walsh, a microbiologist at the University of Oxford, who has previously studied antimicrobial resistance and microplastics.

When bacteria encounter a surface—a sliver of wood floating in water or a door handle—they stick to it and to one another, forming a biofilm. As they attach, “they grow and proliferate,” says Muhammad Zaman, a biomedical engineer at Boston University. In a biofilm, bacteria live close together, making it easier to transfer genetic material from one cell to another in what’s “basically bacterial sex,” says Emily Stevenson, a public health researcher at the University of Exeter in England and lead author of the August review paper. The more chances microbes get to swap genes in general, the more chances they have to spread DNA that codes for antibiotic resistance.

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Scientists in Germany, Costa Rica and the U.K. first detected the troubling trend in 2018, when they showed that bacteria on microplastics were more inclined than free-living bacteria to exchange genes encoding resistance to trimethoprim, an antibiotic commonly used to treat bladder infections and traveler’s diarrhea. Other research has since shown that antimicrobial resistance genes were more common in the plastisphere—the community of microbes living on plastics—compared with those in water or soil.

But scientists have questioned whether detecting high quantities of antimicrobial resistance genes actually translates to higher amounts of resistant bacteria—pathogens surviving despite being doused with drugs.

A study published in March offers strong evidence that it does. Scientists tested lab-grown Escherichia coli biofilms in various environments, including on microplastics, on tiny glass fragments and in cell culture media. They found that pathogens in the plastisphere not only grew faster—reconfirming earlier research—but, importantly, were harder to kill when treated with several different antibiotics. The effects “were significantly larger than what we were expecting,” says Zaman, who was senior author of the study. For example, after applying the widely used antibiotic ciprofloxacin, resistance was 75-fold higher among E. coli grown on microplastics than those grown alone.

These effects do not appear to be limited to lab-grown bacteria. Researchers in Germany and Poland added microplastics to water samples taken from the Oder, a major European river. The results, published in May in Scientific Reports, revealed that after a week of incubation, disease-causing bacteria—such as E. coli, Klebsiell pneumoniae and Salmonella—were more abundant in samples with the added plastics than in those without them. What’s more, the number of antibiotic resistance genes was also higher among the plastisphere.

Stevenson points to another insightful study, published in May, that involved a bay in Xiamen, China. Scientists submerged individual bags of different microplastics and natural surfaces into the water and performed various tests on the resulting biofilms—running analyses on biofilm formation, metabolic activity, antibiotic resistance genes and actual resistance against three antibiotics. The results suggest that biofilms containing metabolically active, resistant bacteria were about 10 times more likely to form on microplastics than on natural surfaces, equating to about a 10-fold higher human health risk, according to the study authors.

So how do antibiotic resistance genes arise on microplastics in the first place? A July study in Scientific Reports investigates one theory: antibiotics can cling to microplastics, too. The study’s authors showed that common antibiotics, such as amoxicillin and tetracycline, stuck to microplastics—and that, the older the microplastics were, the more readily the antibiotics attached to them. As they age, microplastics become rougher and more electrostatic, which makes them even better at trapping antibiotics. The combination of antibiotics and pathogenic bacteria biofilms on microplastics could theoretically drive the evolution of antimicrobial resistance.

Stevenson says that the question of whether microplastics are significantly better at spurring antimicrobial resistance than other surfaces such as wood or glass is far from settled, but the mere fact that plastics have the potential to carry both dangerous pathogens and antibiotics is reason enough to worry.

Microplastics have been detected in nearly everything from air, water, plants and food, and as they are ingested, they bioaccumulate in animals’ tissues—including human brains. A preliminary mouse study suggests that microplastics in the gut microbiome may also be fertile ground for antibiotic-resistant bacteria: mice exposed to both microplastics and tetracycline had more antibiotic resistance genes in their gut microbiota than rodents exposed only to the antibiotic. “These kinds of things merit serious investigation,” Zaman says.

What’s more, microplastics travel and do not degrade. Pieces of plastic carrying multidrug-resistant bacteria have been found as remotely as Antarctica. Microplastics’ potential to shuttle antimicrobial-resistant bacteria around the globe makes Zaman particularly worried about plastic pollution that may pick up pathogens in wastewater treatment plants, hospital sewage or refugee camps—the latter of which he is currently investigating.

“I think it’s pretty clear that you see an enrichment of antibiotic-resistant bacteria and antibiotic resistance genes on plastic particles,” says Johan Bengtsson-Palme, a microbiologist at Chalmers University of Technology in Sweden, whose research focuses on antibiotic resistance. But how much of a threat plastic-derived drug-resistant pathogens pose to humans is a question that remains to be fully understood, he says.

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