Are microplastics spreading infectious disease?

Definitive evidence remains elusive, but early results suggest there’s good reason for vigilance.

In 2010, a team of marine scientists based in Woods Hole, Massachusetts, began dipping fine mesh nets into the sea in the middle of the North Atlantic. As each net skimmed the ocean surface, it amassed tiny plastic remains from far-off shores.

As microplastics become nearly ubiquitous in our oceans, researchers are struggling to understand how they might impact disease transmissions. Image credit: Shutterstock/chayanuphol.

When the researchers viewed their catch under a scanning electron microscope, they discovered that these microplastics were teeming with life. Algae, protists, and bacteria all lived together on plastic specks (1). The study’s senior author, marine microbial biologist Linda Amaral-Zettler of the Marine Biological Laboratory (MBL), dubbed these rich communities of plastic inhabitants “the plastisphere” (1).

In a preliminary census, the team discovered one alarming plastisphere resident: Vibrio, a genus of bacteria that includes strains pathogenic to humans. At the time, the researchers couldn’t tell whether this particular Vibrio found thriving on a fragment of polypropylene was truly a threat. But Amaral-Zettler, now at MBL and NIOZ, the Royal Netherlands Institute for Sea Research, and her team recently analyzed its full genome and discovered genes known to underlie pathogenic behavior, including many seen in the Vibrio that causes cholera (2).

Indeed, it turns out that a host of potentially infectious disease agents can live on microplastics, including parasites, bacteria, fungi, and viruses.

With microplastic pollution growing—one recent estimate suggests that between 82 and 358 trillion pieces are afloat just in the planet’s oceans (3)—researchers worry that pathogens may use these durable and buoyant fragments to hitchhike long distances in water or air. Animals, including humans, could inhale or ingest pathogen-coated microplastics and contract infectious diseases. Other evidence suggests that microplastics can encourage the evolution of antibiotic resistance and skew host immune responses, altering the conditions for disease transmission less directly.

Definitive proof that any of this leads to increased disease is lacking, though. “There are probably more papers that have tried to put forward the reasons why we have to worry about microplastics, and far less that have demonstrated that microplastics are indeed dangerous in terms of the possibility that they can spread infectious diseases,” says Gabriele Sorci, an expert in the ecology and evolution of host–pathogen interactions with the French National Centre for Scientific Research, who coauthored a 2022 discussion paper weighing whether microplastics can promote infectious disease emergence (4).

To assess the threat, microbial ecologists and infectious disease experts are probing the sea and its creatures for microbes linked to microplastics and challenging lab animals and human cells with a host of pathogen-coated microplastics. But researchers are up against the great diversity of microplastic sizes, shapes, and chemical compositions, as well as myriad pathogens, whose individual success may hinge on their own genetic variations and the particulars of the plastics they encounter. Researchers already know that disease spread through microplastics could happen, Sorci says. “Therefore, it is important to study what is the probability that this will happen.”

The Plastic Problem

Since the 1950s, industry has pumped out plastic products at an increasingly rapid clip (5). Production now reaches about 400 million metric tons per year (5). Half of these plastics are intended for a single use, such as polystyrene plates and polypropylene grocery bags, of which the vast majority are not recycled.

Once tossed, plastics can remain in the environment for centuries, slowly breaking down into smaller pieces. Although plastic is long-lived, some plastic types can begin eroding within months or even weeks when exposed to the elements on a beach or ocean surface (see “Opinion: We need better data about the environmental persistence of plastic goods,” https://www.pnas.org/doi/10.1073/pnas.2008009117). When pieces measure less than 5 millimeters, researchers call these fragments microplastics. Industry also produces microplastics directly for consumer use, such as beads in exfoliating face washes and fibers in synthetic textiles.

Even if industry halted all plastic production, microplastics would continue to swell as our existing plastics slowly break down. Microplastics have been found in sea ice in the Arctic Ocean (6), air in the Antarctic (7), and dust in homes (8). They’ve turned up in the guts of seals and dolphins (9) and in seafood (10) and bottled water (11). They’ve even been found in human blood (12), stools (13), lungs (14), and placenta (15). “We’re swimming in a mess of our own making,” says marine ecologist Randi Rotjan of Boston University.

Exactly what these bits may be doing to the health of humans and wildlife isn’t clear. Plastics are often imbued with toxic chemicals like flame retardants. They can also attract toxic chemicals from the environment. Some prior microplastic studies have explored how these toxins, as well as the physical structure of plastics, could potentially wreak havoc on a body (16). In May, a team of UK- and Australia-based researchers coined “plasticosis” to describe the fibrotic disease caused by plastic-induced scarring they observed in the stomachs of seabirds (17).

A micrograph shows Toxoplasma gondii (arrow) and Giardia (arrowheads) within a biofilm formed on a polyester microfiber that’s suspended in seawater. Image credit: Karen Shapiro.

Microbial Hitchhikers

As toxicity concerns mount, a growing community of researchers worries that microplastics may also be conduits for infectious disease. Testing this hypothesis isn’t easy. First, researchers need to show that pathogens colonize microplastics. “You have to be sure that these are indeed pathogens because, for the same bacteria, you have nonpathogenic and pathogenic strains,” Sorci says. “Then, you have to show that these bacteria, once they colonize the microplastic, can survive on these substrates long enough to get into potential hosts. They have to infect the host, and then they have to produce disease.”

Few researchers have tracked a pathogen from its entry into the plastisphere to when it infects a host. But in 2019, Rotjan and colleagues reported that pathogens can make this journey, at least in the lab (18). The team fed the coral Astrangia poculata polyethylene beads. Coral that ate these microplastics without microbes survived. Those that ate microplastics coated in a pathogenic strain of Escherichia coli died within a month. Because the team used genetically modified E. coli that fluoresces, they could follow its spread. “Even if [coral] spit the bead back out, the E. coli can stay inside the coral and actually move from polyp to polyp,” Rotjan says.

A variety of microbes can congregate on microplastics, as shown in this SEM image of a diatom (center) surrounded by bacteria and other protists. Taken from a sample collected off of the US East Coast, the microbes are situated amongst a network of thin filaments exuding from the diatom’s pores that help it attach to substrates—in this case, plastic. Image credit: Erik Zettler (NIOZ Royal Netherlands Institute for Sea Research, Texel, Netherlands. Imaged at MBL Central Microscope Facility, Woods Hole, Mass.)

Although the experiment offered a proof of principle, E. coli is not a pathogen that corals would typically encounter in the wild. Even so, the team was able to test what wild corals are likely to do when they encounter microplastics. Researchers offered coral a choice of microbe-free microplastics or brine shrimp eggs of the same size. Strangely, the coral much preferred the microplastics over the actual food (18). By dissecting corals of the same species collected off the coast of Rhode Island, Rotjan learned that they indeed consume copious amounts of microplastics (18). The average coral polyp contained over 100 particles. Whether these particles ever deliver disease remains unknown, Rotjan says.

Parasites, too, can catch a ride on microplastics, according to recent work from infectious disease expert Karen Shapiro at the University of California, Davis. Her team added three parasites—Toxoplasma gondii, Cryptosporidium parvum, and Giardia enterica—to glass bottles containing seawater and microplastics in the lab (19). They saw that all three of these parasites could attach to microplastics and that the proportion of parasites clinging to microplastics, versus floating in the water, increased over the course of a week.

In nature, these three parasites, which are known to concentrate in shellfish and sicken humans who eat raw shellfish, can all move from land to sea via water. To what extent they travel via microplastics is unclear. “The risk to people or animals is really dependent on [the parasites’] transport behavior, which is why I think it is so critical to understand the movement of how these parasites end up in the sea,” Shapiro says. She’s now testing whether microplastics increase the concentration of the parasites in oysters in the lab.

But simply inhaling or ingesting ambient microplastics (13, 14) could, theoretically, deliver pathogens that cause infections in humans. In March, for example, a team of researchers based in Guangdong, China, showed through an in vitro study that influenza A can colonize polystyrene microplastic and ride it into human lung cells via endocytosis of the plastic particle. Once inside, the microplastic also hindered the cell’s ability to produce proteins that could otherwise keep the virus’s spread in check (20).

European teams are now testing how human cells respond to pathogen-coated microplastics as part of a broader microplastics initiative called the European Research Cluster to Understand the Health Impacts of Micro- and Nanoplastics (CUSP). One series of in vitro studies, not yet published, suggests that Vibrio parahemolyticus, a species that frequently contaminates seafood, can attach to microplastics and then infect human gastrointestinal cells at higher rates than the pathogen without microplastics. This work is ongoing, says toxicologist and CUSP team member Alberto Katsumiti of the GAIKER Technology Centre in Zamudio, Spain. But “our results are showing that microplastics can act as vectors of these pathogens.”

Beyond their potential for direct delivery of infectious agents, there’s also growing evidence that microplastics can alter the conditions for disease transmission. That could mean exacerbating existing threats by fostering resistant pathogens and modifying immune responses to leave hosts more susceptible.

Reservoirs of Resistance

Researchers in Germany witnessed just how quickly bacteria can evolve antibiotic resistance on microplastics. When they placed Pseudomonas bacteria from a German lake into tanks of water with resistant E. coli, up to 1,000 times more individual Pseudomonas bacteria received an antibiotic resistance gene from the E. coli donor when living on microplastic compared to when living in a plastic-free tank (21).

Why the big discrepancy? These small flecks of plastic may foster resistant bacteria by serving as hubs for gene exchange, says aquatic microbial ecologist Hans-Peter Grossart, who led the experiment. When bacteria colonize microplastics and other surfaces, they often form a biofilm—a layer of densely packed bacteria within a protective matrix of polysaccharides and other molecules. Close proximity to neighbors makes it easy to trade genes, including those conferring antibiotic resistance, on circular pieces of DNA called plasmids. “They can be exchanged quite rapidly,” says Grossart, who’s at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin.

His team also left microplastics in mesh cages in the same lake and waited for biofilm to form. They then harvested the bacterial colonies and learned that this diverse community of species was also far more likely to take in the E. coli ’s plasmid than bacteria found floating freely in the lake (21). (Grossart cautions that his team used a plasmid created in a lab; with a natural plasmid, the outcome could be different.)

“Cleaning up microplastics is not a viable solution. They are ubiquitous in our environment. And macroplastics are going to break down to microplastics for millennia. What we can do is try to understand the risk.” —Randi Rotjan

Despite such disquieting findings, it’s still not clear whether microbes colonize microplastics more readily than other forms of natural debris, like wood particles. Some research, including from Grossart’s group, suggests that there are more potential pathogens on microplastics than on natural materials in some environments (22). But a 2020 meta-analysis in the Annual Review of Marine Science concluded that microplastics are no more likely to harbor potential pathogens than natural surfaces (23).

“It is certainly on our radar for future research,” says marine scientist Erik Zettler of NIOZ, the Royal Netherlands Institute for Sea Research, a member of the Woods Hole-based team that identified Vibrio on microplastics. Regardless, he notes that plastic typically lasts and floats longer than natural substrates. “So it has the potential to transport attached organisms, including nonnative species and pathogens, further around the globe,” he says.

While in transit, these plastic bits may do more than just serve as a hub for gene exchange. Antibiotics in waste streams can adhere to microplastics, which could kill off susceptible bacteria, encouraging the proliferation of those that resist the drugs (4). Heavy metals mixed with microplastics—such as the bits that fly off tires over time—can make things worse. When microbial communities on these fragments evolve resistance to the toxic metals, antibiotic-resistance genes, often located nearby in the bacterial genome, can get pulled along for the ride. As a result, heavy metals present a selective pressure for antibiotic resistance genes, Grossart explains.

Of course, bacteria can also enter the plastisphere already resistant to antibiotics. Microplastics frequently ride into rivers in wastewater rife with antibiotic-resistant bacteria that could colonize the tiny life rafts (4). “The water runs down the river and gets diluted,” Grossart says. “When you have microplastic, you keep basically a microenvironment where the respective bacteria, including probably multiresistant bacteria, still grow.”

Weakened Defenses

Even if microplastics are disease-free, they may have toxic effects that leave animals more susceptible to the pathogens they encounter. At the Virginia Institute of Marine Science (VIMS) in Gloucester Point, researchers placed different types and concentrations of microplastics into tanks of rainbow trout and then exposed the fish to an aquatic virus known as infectious hematopoietic necrosis virus (IHNV) 4 weeks later.

The team’s results, reported in March, showed that microplastics alone had no impact on trout survival (24). But when fish swimming with specific types and concentrations of microplastics were exposed to the virus on a day when the tanks were free of plastic, they died at higher rates than fish that had never experienced microplastics. Nylon fibers at the study’s highest concentration—10 milligrams per liter—had the biggest impact. About 80% of fish in this treatment died from the virus, compared to 20% of unexposed fish.

It’s not as though the water had been thick with plastic. Imagine piling nylon filaments onto a quarter and then sprinkling them into a 6-liter tank, says marine scientist and environmental chemist Meredith Seeley, who conducted the research as a PhD student at VIMS. That amount of microplastic was all it took for the dramatic increase in mortality. This concentration of microplastic represents a sudden, high influx into a habitat at a level that is “not outside the realm of possibility” in nature, explains Seeley, now a National Research Council postdoctoral fellow at the National Institute of Standards and Technology in Waimanalo, Hawaii.

In this study, the microplastic likely wasn’t the vehicle for the virus. The team hypothesizes that the prior exposure to microplastics irritated the fish’s gills or digestive tract, causing damage to protective membranes, which gave the virus easier entry. This irritation could also trigger the immune system to launch an inflammatory response that made it harder for fish to subsequently fight off the virus. And if microplastics are carrying pathogens, the inflammatory effects may be even worse, according to work from Katsumiti of CUSP.

Whatever the exact mechanism, the outcome suggests that microplastics in fisheries could be costly. IHNV is “really prevalent in salmonid fisheries in both aquaculture and wild populations,” Seeley says. “It leads to huge financial losses.”

Myriad Microplastics

Despite all the suggestive findings, conclusions are hard to come by. Part of the challenge is methodological. Lab experiments can only explore a select few of the great many microplastic forms. Studies often use perfect spheres manufactured for industry, even though many microplastic pollutants are irregular fragments of larger “macroplastics.” Seeley, in her follow-up work, found that microfibers, for example, increase the rainbow trout’s susceptibility to the virus more than spheres.

Size could confound results, too. Plastics less than 100 nanometers are called nanoplastics. These offer less surface for bacteria to exchange genes and develop resistance, but they enter cells more easily than larger microplastics, notes food microbiologist and CUSP team member Andreja Rajkovic of Ghent University in Belgium.

Claire Loiseau, an ecologist at the University of Montpellier in France and coauthor with Sorci of the 2022 discussion paper, is grappling with the challenge of how such differences might affect outcomes, while designing her own upcoming study to test whether microplastics shift the microbiomes of mosquitoes in a way that could alter the bugs’ ability to transmit pathogens such as malaria. “There are different types of polymers. There are different types of sizes. There are different types of shapes,” she says. “What does it mean, ‘microplastic’?”

Composition, whether polypropylene or polystyrene, influences which bacteria thrive, Rajkovic says. His work, not yet published, suggests that some strains of Staphylococcus aureus form biofilms on microplastics, while others don’t. Rajkovic and his team have sequenced the entire genomes of these and other bacteria and are now searching for the genes that determine which strains will colonize which microplastics.

Complicating matters, many early studies did not sequence bacterial communities extensively enough to determine whether strains on microplastics were truly pathogenic. But today, with deeper sequencing, more data are coming. Grossart, also a CUSP team member, is partnering with researchers around the globe to leave microplastics in the sea for months at a time. Once the team retrieves the plastics, they’ll use long-read sequencing technology to more closely identify bacterial inhabitants and any antibiotic-resistance genes they carry.

Sorting through all of these variables takes time. “I don’t think in general there is enough information and enough scientifically credible evidence to support any final conclusion,” Rajkovic says. He questions whether microplastics, if they really could spread disease, wouldn’t already be doing so at noticeable levels. “I don’t think that we are seeing more infections than before,” he says. “Not certainly in terms of food safety.”

Still, the scope of the potential problem will only get bigger, along with the volume of microplastics. “Cleaning up microplastics is not a viable solution. They are ubiquitous in our environment. And macroplastics are going to break down to microplastics for millennia,” says Rotjan at Boston University. “What we can do is try to understand the risk.”