To find out if the microbes being introduced might be “good” or “bad,” Fackelmann and her colleagues analyzed the microbiomes and looked up individual types of microbes in databases to learn what they do. They found that with more plastic, there were more microbes that are known to break down plastic. There were also more microbes that are known to be resistant to antibiotics and more with the potential to cause disease.
Fackelmann and her colleagues didn’t assess the health of the birds, so they don’t know if these microbes might have been making them unwell. “But if you accumulate pathogens and antibiotic-resistant microbes in your digestive system, that’s clearly not great,” says Wagner.
The study, which was published in the journal Nature Ecology and Evolution, shows that the levels of plastic already present in the environment are enough to affect animals’ microbiomes, says Fackelmann. The next step is to work out what this might mean for their health and the health of other animals, including humans, she says.
“When I read [the study], I thought about the whales we find beached with kilograms of plastic debris found in their bellies,” says Wagner. “It’s probably quite comparable to what birds have in their digestive systems, so it would be interesting to know if this happens in whales, dolphins, [and other marine animals] as well.”
We don’t yet know if the amount of plastic that humans ingest might be enough to shape our microbiomes. People ingest a lot less plastic than seabirds do, says Richard Thompson, a professor of marine biology at the University of Plymouth in the UK. The amount of plastic that gets into our bodies also depends on where we live and work. People who work in textile factories will have a higher exposure than those who work outdoors, for example.
And we don’t know the consequences of ingesting microbes that cling to the microplastics that get into our bodies. Humans are already exposed to plenty of disease-causing microbes that aren’t on plastics, Thompson points out. For example, we might worry that tiny bits of plastic might pick up nasty bugs in wastewater, and that these might somehow end up in our bodies. But overflows of wastewater regularly contaminate beaches and drinking water directly.
There’s a chance microbes that break down plastic will end up residing in our guts too. It’s difficult to know how—or whether—this will affect us. Microbes can evolve quickly, and they can swap genes with neighboring bugs. “Are we going to evolve to eat plastic? My answer would likely be no,” says Fackelmann. But the possibility that our guts will become home to more microbes that can break down plastic is “not beyond the realm of possibility,” she says.
There’s also the possibility that plastic pollution will affect us indirectly. Introducing more pathogenic microbes to birds and other animals could cause disease outbreaks, and one of the microbes that the team found to be correlated with plastic in the birds’ guts is thought to be able to jump from animals to humans. Wagner thinks it is unlikely that microbes seabirds pick up from floating plastic could eventually cause disease outbreaks in people. “But the more we disturb natural systems, the higher the likelihood of zoonosis [a disease jumping from animals to humans],” he adds.
Given the ubiquity of microplastics, studies like these are desperately needed to help us understand how plastic pollution affects living creatures, including humans, the researchers say.
“We’ve basically plasticized the globe,” says Wagner. “Everybody is exposed to microplastics and the chemicals in plastics—it’s just a matter of time until we figure out what it’s doing to our microbiome as well. And I cannot see any argument for why plastic ingestion would be beneficial.”