Human pathogens found on waste at sea

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This article was originally published on Hakai Magazine, an online publication on science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.

The plastics had only been submerged in the ocean off Falmouth, England, for just a week, but during that time a thin layer of biofilm, a slimy mixture of mucus and microbes, had already grown inside. their surface. Michiel Vos, a microbiologist at the University of Exeter in England, had cast five different types of plastic as a test. He and his colleagues wanted to know which of the myriad microbes living in the ocean would reflect on these introduced materials.

The main concern of Vos and his colleagues was pathogenic bacteria. To understand the extent to which plastic can be colonized by potentially deadly bacteria, scientists injected the biofilm into wax moth larvae. After a week, four percent of the larvae are dead. But four weeks later, after Vos and his team had left the plastics to simmer in the ocean a bit longer, they repeated the test. This time, 65% of the wax moths died.

The scientists analyzed the biofilm: the plastics were covered in bacteria, some of which are known to make us sick. They discovered pathogenic bacteria responsible for urinary tract, skin and stomach infections, pneumonia and other illnesses. To make matters worse, these bacteria also carried a wide range of antimicrobial resistance genes. “Plastics you find in water are quickly colonized by bacteria, including pathogens,” Vos says. “And it doesn’t matter what plastic it is.”

It’s not just bacteria that hitchhike on plastics. Biofilms on marine plastics can also harbor parasites, viruses and toxic algae. With marine plastic pollution so ubiquitous — it’s found everywhere from the bottom of the Mariana Trench to Arctic beaches — scientists fear that plastics are carrying these human pathogens into the oceans.

But it’s hard to answer the question of whether the plastics contain populations of pathogens dense enough to be actually dangerous, and whether they transport them to new areas.

There are good reasons to believe that plastics accumulate and spread pathogens around the world. Linda Amaral-Zettler, a microbiologist at the Royal Netherlands Institute for Marine Research, who coined the term plastisphere for the new ecosystem created by plastics, says that plastic is different from other hard surfaces often found in the ocean – such as logs, shells and rocks – because plastic is durable, has a long lifespan and a large part floats. “It gives him mobility,” she says.

Plastics can travel long distances. After the 2011 earthquake and tsunami in Japan, for example, many identifiable Japanese objects washed up on the west coast of North America. This litter, says Amaral Zettler, has “the potential to carry anything that comes with it.”

Recent laboratory work also shows that some typically terrestrial parasitic pathogens can survive in seawater and infect marine mammals. Karen Shapiro, an infectious disease expert at the University of California, Davis, has shown that these protozoan parasites, in particular, Toxoplasma gondii, Cryptosporidium parvumand Giardia enterica— can attach to microplastics in seawater. This could alter where, when, and how these parasites accumulate in the ocean.

“If they hitch a ride on plastics that are in the same sewer outlet, or in the same river, or in storm sewer runoff, they’ll end up where the plastic ends up,” Shapiro says. It could be in the seashells on the seabed or floating on the currents in the middle of the ocean.

The next step, Shapiro says, is to look for a similar association between parasites and plastics outside of the lab.

The fact that microplastic pollution appears to be a breeding ground for pathogens also raises a long-term concern for Vos that plastics could promote the spread of antibiotic resistance. Bacteria can swap genes, and because bacteria are in close contact on the surface of tiny microplastics, the level of horizontal gene transfer between them is high, he says. Plastics can also bring bacteria into close contact with pesticides and other pollutants, which also adhere to biofilms. This promotes the development of antimicrobial resistance.

“We don’t know much about it,” Vos says, “but there are potentially interesting ways in which bacteria can undergo stronger selection. [for antimicrobial resistance] on plastics, but also have more opportunities to exchange genes that might confer resistance.

In addition to posing potential risks to human health, plastic-borne pathogens could threaten marine ecosystems and food supply chains, says Amaral-Zettler. Millions of people depend on seafood as a source of protein, and many pathogens infect the fish and shellfish we eat. According to Amaral-Zettler, it could be possible for microplastics to spread disease between different aquaculture and fishing areas.

Even if we don’t fully understand the risks, these studies are yet another good argument for limiting plastic pollution, Vos says. “There can’t be anything positive about plastics with floating pathogens.”

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