Human pathogens found on litter in the sea

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 plastic had been submerged in the ocean off Falmouth, England, for only a week, but by that time a thin layer of biofilm, a slimy mixture of mucus and microbes, had already developed on their surfaces. Michiel Vos, a microbiologist at the University of Exeter in England, had sunk 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 focus 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 life-threatening bacteria, the scientists injected the biofilm into wax moth larvae. After a week, four percent of the larvae died. But four weeks later, after Vos and his team let the plastic stew in the ocean for a little longer, they repeated the test. This time, 65 percent of the wax moths died.

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

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

But whether plastics are enduring populations of pathogens dense enough to be actually dangerous and whether they are transporting them to new areas are difficult questions to answer.

There are good reasons to believe that plastics are accumulating and spreading pathogens around the world. Linda Amaral-Zettler, microbiologist at the Royal Netherlands Institute for Sea Research, who coined the term plastic for the new ecosystem created by plastic, he says that plastic is different from other hard surfaces often found in the ocean, such as logs, shells, and rocks, because plastic is durable, long-lasting, and much of it floats. “This gives him mobility,” he says.

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

Recent laboratory work also shows that some disease-causing parasites typically on land can survive in seawater and infect marine mammals. Karen Shapiro, an infectious disease expert at the University of California, Davis, has shown that these parasitic protozoa, in particular, Toxoplasma gondi, Minor cryptosporidiumAnd Giardia enteric—Can attach itself to microplastics in seawater. This could alter where, when and how these parasites accumulate in the ocean.

“If they’re hitchhiking on plastic that’s in the same sewer, or river, or overland runoff from a storm drain, then they’re going to end up where the plastic ends up,” says Shapiro. It could be in crustaceans at the bottom of the sea or floating on currents in the middle of the ocean.

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

The fact that microplastic pollution appears to be a breeding ground for pathogens also raises a long-term concern for Vos: plastic could promote the spread of antibiotic resistance. Bacteria can exchange genes, and because bacteria are in close contact on the surface of tiny microplastics, the level of horizontal gene transfer between them is high, she says. Plastic can also put bacteria in close contact with pesticides and other pollutants, which also attach themselves to biofilms. This encourages the development of antimicrobial resistance.

“We don’t know much about it,” says Vos, “but there are potentially interesting ways that bacteria can experience stronger selection. [for antimicrobial resistance] on plastic, but they also have more possibilities to exchange genes that could 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 rely on seafood as a source of protein, and there are many pathogens that infect the fish and shellfish we eat. It may be possible, says Amaral-Zettler, that microplastics spread disease between different areas of aquaculture and fisheries.

While we don’t fully understand the risks, these studies are another good argument for limiting plastic pollution, says Vos. “There can be nothing positive about plastics with pathogens floating around.”

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