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Two scourges of 20th century public and environmental health—microplastics and antimicrobial resistance—seem to be teaming up to birth a whole new worry.

The ocean is teeming with microorganisms. And since the mid-20th century, plastic. Scientists have previously discovered how plastic creates habitat and a handy transport system for marine microbes, including potentially harmful human pathogens. Now, researchers led by Sasha Tetu, a microbiologist at Macquarie University in Australia, have shown that chemicals leaching from marine microplastic pollution can alter the composition of microbial communities, making them more virulent and increasing the prevalence of antimicrobial resistance.

Tetu and her colleagues conducted their work in the lab. They collected samples of seawater, mixed them with the leachates from polyvinyl chloride (PVC), and analyzed how the DNA of the bacteria living in the water changed over six days.

PVC is one of the most common plastics, and like many other plastics, it leaches additives like metals, dyes, and stabilizers—compounds added to improve the plastic’s performance.

The scientists aren’t exactly sure how or why, but the plastic-addled bacteria increasingly carry genes related to higher virulence and antimicrobial resistance.

When they dug deeper into their results, they found that the effect is more complex than a blanket increase in resistance to all antimicrobials. Instead, the bacteria showed an increased abundance of genes that offer resistance to some classes of antimicrobials and a decreased prevalence of genes protecting them from other groups of antibiotics.

Likewise, the prevalence of genes that encode for certain types of virulence, such as mechanisms to suppress a host’s immune response, increased significantly, while those linked to other harmful activities decreased.

Many of the genes for antimicrobial resistance were identified in bacteria that are not known to be human pathogens. But this does not mean they do not pose a risk to human health, Tetu says. “Microbes have many different ways of sharing genes, often across distantly related lineages.”

Tetu says her research is a first step and that more investigation is needed to determine how plastic leachates may be affecting microbial communities. But the gist is that plastic exposure seems to select for hardier microbes, with increasing antimicrobial resistance being an unhappy little accident.

“Exposure to such leached chemicals,” says Tetu, “selects for a suite of opportunistic environmental microbes that are likely more metabolically versatile and also happen to carry a variety of genes associated with antimicrobial resistance.”

Emily Stevenson, a graduate student at the University of Exeter in England, studies how microplastics affect the spread of antimicrobial-resistant bacteria. She says this new research gets at important questions around how plastic exposure changes microbial communities. How well the laboratory findings translate into real-world conditions is hard to know, but Stevenson says it’s worrying.

“What I am particularly concerned about,” she says, “is where these leachates are antimicrobials themselves.” A lot of plastics are impregnated with known antimicrobials, such as triclosan. “If that is bioavailable to the bugs, then that would be adding a selective pressure, so they are likely to evolve antimicrobial resistance,” she adds.

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