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Each year, millions of tonnes of plastic enter our oceans. Many of the negative effects of all this plastic are well known: it poisons fish, entangles sea turtles, and has been implicated in the deaths of sperm whales.

New research raises yet another concern: marine plastics may be spreading antibiotic resistance. A recent study in Northern Ireland found that almost all of the marine plastics studied harbored antibiotic-resistant bacteria.

Researchers from Queen’s University Belfast in Northern Ireland and the Northern Ireland Public Health Laboratory collected plastic food packaging larger than five millimeters in diameter from several locations along the Irish coast.

The scientists collected bacteria from the plastic packaging and tried to kill them with 10 common antibiotics. They found that 98 percent of the plastic pieces they collected harbored bacteria that were resistant to ampicillin, while only 16 percent carried bacteria resistant to minocycline. The prevalence of resistance to other antibiotics fell between these two extremes.

The researchers also identified the bacteria and found that while some are relatively harmless, many are known to cause disease in plants, fish, and even humans.

Adam Martiny, an environmental microbiologist at the University of California, Irvine, who was not involved in the study, says bacteria will readily grow on any surface in the ocean. So by filling the ocean with plastics, we’re creating lots of new habitats.

“This idea that we are greatly increasing the surface area that microbes can colonize in the ocean is concerning from a variety of different perspectives,” he says.

The rapid spread of antibiotic resistance is mostly caused by human activities that take place on land, such as the overprescription of antibiotics. Martiny says it’s unlikely that terrestrial bacteria can survive for long at sea, but because bacteria have the ability to swap DNA with each other, genes for antibiotic resistance can be passed to other bacteria.

As the authors write in their study, once these genes get into oceangoing bacteria, mobile surfaces such as plastic debris can help them travel great distances, potentially facilitating the international spread of dangerous antibiotic resistance.

Gabriel Perron, a microbiologist at Bard College in New York who was not involved in the study, says he is concerned about the potential effects that ingesting plastic laden with antibiotic-resistant bacteria might have on animals, including people.

The authors note that antibiotic-resistant bacteria on marine plastics could be ingested by fish or other marine creatures that humans eat. Yet the authors emphasize the need for more research as it’s unknown exactly how likely it is that these contaminated plastics could cause disease.

Martiny says that along with reducing the amount of plastic that enters the ocean, using plastics that biodegrade in ocean water may help combat this potential public health problem. Perron adds that “antibiotic resistance in the environment is definitely contributing to the antibiotic resistance crisis and mitigating it has to be part of the solution.”