Swimming in the ocean can change your microbiome. And that might make you sick

Swimming in Grand Strand swashes and ocean outfalls unsafe

Swimming in swashes and ocean outfalls along the Grand Strand is unsafe as water may contain high levels of bacteria after heavy rains.
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Swimming in swashes and ocean outfalls along the Grand Strand is unsafe as water may contain high levels of bacteria after heavy rains.

What happens to human skin during a swim in the ocean? In particular, what happens to that layer of microbes that live all over human bodies? And what does that mean for swimmers off the NC coast?

Scientists know that swimming in the ocean increases the risk of infection. Big changes in colonies of skin microbes can have the same effect. One California scientist set out to discover what happens to organisms living on human skin when exposed to ocean microbes, and was surprised at just how much they changed.

Microbiomes, swimmers, and infections

Human microbiomes always seem to be in the news, whether in reports about parents overzealously sanitizing the house , or fecal transplants leading to serious illness and death from E. coli infection. The microbiome is a vast array of organisms that make vitamins and help digest humans’ food, among other things. Colonies of bacteria, fungi, viruses and mites naturally live all over human skin, and can help keep out invading organisms.

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However, according to Marisa Chattman Nielsen, a doctoral student at the University of California - Irvine, the ocean microbiome is very different from what is found on human bodies on a regular basis. While those tiny ocean organisms might not cause any harm, changes to the microbiome seem to make humans more susceptible to diseases. That’s because the creatures who often live on the skin’s surface can ward off harmful organisms.

Battle of the microbiomes

Nielsen went to a California beach armed with swabs, and looked for volunteers who weren’t wearing sunscreen, hadn’t bathed in the past 12 hours, didn’t go to the beach much and hadn’t recently used antibiotics. She found nine volunteers, swabbed the back of their calves to collect microbiome samples before they got into the water and then swabbed again after a 10-minute swim.

So what happened after hanging out in ocean water? Before swimming, each participant had different skin microbes, as expected. After swimming, they shared the same species of skin organisms: all of them from the ocean, all completely different from their original skin swabs.

Six hours after the swim Nielsen tested again, and found the swimmers’ original microbiomes were making a comeback. After 24 hours, their original microbes were well on their way to recolonizing their bodies.

Use caution and keep swimming

What caused concern for Nielsen was that some bacteria seemed to stick to the skin after swimming: a species called Vibrio. Most Vibrio are harmless, but the Vibrio genus includes the infectious disease cholera, which can cause diarrhea, dehydration and death. Nielsen didn’t study swimmers’ infection rates, but did suggest that future studies look at whether pathogenic bacteria from polluted areas stick to swimmers, and if microbiome changes do indeed put swimmers at risk of infection.

“Everybody’s asking me, ‘What do I do?’” Nielsen said. “Go in the ocean. I go in the ocean. I’ll let my toddler swim in the ocean, but we take a post-swim shower.”

Lawrence David, an assistant professor at Duke University School of Medicine, also works with human microbiomes. He agrees that this research isn’t going to keep him out of the water. He explained that, since the ocean is so different from human skin, ocean microbes probably have a hard time gaining a foothold.

“If anything, what’s really fascinating to me is how resilient our natural bacterial communities on the skin appear to be,” David wrote in an email to the News & Observer. Even after the challenges of an ocean environment, they keep coming back.

Nielsen’s work took place on a relatively clean California beach. She said that, though she didn’t want to make people swim in dirty water for the sake of research, polluted areas might yield different kinds of bacteria. She wants to continue researching human microbiomes by studying if frequent ocean swimming causes human microbiomes to reflect exposure to ocean organisms.

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Jennifer DeMoss is a science intern at The News & Observer through a fellowship with the American Association for the Advancement of Science. Jennifer, an anthropologist with training in forest ecology and botany, is looking forward to covering the latest research in the North Carolina area.