A small but striking study has found that a short swim in the ocean can dramatically alter the composition of a person's skin microbiome for at least 24 hours. It is unclear what the implications of these microbiome changes are but the researchers hypothesize the alterations could leave a person more susceptible to infection.

The new study was inspired by recent research finding people who swim in the ocean have a higher risk of contracting a variety of illnesses and infections. Other work has suggested the large population of microorganisms that live on our skin, often referred to as the skin microbiome, can play a role in our susceptibility to infection.

Marisa Chattman Nielsen, from University of California, Irvine, set out to investigate whether swimming in the ocean can directly alter the composition of our skin microbiome, and if so, for how long? Nine subjects were recruited, and their skin microbiomes were tracked before, immediately after, and 24 hours after, a 10-minute ocean swim.

The results revealed a short ocean swim distinctly altered all the participants' skin microbiomes. Following the swim all nine subjects showed similar bacterial communities on their skin, and very different compositions from what was there pre-swim. The alterations remained for up to 24 hours, although in most subjects the microbiome did consistently return to a pre-swim composition after about a day.

"One very interesting finding was that Vibrio species – only identified to the genus level –were detected on every participant after swimming in the ocean, and air drying," explains Nielsen, lead author on the study. "While many Vibrio are not pathogenic, the fact that we recovered them on the skin after swimming demonstrates that pathogenic Vibrio species could potentially persist on the skin after swimming."

It's important to note that the primary goal of the study was to simply investigate how ocean water can affect the human skin microbiome, and there is no evidence that this microbiome disruption enhances a person's receptivity to infection. This study is also not yet published or peer-reviewed, as the researchers only just presented initial data at the recent annual meeting of the American Society for Microbiology.

It is also reasonable to ask how this exposure to ocean water disrupts the skin microbiome any more significantly than a thorough shower or bath. The implication in the study is that while any water exposure will obviously slightly disrupt the skin microbiome, it is this process, in addition to the major volume of bacterial pathogens present in ocean water, that makes a swim in the sea more potentially harmful.

In the end, this research should not fundamentally scare people off a swim in the ocean. There is a large array of science pointing to an assortment of health benefits that can be garnered from a dip in the sea. However, this is a solid reminder that the ocean is not akin to a sterile salt water bath, but in fact it can be filled with a large volume of different opportunistic and pathogenic microbes.