Japanese gut bacteria have picked up genes that help their hosts digest the seaweed used to wrap sushi

The next time you order sushi in a Japanese restaurant, raise a glass of sake to the countless marine microbes that might be clinging to it.

Bugs that live on the seaweed used to wrap sushi have given some of their genes to bacteria that live in the human gut, and in doing so, help them to digest the food.

Scientists stumbled on the discovery after sequencing the genetic make-up of marine bacteria that live on Porphyra seaweed and searching DNA databases for matches.

Eleven genes used by the bugs to break down carbohydrates in seaweed also showed up in bacteria isolated from the intestines of Japanese people, but were absent from the gut microbes of North Americans.

Writing in the journal Nature, the scientists describe how a long tradition of seaweed eating in Japan that dates back at least to the 8th century had caused their gut bacteria to adapt to the national diet. Marine microbes probably swap their genes with bacteria in the gut, giving some the ability to digest seaweed.

The human intestine is home to trillions of bacteria, of which there are an estimated 500 different species. Many benefit their hosts by helping to break down food that would otherwise be impossible to digest.

"This gives us a hint at how the diversity of bacteria we have in our guts arises and evolves," said Mirjam Czjzek at the University of Pierre and Marie Curie in Paris. "What we eat and how we prepare it can have an influence on our microflora."

Czjzek's team analysed the genes of the marine bacterium Zobellia galactanivorans, which lives on Porphyra algae, and found several that help the microbes digest seaweed. Further searches found the same genes had been incoporated into a common gut microbe called Bacteroides plebeius.

The scientists have yet to work out whether Japanese people benefit from bacteria in their gut that carry the genes, or if the only advantage is for the bacteria themselves.

In an accompanying article, Justin Sonnenburg at Stanford University School of Medicine writes: "The next time you take a bite of an unfamilar food, think about the microbial inhabitants you may also be ingesting, and the possibility that you will be providing one of your 10 trillion closest friends with a new set of utensils."