The old adage, "You are what you eat," has a bacterial component.

In a neat confluence of human history, stomach bacteria and food, researchers have found that the intestinal microbes of Japanese people may be souped up for eating seaweed.

"In a marine bacteria, we identified an enzyme that is very specialized for degrading algal cell walls," said Mirjam Czjzek, a biologist at France's Station Biologique de Roscoff. "The only other place we find this enzyme is in the human-gut bacteria of Japanese individuals."

The discovery, described April 7 in Nature, started with Roscoff biologist Jan-Hendrik Hehemann's analysis of Zobellia galactanivorans, a common marine bacteria. In it, he found an enzyme that breaks down porphyran, a carbohydrate found in the cell walls of red algae.

The gene that codes for the enzyme has been found in one other place: the genome of Bacteroides plebeius, a microbe found in human intestines. However, not all B. plebeius strains produce the algae-crunching enzyme. It has only been found in Japanese people.

According to the researchers, the enzyme helps Z. galactanivorans eat red algae, which westerners know best as the nori seaweed wrapping around sushi rolls. At some unknown points and in some unknown stomachs in the Japanese past, the enzyme-coding gene passed from Z. galactanivorans and into B. plebeius. That lucky microbe would have benefited from a new-found ability to process red algae, spreading through its stomach environment and eventually through the human population, which in turn derived more nutrients from an algae-rich diet.

Humans are known to benefit from digestive enzymes produced by the trillions of microbes in each person's intestines, but "I don't think anyone's ever shown an ethnic difference like this," said Andrew Gewirtz, an Emory University immunologist who studies the role of gut bacteria in obesity. "It's perfectly logical, and fits with ideas that scientists have kicked around."

How much the new gene helps people digest seaweed hasn't yet been quantified. The microbes' fate in people with seaweed-free diets is uncertain.

The researchers also don't know when the gene jumped from marine to human microbes, though Czjzek suspects it happened long ago. As for whether other people have seaweed-processing strains, the study isn't absolutely conclusive. It looked only at the gut microbes of 18 westerners – enough to suggest a pattern, but not a final word, though the chances are probably low.

"Often the question comes, 'I've been eating sushi for two years now. Do I have this enzyme?' The answer is, these are very rare events," said Czjzek. "In the early days, seaweed wasn't sterilized. Nowadays, it's cooked, roasted and prepared. The chance to have this type of transfer is much lower."

That's likely the case with most types of food, said Gewirtz. As for whether "that's a good or a bad thing, it's hard to say," he said. But Justin Sonnenburg, a Stanford University microbiologist who wrote a commentary accompanying the findings, is concerned.

"Consumption of hyper-hygienic, mass-produced, highly-processed and calorie-dense foods is testing how rapidly the microbiota of individuals in industrialized countries can adapt while being deprived of the environmental reservoirs of microbial genes," he wrote.

However, globalized diets do give people a chance to eat foods they wouldn't have found before. "The next time you take a bite of an unfamiliar 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," wrote Sonnenburg.

Image: Javier Lastras/Flickr.

See Also:

Citations: "Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota." By Jan-Hendrik Hehemann, Gaelle Correc, Tristan Barbeyron, William Helbert, Mirjam Czjzek, & Gurvan Michel. Nature, Vol. 464 No. 7290, April 8, 2010.

"Genetic pot luck." By Justin L. Sonnenburg. Nature, Vol. 464 No. 7290, April 8, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.