Japanese gut bacteria gain special powers from sushi

Sushi arms the guts of the Japanese with new digestive powers. A seaweed-eating enzyme seems to have jumped from marine bacteria to the harmless bugs that call the intestines of sushi-eaters home. This is the first evidence that food bacteria can transfer genes to our own gut bacteria, and could help us extract more energy from food, says Mirjam Czjzek at the National Centre for Scientific Research (CNRS) in Roscoff, France.

Czjzek's group uncovered the genetic swap while hunting the genes for certain enzymes produced by bacteria. These enzymes break down carbohydrates in the cell walls of the algae that the bacteria feed on.

One enzyme, porphyranase, breaks down a polysaccharide that makes up around 40 per cent of the cell walls of Porphyra, a red alga used to make the nori sheets that wrap around sushi. The carbohydrate is rare in most other marine plants, however.

The enzyme turned up in samples of marine bacteria, but also, curiously, the gut of a Japanese person. "We thought this was a funny coincidence, especially as Porphyra is used in sushi," says Czjzek.


The guts for sushi

To see if a gene for the enzyme was present in the microflora of other Japanese people, Czjzek's team looked up a previous study of Japanese and North American guts. The gene was not found in any North American, but the group found porphyranase genes in 5 of the 13 Japanese.

"The number is low," says Czjzek. "But we can say that apparently this enzyme is present only in Japanese and not in [North] Americans." She thinks the difference is due to the seaweed-rich diet of Japanese people, who eat an average of 14.2 grams of the stuff a day.

Genes regularly shuttle between different bacteria, offering each other new traits such as drug resistance. But this is the first time a gut bacterium has been found to have got new genes from its host's food. In theory, Japanese people with the porphyranase enzyme can digest seaweed, while it passes straight through the North American gut.

"It's a really nice demonstration of genetic variation of microbiota between individuals," says Justin Sonnenburg, a microbiologist at Stanford University in California. He agrees that the gut bacteria from the Japanese people probably use the enzyme to break down seaweed carbs.

But non-Japanese readers shouldn't rush off for a sushi fix to "grow your own" porphyranase enzymes just yet. Supermarket seaweeds are commonly roasted to sterilise them, Czjzek says. "If the algae are roasted before preparing sushi, the microbes are destroyed, and the genes are not transferred."

Journal reference: Nature, DOI: 10.1038/nature08937