A genetic gift for sushi eaters

Seaweed-rich diet leaves its mark on gut microbes.

Genes picked up from a microbial hitch-hiker may enable some
Japanese individuals to extract otherwise intractable nutrients from
seaweed.
A study published this week in Nature1 suggests that a marine
microbe — perhaps ingested on a sliver of seaweed — probably
transferred genes that encode algae-munching enzymes to bacteria
that live in the human gut. The enzymes break down algal
carbohydrates including one found in red algae of the genus
Porphyra, best known to sushi lovers as nori.
Although gene transfer to gut microbes is suspected in other cases,
this is the first clear-cut example in which a gut microbe has gained
a new biological niche by snatching genes from an ingested
bacterium, says Mirjam Czjzek, a chemist at the Pierre and Marie
Curie University in Paris, one of the two researchers who led the
study. "Probably there are many more examples," she says. "It's
only because of this exotic niche and the very rare specificity of this
enzyme that we were able to pinpoint where it came from."
You are what you eat
Many of the microbes residing in the human gut are likely to be beneficial to their host. Some
may give their host a calorific boost by breaking down ingested plant carbohydrates that human
enzymes cannot touch. In Japan, where about 14 grams of seaweed are consumed per person
each day, some of these indigestible carbohydrates come from the algae that wrap sushi rolls and
form the basis for a variety of soups and salads.
Czjzek together with Gurvan Michel, a structural biologist also at the Pierre and Marie Curie
University, and their colleagues found a new class of algae-degrading enzymes called 8-
porphyranases while hunting for proteins that break down algal biomass.
In the genome of the marine bacterium Zobellia
galactanivorans they found enzymes that were similar
to those that degrade the algal compounds agarose and

carrageenans. But the enzymes lacked a crucial region
needed to recognize these polysaccharides.
Instead, the enzymes broke down a Porphyra
polysaccharide called porphyran. The team searched
databases for related enzymes and found that they are
all also made by marine microbes — except one found in
the genome of a human gut bacterium called
Bacteroides plebeius.
The bacterium also contained an enzyme that breaks down agarose. Bacteroides bacteria
containing these genes were only found in individuals with Japanese ancestry. And on the basis of
the similarity in DNA sequence and the finding that the two genes appear near others in the B.
plebeius genome that appear to come from marine bacteria, the researchers concluded that the
genes had been transferred from a marine bacterium to microbes living in the human gut.
Genetic riches
The results suggest that ingested bacteria may have provided a valuable genetic resource for gut
microbes throughout human history, says Justin Sonnenburg, a microbiologist at Stanford
University in California.
But as our food becomes increasingly sterile, our exposure to this
genetic treasure chest is dwindling, he adds. "We've gone to great
lengths in the developed world to decrease the microbial burden of
food, and in doing so we have decreased food-borne illness," he
says. "This is good, but it comes at a cost. We've eradicated this
potentially beneficial microbial component."
Western sushi eaters are unlikely to harbour nori-digesting
bacteria, says Michel. Gene-transfer events are extremely rare,
and there would be little need for bacteria exposed to a Western
diet to hang on to such genes, he adds. "The biggest difference in
Japan is the quantity of seaweed that is eaten every day," he says.
"It is far higher than just eating sushi once a week. I don't think the
pressure is high enough to keep the genes in our gut."

References:-Hehemann, J.-H. et al. Nature 464, 908-912 (2010).