Bacteroides plebeius

by Peter Andrey Smith

One morning the summer I was nineteen, I woke before dawn, downed a grainy cup of cowboy coffee and trudged through the wild angelica growing along the Oregon coast with a plastic five-gallon bucket. I slipped along the shoreline filling the bucket with seaweed—kombu and dulse and nori and bladderwrack. The sun rose and the tide came in and campers from California parked all along Highway 101. That afternoon, high in the Siskiyou Mountains, I hung the harvest out to dry, like a clothesline of ruffled green washcloths.

The morning is memorable perhaps because it did not repeat itself. My appetite for eating algae, like that of much of the Western world1, pales in comparison to that of the people who live on the islands of Japan. A Silk Road of seaweed once lubricated trade across Asia. To this day, seaweed remains a staple on the dinner table. One of the most recognizable forms, the purplish Porphyra, has been farmed for centuries. Dried and pressed into paper-thin sheets of nori, the seaweed is used to wrap sushi and pickled balls of rice.

Many miles north of my Oregon excursion, in 2010, a researcher named Jan-Hendrick Hehemann, then living in Vancouver, British Columbia, found a particular bacteria living on these seaweeds, a seafaring species of the Bacteroidetes clan known as Zobellia galactanivorans. The bacteria looks like a microscopic prickly pear and it specializes in breaking down the complex carbohydrates, or polysaccharides, in seaweed. When Hehemann looked in GenBank, a big database of the genes of, well, nearly everything, he discovered that the genes that encode for this microbe’s ability to break down these nutrients—its enzymatic power tools—were present in a bunch of these seafaring microbes; but he also saw something unexpected: samples of bacteria from the guts of Japanese people also had similar genes that coded for enzymes that break down the polysaccharides in seaweed. These bacteria were in the genus Bacteroides, gut-dwelling members of the Bacteroidetes clan.

Bacteroides live in many places around the world; they’re found in soils, oceans, lakes, and inside the guts of humans, dogs, gulls, and millipedes. A diverse bunch of Bacteroides dominates the trillions of organisms found living inside our guts.

Bacteroides produce digestive enzymes and are responsible for making Vitamin K. Just which species of Bacteroides you have varies with age and diet and where you live, but in one form or another, they are there. One of the species found in nearly everyone is Bacteroides plebeius. If our colons were ancient Roman cities, these bacteria are, as their names suggests, the plebs, the skilled commoners who manufacture tools to break down the tough stuff at the bazaar: proteins and complex carbohydrates. These commoners are, well, common, but in some folks they come in a special variety.

When Asiatic peoples arrived on the Japanese archipelago some 40,000 years ago, they and the bacteria in their guts probably lacked the ability to fully digest seaweed. As the food became ingrained in their culture, the bacteria of islanders appear to have picked up a knack for breaking down more of the carbohydrates in seaweed, including the very toughest carbohydrates. This is what Hehemann was seeing in the genetic database. As he saw it, the plebs had stolen the genetic tools, the so-called “sushi factor,” from a bacteria such as Zobellia galactanivorans living in the ocean, bacteria that, once or twice upon a time, arrived in some unsuspecting eater’s gut on a seaweed or a fish that had feasted on seaweed. The resident bacteria could now extract additional benefit from a staple food. If you want to crack a nut, it helps to have a nutcracker. If you want to digest some seaweed, it helps to have a carb-cracking Bacteroides plebeius.

Perhaps an ocean-going Bacteroidetes like Zobellia galactanivorans first appeared in the gut as a transient. It wouldn’t have had the right stuff to stake out territory and build a settlement there, but enough of them might have stuck around for long enough to trade genes with the resident plebs. Bacteria don’t have sex per se; they are generous social networkers, traders at the bazaar. They swap and trade genes with whomever is living in close proximity. Horizontal gene transfer, it’s called. Bacteria do this even with distant microbial relatives and especially when in close proximity such as the gut. It seems that in this way one lucky Bacteroides plebeius in an ordinary Japanese gut picked up the genes requited to digest seaweed and, in doing so, thrived.

1-Well, as long as one does not count seaweed-based emulsifiers that food engineers add to ice cream to stop the crystal formation known as freezer burn. It’s also worth pointing out that agar-plated Petri dishes, used to culture and grow bacteria, derive from seaweed. The stuff is everywhere, just highly refined.


Jan-Hendrik Hehemann et al. Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota. Nature 464, 908-912 (8 April 2010). doi:10.1038/nature08937

Justin L. Sonnenburg. Microbiology: Genetic pot luck. Nature 464, 837-838 (8 April 2010). doi:10.1038/464837a

Chris S. Smillie et al. Ecology drives a global network of gene exchange connecting the human microbiome. Nature 480, 241–244 (08 December 2011). doi:10.1038/nature10571

Ole G. Mouritsen. Seaweeds: Edible, Available, and Sustainable. The University of Chicago Press (2013).

About the Author

Peter Andrey Smith writes about food, science and technology. His work has appeared in The New York Times, Wired and The Walrus and online at The New Yorker and Smithsonian. He’s written about experimental lakes, motion sickness, black-market eels, and the early advocates in the fight against noise pollution. His spirit animal lives in the ocean; he lives in Brooklyn and tweets at @petersm_th.