In much the same way that every person has one of eight common blood types, each of us may contain one of several possible bacterial communities, suggests new research.

Why these communities exist, and what they may do, is unknown.

“There are hundreds of questions that need to be explored,” said Mani Arumugam, a geneticist at the European Molecular Biological Laboratory. “Why these three? What’s special about them? Is it only these three, or are there more?”

Published Apr. 21 in Nature, the findings add a tantalizing wrinkle to the fast-growing field of human microbiomics, or the study of how microbes colonize our bodies.

People are less discrete organisms than teeming symbiotic entities: In every person, Homo sapiens cells account for a small fraction of the whole. And in exchange for a stable, nutrient-rich home, bacteria and viruses help process nutrients, calibrate our immune systems and generally keep people running.

Over the last decade, researchers have moved from acknowledging our microbial symbiosis to investigating its purposes to mapping the so-called microbiome — a task more complicated than mapping the human genome, and far from complete. In tagging all the bugs in our guts, researchers have generally reached the genus level: They know most of what’s there, but haven’t yet gone through it in species-by-species detail, and don’t quite know how groups of bugs interact.

In the latest study, Arumugam, fellow EMBL bionformaticist Peer Bork and dozens of other researchers sequenced every gene they could find in fecal samples from 22 people from Denmark, France, Italy and Spain. Then they searched the data for patterns, looking to see if certain arrangements of bacteria tended to be found in certain people.

‘Exactly what they are doing in there is still to be explored.’

The search returned three distinctive “enterotypes,” or bacterial communities dominated by a distinct genus — Bacteroides, Prevotella or Ruminococcus — each of which is found with a particular community of bacteria (see picture above).

“One analogy that people draw — I don’t know how accurate it is yet — is blood type,” said Arumugam. “It’s not exactly the same. Blood types don’t change, but we don’t know if enterotypes do.”

Further analysis of microbiomes from 13 Japanese and four Americans returned the same three clusters, suggesting the patterns are widespread and unconnected to ethnicity, age or gender. With such a limited sample size, however, containing no microbiomes from South Asia, Africa, South America and Australia, it remains to be seen whether other enterotypes exist.

Beyond identifying the enterotypes, “anything we say now will be a hypothesis,” said Arumugam. In terms of function, each of the enterotype-defining genera has been linked to nutrient-processing preferences — Bacteroides to carbohydrates, Prevotella to proteins called mucins, or Ruminococcus to mucins and sugars — but far more may be going on.

“Exactly what they are doing in there is still to be explored,” said Arumugam, who also mentioned enterotype-based differences in drug metabolism as another possible implication of the findings.

As for how enterotypes form, each may represents an especially stable arrangement, perhaps similar to how different species compositions prevail in nature. A field, for example, has a different ecological community than a forest, which in turn differs from a swamp. But this too is an open question.

“It’s an exciting time,” said Arumugam.

Image: Above, abundance measurements of the key bacterial genus in each enterotype above; below, the bacterial communities associated with each enterotype (Nature).

See Also:

Citation: “Enterotypes of the human gut microbiome.” By Manimozhiyan Arumugam, Jeroen Raes, Eric Pelletier, Denis Le Paslier, Takuji Yamada, Daniel R. Mende, Gabriel R. Fernandes, Julien Tap, Thomas Bruls, Jean-Michel Batto, Marcelo Bertalan, Natalia Borruel, Francesc Casellas, Leyden Fernandez, Laurent Gautier, Torben Hansen, Masahira Hattori, Tetsuya Hayashi, Michiel Kleerebezem, Ken Kurokawa, Marion Leclerc, Florence Levenez, Chaysavanh Manichanh, H. Bjørn Nielsen, Trine Nielsen, Nicolas Pons, Julie Poulain, Junjie Qin, Thomas Sicheritz-Ponten, Sebastian Tims, David Torrents, Edgardo Ugarte, Erwin G. Zoetendal, Jun Wang, Francisco Guarner, Oluf Pedersen, Willem M. de Vos, Søren Brunak, Joel Dore, MetaHIT Consortium, Jean Weissenbach, S. Dusko Ehrlich & Peer Bork. Nature, Vol. 472 No. 7343, April 21, 2011.