Bugs Inside: Which Type are You?

I’ve written before about the important, yet poorly understood, role  that bacteria living on or in us play in our overall health.  These organisms are more numerous than our own cells; we have ~10 trillion cells, but ~100 trillion resident bacteria.  (The National Institutes of Health is sponsoring the Human Microbiome Project [data here], which is attempting to identify and classify these organisms, and their roles.)   We’ve also seen how disruption of a person’s normal microbiome can cause serious health problems.

A new research paper [abstract], published this week in Nature, and reported in an article at the PhysOrg.com site, suggests a potentially fascinating new insight into our “inner” environment.  Rather than  more-or-less random variations in bacteria from person to person, the researchers found that people’s internal ecosystems tended to fall into one of three clusters (which they call enterotypes), each with a different dominant genus of bacteria.  Why this should be so is, at present, something of a mystery.

The scientists don’t yet know why people have these different gut types, but speculate that they could be related to differences in how their immune systems distinguish between ‘friendly’ and harmful bacteria, or to different ways of releasing hydrogen waste from cells.

The three “defining” genera of bacteria are Bacteroides, Prevotella, and Ruminococcus.  The graphs below show the different relative abundance of each in genus in the three enterotypes:

There may be some significance to the fact that the three defining bacterial genera have differing nutrient preferences.

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.

The different bacterial populations also produce different vitamins as by-products, but the significance of this is also unclear.

This study was based on a relatively small sample, and it is certainly possible that further work will reveal a more complicated picture.  Still, it is an interesting first step toward exploring a landscape that, though it is literally under our noses, is still to a significant extent terra incognita.

(There are also articles on this research at New Scientist and Wired.)