Monday, January 10, 2011


Who am I? A soma of a trillion cells that developed from a fertilized egg. But what about the 10 trillion cells in me and on me that aren't me? Or are they me, since apparently I couldn't get along without them? By some estimates, there is a kilogram's worth of bacteria in my gut. A kilogram! Is that possible? Commensal -- eating at the same table. That's a lot of bugs.

Here is a diagram from a recent issue of Science (December 24), showing the species of bacteria that live in various parts of the body. The red circles are species whose genomes have been sequenced. Give a big welcome to all your visitors!

The authors of the article that accompanies the diagram, Yun Kyung Lee and Sarkis K. Mazmanian, write:
Although microbes have been classically viewed as pathogens, it is now well established that the majority of host-bacterial interactions are symbiotic. During development and into adulthood, gut bacteria shape the tissues, cells, and molecular profile of our gastrointestinal immune system. This partnership, forged over many millennia of coevolution, is based on a molecular exchange involving bacterial signals that are recognized by host receptors to mediate beneficial outcomes for both microbes and humans.
Apparently we arrive in the world pristine. But no sooner do we poke our noses out of the womb than bacteria, fungi and viruses start colonizing our nooks and crannies, like the colonization of a new island arisen from the sea. It's called the human microbiome. Wave after wave of pioneers, enter our bodies by every access. Millions of genes in there, compared to the mere 30,000 or so that I inherited from my parents.

What happened to my microbiome several years ago when I was given a month-long course of antibiotics for lyme disease? How did my kilogram of symbiotic inhabitants deal with the trauma, if indeed it was a trauma? How long did it take them to recover?

It is now possible to transplant an entire digestive track into someone whose system has been ruined by disease. It used to be that the physicians first flushed the new track clean of microorganisms before transplanting, then relied on recolonization. But now it seems best to leave in the microbiome that came from the donor. A transplanted gut and transplanted bugs. Someone else's bugs. Every gut its unique biome.

I trust my kilogram of microcreaturedom is doing me enough good to warrant lugging around all that extra weight.

(A tip-'o-the-hat to Mo.)