How the Human Microbiome Works

Julie Douglas

Welcome to the microbial jungle my friends, where you are the jungle - home to 100 trillion bacteria. This rich microbic universe began the second you emerged from the womb, with a slathering of your mother's beneficial microbes on your way through the birth canal.

Your overall microbiome would have stabilized by age 3 and matured in concert with your immune system, which learned not to attack roving bacterial cells. And it's this symbiotic ever-changing relationship that researchers are most interested in.

It's hardly news that bacterial cells outnumber our cells 10 to 1 - we're essentially human wrappers for the bacterial goodness contained inside.

But our burgeoning understanding of how microbes permeate every aspect of our physiology could lead to a kind of Unified Theory of Everything for the human body -- not just determining our body odor, our moods or our digestion, but perhaps curbing debilitating diseases before they begin.

Thanks to the Human Microbiome Project (funded by the National Institute of Health) a kinder, more awe-inspiring view of bacteria is emerging. In an effort to make headway on what a "typical" microbiome might look like, 200 scientists at 80 institutions sequenced the genetic material of bacteria taken from 250 healthy people.

We're talking about a massive amount of data taken from stool samples and saliva as well as up to 18 different places on the body, which generated more than 11,000 samples. This led to the discovery that each human contains up to 10,000 strains of bacteria with eight million bacterial genes (in contrast to just 22,000 human genes).

In addition each person harbors his or her own tailor-made microbiome with bacteria specialized to the area it resides in. Think of your bacterial cells as diverse as the flora and fauna you might find in the Amazon rainforest compared to the Sahara desert.

Now think of your cultural eating habits. Live in Japan and eat sushi? There's a gut microbe for that -- one that borrowed a gene from a marine bacterium in order to break down seaweed.

Mind-altering microbes: how the microbiome affects brain and behavior

Researchers from the Human Microbiome Project found genetic signatures of disease-causing bacterial strains in everyone's microbiome. But these harmful bacteria appear to coexist with benign or helpful microbes, perhaps kept in check by them. This is recasting the immune system's role in the body as not just a disease- and germ-fighter but as a communications expert dealing with resident bacteria to keep pathogens at bay.

The main idea here is that microbes aren't just using humans to hitch a ride; they're metabolically active, responding to your environment with astonishing speed. According to Michael Pollen in "Some of My Best Friends Are Germs," bacteria can swap genes and pieces of DNA with itself. And this is an incredibly important adaptation that allows the microbiota, exposed to a toxin or food (like the sushi example), to "swiftly come up with precisely the right gene needed to fight it - or eat it."

As Pollen points out in his article, researchers aren't shouting from the rooftops just yet about how the human microbiome could be a medical game-changer. But there is mounting evidence - and mountains more of it to sift through - that we might just need to give bacteria a metaphorical wing of the building in our bodies, emblazoned with the sign, "Ecosystem Services."

[For more information on findings from the Human Microbiome Project and how diet affects the microbiome, check out Michael Pollen's article "Some of My Best Friends Are Germs."]