It seems we have met the enemy and he is us.
Bacteria in our intestines are what digest most of our food, and the contents of our colons differ.
It turns out there is more to it than that.
Jeremy Nicholson (above, from his Web site) of the Imperial College in London, England, calls the human being a superorganism. Most of the cells inside us are microbes, not human cells at all, he wrote in a ground-breaking 2004 paper.
Our gut bacteria alone include over 500 individual species, not only digesting our food but helping ward off infections. Japanese folks can digest the nori around their sushi because they have the right gut bacteria for the job. The efficiency, or inefficiency, of our gut bacteria determines how much nutrition we get from what we eat.
Where you live, what you eat and your species all determine what bacteria help you live, a primate's gut bacteria being notably different from our own. It's thought that controlling the mix of gut bacteria can aid in weight loss.
Nicholson's research has spawned the birth of two new scientific disciplines. Metabolomics studies the chemical products of our metabolism for knowledge on diet, drugs and disease. Metabonomics extends this into the study of other environmental factors.
All this means we have to change our attitude about bacteria. Yes, some induce disease. But some fight it. Some bacteria want to kill us, but others are devoted to helping us live.
A hint of the truth lies in H.G. Wells' novel "The War of the Worlds," (spoiler alert) in which an alien invasion is stopped because the aliens are susceptible to infection by Earth bacteria.
The old joke of people searching for extra-terrestrial life was "we're not alone out there."
Well, we're not alone in here either. And this is dramatically changing the medical science of the 21st century.