Gut bacteria and the brain: Are we controlled by microbes?

The gut has defenses against pathogens, but, at the same time, it encourages the survival and growth of "healthy" gut bacteria.

The vast majority of these single-celled visitors are based in the colon, where no less than 1 trillion reside in each gram of intestinal content.

Estimating the number of bacterial guests in our gut is challenging; to date, the best guess is that 40 trillion bacteria call our intestines home - partially dependent on the size of your last bowel movement (poop's major ingredient is bacteria).

To put that unwieldy number into perspective, our bodies consist of roughly 30 trillion cells. So, in a very real sense, we are more bacteria than man.

Most of our gut bacteria belong to 30 or 40 species, but there can be up to 1,000 different species in all. Collectively, they are termed the microbiome.

Of course, bacteria do benefit from the warmth and nutrition in our bowels, but it is not a one-way relationship - they also give back.

Some species benefit us by breaking dietary fiber down into short-chain fatty acids that we can then absorb and use. They metabolize a number of compounds on our behalf and play a role in the synthesis of vitamins B and K.

On the other side of the fence, recent research infers that dysregulation of gut bacteria might be an important factor in inflammatory and autoimmune conditions.

The microbiome's role in health and disease is only slowly giving up its secrets. The latest and perhaps most remarkable finding is the ability that gut bacteria have to moderate our brain and behavior.