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.

Why should the gut and brain be linked?

The goings on in our guts are a matter of life or death. If the gut is empty, our brain must be told; if there is a problem with our gut that will hinder food processing and therefore nutrition absorption, the brain will need to be informed. If our gut is facing a pathogen attack, our brain should be kept in the loop.

The links between our gut and brain are hormonal, immunological, and neural, via the central nervous system and the enteric nervous system, which governs the function of the gut. Collectively, they are termed the gut-brain axis.

Although, at first glance, the connections between the gut and brain might seem surprising, we have all experienced it in action. The relationship between stress, anxiety, and a swift bowel movement are no stranger to anyone.

These gut-brain conversations have been studied for some time. However, a new level to this partnership has recently been glimpsed; researchers are now considering the influence of our microbiome on the gut-brain axis. In other words, researchers are asking: do the bacteria in our gut affect our psychology and behavior?

Termed, rather clumsily, the brain-gut-enteric microbiota axis or microbiome-gut-brain axis, researchers are only beginning to scratch its surface.

Stress and the gut

In humans, the hypothalamic-pituitary-adrenal (HPA) axis is the primary responder to stresses of any kind. It is one of the major players in the limbic system and is heavily involved in emotions and memory.
Stress activates the HPA axis and eventually results in the release of cortisol - the "stress hormone" - which has a variety of effects on many organs, including the brain and gut.In this way, the brain's response to stress has a direct influence on the cells of the gut, including epithelial and immune cells, enteric neurons, interstitial cells of Cajal (the pacemakers of the bowels), and enterochromaffin cells (serotonin synthesizing cells).