How Gut Bacteria affects Brain Health

Rebecca Lewis October 24, 2014

The human gut is often referred to as the “second brain”. It houses the microbiome – the collection of bacteria living in the gastrointestinal tract which plays a complex and critical role in the health of its host. The microbiome interacts with and influences organ systems throughout the body, including, as research is revealing, the brain. This discovery has led to a surge of interest in potential gut-based treatments for neuropsychiatric disorders and a new class of studies investigating how the gut and its microbiome affect both healthy and diseased brains.

The microbiome consists of a startlingly massive number of organisms. Nobody knows exactly how many or what type of microbes there might be in and on our bodies, but estimates suggest there may be anywhere from three to 100 times more bacteria in the gut than cells in the human body. The Human Microbiome Project, co-ordinated by the US National Institutes of Health (NIH), seeks to create a comprehensive database of the bacteria residing throughout the gastrointestinal tract and to catalogue their properties.

The lives of the bacteria in our gut are intimately entwined with our immune, endocrine and nervous systems. The relationship goes both ways: the microbiome influences the function of these systems, which in turn alter the activity and composition of the bacterial community. We are starting to unravel this complexity and gain insight into how gut bacteria interface with the rest of the body and, in particular, how they affect the brain.

In one experiment, researchers transplanted the human microbiome into germ-free mice (animals that have no gut bacteria) in order to study it in a controlled setting. They found that, simply by changing the carbohydrate and fat content of the mice’s food, they could alter basic cellular functions and gene expression in the microbiome.

Depression is not the only psychiatric disorder in which the microbiome may play a role. Research in rodents, as well as a few preliminary studies in humans, indicates that the state of our resident microbes is tied to our anxiety levels.

Germ-free mice, for example, appear to be less anxious than normal mice on behavioural tests of anxiety, whereas mice infected with pathogenic bacteria behave more anxiously. Interestingly, there seems to be a window during development when the presence of a microbiome leads to normal levels of anxiety in adulthood: germ-free mice that were exposed to microbiome bacteria at three weeks of age subsequently behaved like normal mice, whereas those exposed at ten weeks of age continued to be less anxious than normal animals. Like the data on microbiome-immune interactions, these findings highlight the critical role gut bacteria play early in life.

This research also reveals the complexity of the relationship between the microbiome and psychological state. Although the general trend is that fewer bacteria mean lower anxiety levels, it is not just the number but the identity of the bacterial species that determine how gut dynamics interact with mental state.

For example, adding beneficial bacteria through probiotic treatment may reduce elevated anxiety levels caused by inflammation and infection. A key factor in this relationship is stress and the way the body responds to it.

Source of this article: How gut bacteria ensure a healthy brain – and could play a role in treating depression