The microbiome is the new “It Girl” of biomedical research. Suggestions for its role in our body range from influencing our decision on where to grab lunch to shaping our personality or impacting major depressive disorders. Each individual’s microbiome is a culmination of complex genetic and environmental factors. Perhaps, unsurprisingly, one of these is the food we consume. Our diet influences the subtypes of bacteria that occupy our gut. If we eat a high-sugar diet, for example, the bacteria that feed off sugar multiply in proportion. More importantly, these bacteria then send out signals to our brain to continue supplying them with sugary foods, resulting in us craving the food that this subtype of bacteria feeds on. When we give into this craving, we aid in their multiplication, but also experience the benefit of “feel-good” compounds the bacteria secrete.
How these “feel-good” signals actually reach the brain is hotly debated. It seems likely that the vagus nerve is involved. The word vagus comes from Latin, meaning “to wander”, and is very apt for this trailing nerve that begins at the brainstem and travels all the way to the colon. It relays information between the brain and many of the body’s organs. Alternatively, there could be a role for the endocrine system or the immune system, or even all three, in translating these signals. Considering that a humble bacteria’s sole goal in life is to multiple and spread, could they evolve to make their host more social and outgoing?
“Germ-free mice have been shown to be less sociable and will spend less time investigating an unfamiliar mouse than normal mice.”
It has been demonstrated that the microbiome influences social behaviour in animals. Germ-free mice have been shown to be less sociable and will spend less time investigating an unfamiliar mouse than normal mice. These mice also showed a stark increase in gene activity in the amygdala, the emotion processing centre of the brain, during these encounters. Similarly, a certain kind of bacteria in blow fly have been shown to produce chemicals that encourage the fly to seek new food sources, hence spreading the bacteria to new areas. It must be stressed that this kind of system is likely to be much more complex in hosts that have a more robust and diverse microbiota.
However, some have discredited the concept that bacteria could evolve to make their host more social. These scientists argue that bacteria which expend extra energy in producing neuroactive compounds would be unable to compete with other bacteria in the diverse microbiome of the human gut. The jump from displaying the correlation between host behaviour and bacteria composition to suggesting that the bacteria cause behavioural changes cannot be overstated. The case for both a causative and a correlative role for the microbiota in major depressive disorders has been investigated.
“Examining new avenues of treatment such as gut microbiome manipulation offer hope of novel therapeutics in areas like this where our current approach cannot do enough.”
Rodents receiving a fecal transplant from human patients with major depressive disorders were shown to develop depressive behaviours. Equally, inducing stress and depressive behaviour in rodent models caused a reduced diversity in gut microbiota. This blossoming field of research could lead to new therapeutic options across a wide array of disorders, including major depressive disorders. A third of patients suffering from depression have what is known as treatment-resistant depression, meaning even after months of trying different therapies they do not achieve remission. Examining new avenues of treatment such as gut microbiome manipulation offer hope of novel therapeutics in areas like this where our current approach does not do enough.
By second guessing our preconceived notions of how the brain and gut interact, we have uncovered a cavernous gap in our understanding. We are redefining how the central nervous system interacts with our bodies, and quite literally rewriting the textbooks.
