We are used to think that all the various roles of bacteria in the microbiota are related to intestinal function only. This is wrong. The intestine is a slightly mistreated organ, but it represents the "second brain", the gateway to our body. We have to consider that organs such as the heart, brain, kidneys, lungs are formed, in evolutionary terms, much later than the intestines. The simplest animal species like as nematodes, are nothing more than… a single long intestine! The gut, and the microbiota that lives within it, work in symbiosis and communicate with other organs to regulate their functionality and health.


There are, therefore, "connecting axes"—as they are defined by the scientific literature—between the gut, microbiota and peripheral organs.
Correctly feeding the gut microbiota and maintaining a state of eubiosis consequently allows one to take care of the whole organism in a systemic and physiological manner.
Among the different axes, the most studied are:


- The gut-liver axis: the liver and the gut communicate via the biliary tract, the portal vein and systemic circulation. The microbiota protects the liver only when it is in a state of eubiosis by reducing the passage of xenobiotics into the bloodstream and consequently reducing hepatic overload and by producing enzymes that facilitate the digestion of fats, alcohol and toxins. Bacteria proliferation in the intestine also helps regulate the intestine-kidney in the same manner as it does with the liver.


- The gut-lung axis: the lungs are not sterile organs but are home to a rich microbial patrimony. A healthy gut microbiota produces molecules like peptidoglycans and SCFA which, through blood circulation, regulate the composition of the lung microbiota and the immune response in the airways.


- The gut-heart axis: the heart and the intestine are also strongly linked by a state of eubiosis of the microbiota. A rich gut microbiota has a positive influence on the production of trimethylamine N-oxide (TMAO), a metabolite that could be the cause of non-optimal functioning of the cardiovascular system and to the accumulation of triglycerides in the liver.


- The gut-urogenital axis: the urogenital system is also teeming with more or less useful bacterial species. The vaginal microbiota, which is rich in lactobacilli, in particular, produces lactic acid, limits the proliferation of bacteria and the translocation of bacterial fragments from the intestine to the urogenital tract that could compromise the state of health of the latter.


- The gut-brain axis: this is certainly the most studied axis of them all. The highway that winds between the two "brains" is the vagus nerve. The brain regulates intestinal motility, visceral responses to pain, fear, anxiety and apprehension primarily through vagal innervation. Not only does the brain affect the intestine, the gut microbiota, in turn, plays an important role in affecting the proper functioning of the brain and its development over time. The bacteria in the microbiota are, for example, capable of producing neurotransmitters like serotonin, polyamines, dopamine and gamma-Aminobutyric acid (GABA). There is currently an ever-growing, exhaustive body of literature that attempts to clarify how the microbiota and the brain "talk" and influence each other. Among all the research centers devoted to studying the gut-brain axis, a world center of excellence, the "APC Microbiome Institute" in Cork, Ireland, deserves special mention.