Microbiota – the forgotten organ

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Today, the idea that gut bacteria affects a person’s health is not new. Most of us know that these microbes influence digestion, allergies, and metabolism. New books explaining which diet will lead to optimum bacterial health are published regularly and most of us have at least heard one time about its importance for our health. Recent studies even suggest that one of the benefits of the Mediterranean diet, recognized as one of the healthiest in the world might come from its favorable impact on gut flora (De Filippis 2015). But lately a new concept appeared, far less obvious than the previous, which suggests that these microbes may extend their reach much further, even into the human brains.

Microbiota – the forgotten organ
The term microbiota basically refers to all the bacteria living in our digestive system and consists of a complex community of microorganisms. The gut flora is composed by the largest (trillions of cells) and most diverse (thousands of different species) reservoir of microorganisms associated with animals.
Although people can survive without gut flora (Steinhoff 2005), its absence would lead to numerous problems and complications. Normally the relation between humans and their gut flora creates a win-win situation: The microorganisms benefit from a safe home and a secure food supply. While on the other side we rely on the crucial help they provide in various metabolic and digestive functions. Gut flora is able to process food (by fermentation) that we would otherwise be unable to absorb and plays an important role in synthesizing vitamins such as vitamin B or vitamin K (LeBlanc 2013).

Gut flora also has a so-called "barrier effect" which prevents other microorganisms to develop in the host. The barrier effect protects humans from both invading species and species normally present in the gut at low numbers, whose growth is usually inhibited by the gut flora (Guarner 2003).
Taken together all the activities performed by these bacteria resemble those of an organ, which led some to label gut flora the "forgotten" organ. The total weight of the intestinal bacteria is about 1 kg, which is about the same weight as a human brain or a liver. The aggregate of all the genes contained by the gut flora is estimated to be hundred times more numerous than those contained in the human DNA, showing how complex this "forgotten" organ can be (McWhinney 1987).

Gut flora – the never ending story
Gut flora is formed over time and from the day you're born. The flora diversity changes whether you’re born by cesarean section or vaginal delivery, then whether you're breast fed or formula fed. The diet you eat throughout your life and the environment in which you are living also play a role, as it has been shown that people living in different geographical locations have different gut flora (De Filippo 2010). As a direct consequence a person’s gut flora can change over time either by moving to a different location or simply by altering the diet. Finally, overuse of antibiotics and excessive sanitizing can lead to a depletion of bacteria diversity in the gut.

A delicate balance
The intestinal bacteria can be classified into two main groups: good bacteria (useful bacteria) such as lactic acid bacteria and bad bacteria (harmful bacteria) which must be kept at bay (see illustration Good and Bad bacterial flora). Balance of gut flora can be easily altered by various factors and has been linked to numerous diseases bay (see illustration Gut flora and diseases) including obesity, inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), non-alcoholic steatohepatitis (NASH), colon cancer, liver cancer, type II diabetes mellitus and allergic diseases (Guarner 2003). Some studies have also suggested that intestinal bacteria are involved in the development of cardiovascular disease (Jingyuan 2015).