Brainy Guts and Gutsy Brains

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We are not alone. Whether such statement applies to life forms beyond the planet Earth, is an area of active scientific research, but not the topic of this article. What we can say, however, is that these four words certainly hold when redirecting our attention to our inner being.

Our physical inner being, that is. As it turns out, we are accompanied by billions or perhaps even trillions of very small organisms that live inside us, for better or for worse. And it seems that our intestines are their most popular gathering place.

Who are they? Are they unique to each of us? What is their role within our intestines? And how do they manage to influence an organ that is far removed from their usual living environments?

Let’s dig up some answers.

Entire Worlds Inside Our Gut

Our body is teeming with microorganisms (also called microbiota or microbiome), including bacteria, archaea, fungi, parasites, and viruses. The largest and most diverse habitats of these tiny creatures are located within our small and large intestines and are mostly confined to the interior of the intestines (lumen) or onto the innermost layer of the gut walls (the epithelial surface).

As a matter of fact, Gail Cresci points out that the small intestines carry between 10⁴ and 10⁸ colony-forming units (CFU) per millilitre (mL) — CFU is a way in which microorganisms can be counted — while the number in the colon rises to approximately 10¹⁰–10¹² CFU/mL. In other words, the number of microbiota in our intestines oscillates between tens of thousands and several trillions. Other researchers, such as Iurii Koboziev et al. even mention estimates in the range of hundreds of trillions.

Within the human gut, most of the microorganisms appear to be bacteria. For instance, based on genetic analyses, Junjie Qin et al. find that 99.1% of the microbiota have a bacterial origin, whereas archaea (0.8%) and other microbiota (0.1%) form a relative minority. This may help explain why bacteria usually make up the main focus of research efforts.

Fig. 1. The lumen (on top) is the innermost part of the intestines where the food content passes through. The epithelium is the physical barrier generally keeping the microbiota from entering the surrounding tissue or blood vessels. (Source: Karen Madsen).