Abstract: The human body is populated by a large number of microbial colonies, with an estimated 10–100 trillion microbes. The total genome size of human microbial colonies by far overwhelms the size of the host’s genome. This heterogenous group of microbial colonies (primarily bacteria, but also archaea, eukaryotes and viruses) is referred to with the term microbiota, and although most of them populate the gut, microbes are also detectable in many other organs of the body, especially in the distal tracts of the genitourinary system and the skin. Over the last years, an increasing amount of evidence has been accumulated on how the microbiota exerts a significant influence on the development and physiology of the human body. The nervous system interacts extensively with the microbiota. To refer to communication between gut microbes and neurons, we have recently suggested that the traditionally established term microbiota-gut-brain axis be replaced with a more specific brain-bacteria axis, which emphasizes the direct interrelationship between these two entities (Herrera-Rincon et al., 2020; Murciano-Brea et al., 2021).