Just like animal species that live within the same natural habitat, the bacteria of the gut microbiota also interact with each other. They collaborate in the digestion of nutrients or compete for territory and the available resources. In non-optimal conditions it is often the less beneficial bacteria that prevail and grab all the available resources, while in condition of eubiosis there is a greater balance between all the species that make up the bacterial flora. The bacteria in the microbiota interact, for example, via cross feeding, quorum sensing and bacteriocins.


1. Cross-feeding is a symbiotic nutritional interaction through which a bacterial strain is able to grow in the body only if another strain provides it with the necessary support. For example, some bacteria such as Bifidobacteria are able to degrade oligofructose and produce acetate and lactate, but not the very important butyric acid. The latter is produced by butyrate-producing bacteria like Roseburia intestinalis, which however, can multiply only in the presence of carbon sources such as acetate and lactate produced by the Bifidobacteria. Through cross-feeding, the different bacteria closely collaborate with each other, generating that effective synergy for maintaining homeostasis.


2. Quorum-sensing is a physiological mechanism of self-regulation of gene expression within the bacterial population. It is a cell-to-cell communication which, like in a group chat, allows bacteria to exchange information in the form of chemical signals. Through quorum-sensing, the microbiota monitors fluctuations in the composition of the species that constitute it. If the number of a bacterial species is too large chemical signals are sent that reduce, for example, the speed of replication of its genome and therefore lower its growth curve. Through this system the microbiota is smart enough to balance its own composition.


3. The bacteria in the microbiota cooperate with each other, but they also compete for the available space and resources. One of the main tools of territorial competition is provided by bacteriocins. Bacteriocins are small peptides (protein molecules) synthesized by bacterial ribosomes that show bacteriostatic or bactericidal activity against "competitive" microbial strains. Through bacteriocins, the different species of the microbiota guard the epithelium and, in many cases, also protect the human body from particularly aggressive undesirable bacteria.