Barrier effect of the microbiota : symbiosis and antibiosis

This lecture has considered the elements that ensure the colonization resistance property of the intestinal microbiota, a considerable effect that can, in animal models, require an increase of several orders of magnitude in the size of the inoculum to ensure the success of an infection compared with an animal devoid of microbiota. This barrier effect is due to the combination of a stable metabolic network established by the resident flora, which deprives the intruder of the nutrients required for growth, and the production of bactericidal molecules and systems such as colicins and type 6 secretion apparatuses, which are capable of perforating bacterial membranes and, in the case of the latter, injecting enzymes and toxins into the cytoplasm of target bacteria. The precise role of bacteriophages in maintaining these balances remains to be demonstrated.