It is now generally agreed that the gut flora or microbiota play a key role during IBD. Various observations illustrate this fact:

• In patients with IBD, several bacterial populations are modified (dysbiosis). Potentially pathogenic bacteria (such as Escherichia coli, Listeria monocytogenes, Yersinia enterocolitica and Mycobacterium paratuberculosis) are found in excessive amounts, while the concentrations of beneficial bacteria from the Firmicutes group are decreased both in terms of the species and the proportions. The most frequent localizations of IBD coincide with the zones having the highest density of digestive tract bacteria (the terminal ileum and the colon).
• In patients for whom surgery had proven to be necessary, the risk of early relapse increases concomitantly with the drop in the bacterium called Faecalibacterium prausnitzii. In animal models of IBD, administration of this bacteria, or the molecules it secretes, can reduce intestinal inflammation and drastically improve survival. Likewise, an IBD will not develop if the animals are maintained in sterile medium from birth.

All of these observations open up the pathway toward new research perspectives, development of novel diagnostic tools and new therapeutic and preventive possibilities.