Summary: The human digestive tract is a habitat for a variety of microorganisms that perform many important functions in it. All microorganisms (including bacteria, fungi, eukaryotes and viruses) are described as "microbiota". Modification of the intestinal microbiota by the use of probiotics has been of interest to scientists over the years.. There is no clear evidence of their effectiveness in Crohn's disease, while more studies confirm the usefulness of probiotics in ulcerative colitis.