These include Crohn's disease and ulcerative colitis (1). The exact cause of these diseases is not known. The common view is that these diseases are the result of an exaggerated immune response to microorganisms in the gut, triggered by a combination of genetic predisposition and environmental factors (2). Interestingly, the identified risk-increasing genes are often involved in the recognition of and response to bacteria, and the functioning of the intestinal wall (2). The composition of the microbiome is altered in IBD patients and the absence of certain commensal bacterial species (especially species that make short-chain fatty acids) and the presence of pathogenic species correlates with disease activity (3,4). In addition, unlike healthy subjects, the composition of the microbiome is not stable over time, even in patients who are in remission (5). So it seems reasonable to say that there is something drastically wrong in the interaction between the immune system and the microbiome in IBD. Modification of the microbiome seems to lead to disease remission, as shown by a study in which 'poo transplantation' was used (6).

This is also called irritable bowel syndrome or spastic bowel. IBS is common in Western countries; Between 10% and 20% of the population suffers from this (7). It is therefore one of the most researched disorders. The exact cause is unknown and several factors are mentioned, including a disturbed communication between brain and gut, hypersensitivity to certain foods, low-grade inflammation (8), and a disturbed microbiome. A strong indication that a disturbed microbiome plays a role is the fact that many cases of the disease are preceded by acute intestinal infection by a bacterium, parasite, or sometimes a virus (7,8). Antibiotic use also seems to be a risk factor for the development of IBS. A second indication of a role of the microbiome is that various studies have found reduced diversity and its composition changed (9). Sterile laboratory animals colonized with faeces from IBS patients show different gut physiology (such as slower flow and increased pain sensitivity) compared to colonization with feces from healthy humans (7). A final clue to the role of the microbiome in IBS comes from the observation that influencing the microbiome with probiotics leads to symptom relief in many cases. Although one form of probiotic is not the same as another (each bacterial species and even strain has different effects) and it is therefore important to be careful not to lump clinical studies together, several meta-analyses have been published that show that certain types of probiotics lead to a reduction in symptoms in some patients (10, 11, 12).

About one in 20 people suffer from this complaint, with about 40% of the cases involving the elderly (60+) (13). In some of these patients, chronic diarrhea is the result of a condition such as IBD, IBS, or celiac disease, but that is not the case for every patient with chronic diarrhea. Chronic diarrhoea can have a wide range of causes, and it is important to find the right cause for proper treatment (13). Some of these causes, such as parasite infection, poor digestion, bacterial disruption in the small intestine, or food intollerance, are undeniably related to the microbiome, given its normal function. In patients whose chronic diarrhea is unknown, the composition and distribution of the microbiome in stool samples has been shown to differ from healthy participants (14). Moreover, treatment with a probiotic causes little or no change in microbiome properties, nor in symptoms or bowel movements in healthy participants, while it leads to clear changes and a decrease in symptoms in patients (14). Even in patients for whom the cause of diarrhoea is known (e.g. IBD, infection with pathogenic bacteria, or antibiotic use), active modification of the microbiome by means of probiotics has been shown to lead to improvements (15), which is a clear indication of the involvement of the microbiome in such cases of chronic diarrhoea.