2. Microbiota, health and diseases

Microbiota affects many physiological processes, while its alteration is thought to render a number of pathologies.

The growing evidence regarding the importance of the microbiome for health and disease and the host-microbe symbiosis at the immunological and metabolic levels become highly challenging for a better understanding of immunopathologies such as autoimmune and inflammatory disorders. Microbiome changes were correlated with a variety of diseases such as inflammatory bowel disease, obesity, type 2 diabetes, autism, and allergies.

Crohn's disease (CD) and ulcerative colitis (UC) are the most prevalent forms of inflammatory bowel disease (IBD), characterized by chronic relapsing inflammation affecting the intestinal mucosa. The etiology of these diseases is unknown, but there are increasing scientific evidences that microbiota influences the pathogenesis of IBD [2].

Patients exhibit a decrease in microbial population and functional diversity, with a decrease of Firmicutes, an increase in Bacteroidetes, and facultative anaerobes such as Enterobacteriaceae [3–5].

The intestinal microbiota was also implicated in several other gastrointestinal-related diseases, such as obesity, type 2 diabetes, celiac disease, and colorectal cancer. The last two of these were associated with changes in microbiota composition, but interestingly, no pattern of alteration was demonstrated [6–10]. Recent studies showed that the expression of the leukocyte antigen DQ2 is a strong risk factor for the development of celiac disease. It seemed that children possessing this haplotype also had an altered microbiota composition (compared to non-HLA DQ2 individuals) prior to clinically apparent disease [11].

On the other hand, there is a bidirectional functional relationship between the intestine and the kidneys, the urinary pH influencing the intestinal microbiota metabolism, while microbialrelated metabolites are involved in the development of the kidney pathologies [12].

Several studies focused on the possibility that the intestinal microbiota may influence the hypothalamus-pituitary-adrenal (HPA) axis, and, therefore, the cognitive function and behavior. HPA is a pathway activated in response to infection and perturbed by psychological stressors (the "gut-brain axis"). There are publications that evoke a direct relationship between enteric infections on one hand and anxiety, depression, and cognitive dysfunctions, on the other hand [13–16].
