The intestine is the last part of the digestive tract. It looks like a variable diameter pipe with flexible walls, folded several times over. The intestine starts with the pectoral opening of the stomach and ends with the anus. It is divided into two portions: the small intestine and the large intestine. The small intestine is in turn divided into duodenum, fasting and ileum; The big one is into the blind, colon and rectum.
The small intestine occupies all the spaces of the abdominal cavity not occupied by other bowels and winds through various inflexions, called intestinal loops. The soft intestine mucosa appears as velvety and has several folds that increase the surface. Below one of these creases, in correspondence with the duodenum, the pancreatic duct opens. Throughout the intestine, there are intestinal villus, formed by thick folds of the intestinal mucosa through which the nutrients of the substances we ingest are absorbed.
In humans there are between 500 and 10,000,000 different species of microorganisms, the most numerous of which are bacteria, but also to a lesser degree, of fungi and viruses.
The human microbial develops during the first days of life and survives, except in the case of illnesses. Intestinal Bacterial Flora is a community of thousands of billions of microbes (innocuous, indigenous) living in the gastrointestinal tract and disrupts nutrient enzymes by regulating the amount and quality of the extracted substances to be sent to body cells. When a portion or some strains of these microbes increase or decrease, it is referred to alteration of bacterial flora (dysbiosis).
Each individual possesses its own microbiota. The researchers of INRA have highlighted the existence of a small number of species shared by all that would be the phylogenetic nucleus of the intestinal microbiota.
The first observations on the positive effects of probiotics on human health date back to the beginning of the 20th century. The Nobel Prize winner Elie Metchnikoff, who worked at the Pasteur Institute in the early twentieth century, suggested that these effects were due to an improvement in intestinal microbial balance by inhibition of pathogenic bacteria.  Since then, knowledge of probiotics has been enriched with large-scale scientific and clinical studies. Therefore, data on specific effects, such as attenuation of chronic intestinal inflammatory diseases,  prevention and treatment of pathogen-induced diarrhea,  urogenital infections,  atopic diseases 
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