**1. Introduction**

The surface of the human gut has a surplus area of 200-250 m2 in order to contain, between intraepithelial lymphocytes and lamina propria, Peyer's patches and lymphoid follicles, the lymphoid tissue, while hosts a flora of about 800 different bacteria species with over 7000 strains. The 99% are obligate anaerobes and varies species were then classified using traditional anaerobic culture techniques. More than 50% of the dominant gut microbiota (corresponding to 10 8-10 11 per gram of faeces) cannot be identified using traditional colture ,but molecular approaches, based on the use of 165 ribosomal DNA molecular (Mai & Morris, 2004). Most of these bacteria colonizes the large intestine (in a range of 10-12 bacteria/g). The bacterial count of the small intestine (duodedum and jejunum) is considerably lower (approximately 104-7 bacteria/ml) than Streptococcus Lactobacillus, Enterobacteriaceae corresponding to the transient microbiota.

The main bacterial species represented in the human large intestine (colon) are distributed with densities higher than 10 9-11 per gram of contents, and these high densities can be explained by the slow transit and low redox potential . In this intestinal tract we can mostly find bifidobacteria and bacteroides ,bifidobacterium clostridium. The fecal microbiota contains 10 9 \_10 11 CFU per gram, and microorganism in about 40% of their weight. The dominant microbiota is represented by strict anaerobes , while the sub-dominant microbiota by facultative anaerobes. In addition to the resident microbiota (dominant and sub dominant), the faeces contain the transient microbiota, that is extremely variable, including Enterobacteriacee (Citrobacter, Klebsiella, Proteus ) and Enterobacter (Pseudomonas) and yeast ( Candida) CFU per gram (Table 1) (Zoetendal et al, 2004).
