**3. Evidences for relationship among intestinal microbiota, probiotics and colon cancer**

### **3.1. The intestinal microbiota**

The gastrontestinal (GI) microbiota undergoes changes in quantity and quality, depending on the location of colonization in the GI. Traditional culture-based characterization may take into account no more than 30% or so of the microorganisms that can be seen and enumerated by microscopic observation. The worldwide species diversity of commensal intestinal bacteria is immense. In that respect, the use of molecular tools has indicated that the majority of the dominant bacterial species observed in the faecal microbiota of an individual (approximately 80%) are specific to this individual [27]. Also, these species are not distributed homogeneously along the length of the GI, so the bacterial activities are considerably variable in different parts of the intestine [28].

The stomach and the small intestine contain few species, whereas the colon contains a complex and dynamic microbial ecosystem, with a great concentration of bacteria. Among these are the bifidobacteria and lactobacilli, considered non-pathogenic or beneficial bacteria [29]. The bacterial population in the large intestine is very large and reaches a maximum count of 10 12 CFU.g-1. In the small intestine, bacterial contents are considerably smaller from 104 to 107 CFU.g-1, while in the stomach only 101 at 10 2 CFU.g-1 are found in function of low pH on this site. In total, the number of intestinal bacteria is approximately ten times the number of cells that make up the human body [30].

On the basis of rRNA sequencing 40,000 strains of intestinal bacteria can be indentified, including non-cultivable bacteria [31]. It was noted that 99% of intestinal bacteria consist of four phyla, Proteobacteria, Actinobacteria, and two main phyla Bacteroidetes and Firmicutes [32]. While the species in the phylum Bacteroidetes show a great variety between individuals, a large number of species in the phylum Firmicutes belong to clusters of clostridial butyrate producers [33].

With advances in molecular biology, it is known that the intestinal microbiome, contains 100 times more genes than the whole human genome [34]. Thus, a close relationship is evolving between the human gut microbiota. The human intestine exhibits to a symbiotic relationship that plays a key role in human homeostasis, including metabolism, growth and immunity [35].

220 Lactic Acid Bacteria – R & D for Food, Health and Livestock Purposes

DNA in skin cells, in both *in vitro* and *in vivo* tests [26].

considerably variable in different parts of the intestine [28].

number of cells that make up the human body [30].

clostridial butyrate producers [33].

the development of CC.

**3.1. The intestinal microbiota** 

**colon cancer** 

There are many types of intestinal bacteria that produce a variety of metabolites that modulate the normal development and functioning of the host. On the other hand, the metabolic activity of intestinal microbiota can generate compounds that are harmful such as reactive oxygen intermediates. These molecules, which include superoxide, hydrogen peroxide, hypochlorous acid, singlet oxygen and hydroxyl radical, can cause oxidative damage to cellular DNA and increase the risk of colon cancer [25]. Studies have shown that *Enterococcus faecalis* can produce superoxide and hydrogen peroxide, causing damage to

Given the role of intestinal microbiota in colon carcinogenesis, it is suggested that factors that modulate beneficially the composition and/or activity of the microbiota could inhibit

**3. Evidences for relationship among intestinal microbiota, probiotics and** 

The gastrontestinal (GI) microbiota undergoes changes in quantity and quality, depending on the location of colonization in the GI. Traditional culture-based characterization may take into account no more than 30% or so of the microorganisms that can be seen and enumerated by microscopic observation. The worldwide species diversity of commensal intestinal bacteria is immense. In that respect, the use of molecular tools has indicated that the majority of the dominant bacterial species observed in the faecal microbiota of an individual (approximately 80%) are specific to this individual [27]. Also, these species are not distributed homogeneously along the length of the GI, so the bacterial activities are

The stomach and the small intestine contain few species, whereas the colon contains a complex and dynamic microbial ecosystem, with a great concentration of bacteria. Among these are the bifidobacteria and lactobacilli, considered non-pathogenic or beneficial bacteria [29]. The bacterial population in the large intestine is very large and reaches a maximum count of 10 12 CFU.g-1. In the small intestine, bacterial contents are considerably smaller from 104 to 107 CFU.g-1, while in the stomach only 101 at 10 2 CFU.g-1 are found in function of low pH on this site. In total, the number of intestinal bacteria is approximately ten times the

On the basis of rRNA sequencing 40,000 strains of intestinal bacteria can be indentified, including non-cultivable bacteria [31]. It was noted that 99% of intestinal bacteria consist of four phyla, Proteobacteria, Actinobacteria, and two main phyla Bacteroidetes and Firmicutes [32]. While the species in the phylum Bacteroidetes show a great variety between individuals, a large number of species in the phylum Firmicutes belong to clusters of One of the primary functions of the intestinal microbiota is the harnessing of energy from elements of the diet that could be lost through excretion [36]. The polysaccharides are not absorbed in the colon, but metabolized by resident microorganisms to short chain fatty acids (SCFA), such as propionate and butyrate, which are absorbed by passive diffusion [37]. SCFA production is dependent on the available fermentation of substrate, such as, starch or other polysaccharides, results butyrate, acetate and propionate [37]. SCFA concentrations are higher on the right side of the colon than on the left and this is probably due to the greater availability of carbohydrates [29]. The SCFA have an important role in the maintenance of the epithelial layer. Studies show that epithelial cells acquire about 70% of their butyrate oxidation [29]. The butyrate also acts as a trophic factor for cells in intact tissues [38]. In addition, it has been proposed that butyrate lowers the risk of colon cancer by its ability to inhibit the genotoxic activity of nitrosamines and hydrogen peroxide, as well as to induce various levels of apoptosis, differentiation and the cell cycle stop colon cancer in animal models [39].

Other researchers also cite the effect of butyrate on mediators of inflammation, it has been proved that this SCFA is able to inhibit the expression of some cytokines (TNF, IL-6, IL-1) and to inhibit the activation of nuclear factor B (NF- B) [40]. Other functions of the gastrointestinal microbiota include digestion of poorly digested nutrients, modification of bile acids, and nutritional supplementation by auxotrophic of mutants additional compounds that cannot be acquired by food consumption, such as folic acid and biotin [41].

The non-pathogenic commensal microbiota has a profound impact on the normal physiology of the GI tract. It ensures the efficiency of bowel motility, intestinal growth and immunity, as well as digestion, nutrient absorption and fortification of the mucus barrier [42].

Researchers have made advances in the characterization of GI microbiota defining the responses that may contribute to the development of inflammatory bowel diseases, such as, colon cancer [43]. Given the importance of a better understanding of intestinal microbiota, the TGI has been often studied. In recent decades, various intestinal simulators have been and are being developed, to facilitate the study of the intestinal microbial ecosystem and its interactions [44, 45].
