SUMMARY

	Obesity is the result of imbalance between intake and energy expenditure, causing an increase in weight and body fat and more likely to develop chronic diseases. It has been observed that in the development of obesity not only involving the diet, but genetic, endocrine, psychological and environmental. In recent decades the intestinal microbiota has also been included among the factors that may have a key influence on obesity and metabolism. The prevalence of obesity especially in children and adolescents is increasing, making it one of the first public health problems worldwide. Therefore, further progress is needed in identifying the factors that may contribute to obesity and the development of alternative strategies for reducin
the prevalence and comorbidity.
   
	The overall objective of the thesis has been the identification of changes in the composition of the intestinal microbiota associated with obesity and assessment of the role of specific components of the microbiota in obesity and associated metabolic and immunological in an animal model.
     
	The first specific objective was to determine the influence of an intervention for the treatment of obesity on the intestinal microbiota of obese and overweight adolescents. Using the techniques of real time PCR or quantitative (q-PCR) and in situ hybridization with fluorescent probes (FISH), quantified the major groups of bacteria of the intestinal tract. Adolescents who experienced greater weight loss, showed increases in concentrations or relative proportions of the Bacteroides group and Bacteroides-Prevotella and Lactobacillus, and reductions in group E. rectale – C. coccoides. The microbiota of adolescents who experienced a significant weight loss was characterized by higher initial concentrations and final group B. fragilis, C. leptum and B. catenulatum and lower group C. coccoides, Lactobacillus, Bifidobacterium, B. breve and B. bifidum, suggesting that the microbiota of the individual can influence the effectiveness of interventions based on changes in
 lifestyle to treat obesity.
 
	 In obese adolescents was an increase in the proportion of IgA associated with intestinal bacteria, related to the proportions of C. histolyticum and E. rectale - C coccoides, which were reduced after surgery to treat obesity, suggesting that some components of the microbiota possibly involved in obesity or its metabolites influence the host immune system.
   
	Obesity is also associated with complications during pregnancy, increasing the risk of child health at birth and suffer from obesity and other related pathologies. The question arose also as to evaluate the composition of the intestinal microbiota of pregnant women with overweight by qPCR and its relationship with biochemical and dietary parameters. Overweight women had lower concentrations of the groups Staphylococcus, Enterobacteriaceae and E. coli, and reductions in the genera Bifidobacterium and Bacteroides. E. increasing coli and reduction of Bifidobacterium spp. and Akkermansia muciniphila also associated with excessive weight gain during pregnancy. In addition, increases in serum cholesterol concentration were associated with increases in Staphylococcus spp., Increases in HDL cholesterol and reductions in total cholesterol were associated with increases in Bacteroides spp., Increases in folic acid were associated with increases in Bifidobacterium spp., levels of ferritin and transferrin showed opposing relationships with low levels of Enterobacteriaceae and Bifidobacterium spp. Therefore, we can say that the intestinal microbiota is related to body weight, with weight gain and metabolic parameters during pregnancy, which may be relevant to the health of the 
mother and child.

After studying the relationship that exists between the intestinal microbiota and metabolism, the next goal that was raised was to study the effect of strain B. pseudocatenulatum CECT 7765 as a potential probiotic in an animal model of obesity. Induced obesity in animals with a diet rich in fat, it was observed that the administration of the bacteria lowered cholesterol, triglycerides, glucose, leptin, hepatic steatosis and adipocyte size compared to untreated obese group with strain. Also improved the function of immunocompetent cells (macrophages and dendritic cells) and modulated the composition of the microbiota, reducing inflammatory properties. Therefore, this strain improved metabolic and immunologic alterations associated with obesity induced by diet in a murine model.