Celiac disease (CD) is an autoimmune enteropathy that involves genetic and environmental factors. It is the most common chronic intestinal disorder and, although it can occur at any age with a variety of symptoms, typical cases with gastrointestinal symptoms usually occur during childhood. In CD patients, the ingestion of gluten from the diet cause chronic inflammation of the small intestine leading to loss the integrity and function of the intestinal mucosa. The intestinal mucosal lesion can cause total villus atrophy, leading to malabsorption syndrome and intestinal and/or extraintestinal manifestations. The only treatment for CD patients is the adherence to a strict lifelong gluten-free diet, but maintaining this dietary recommendation is complex due to the presence of gluten in most processed foods. Therefore, patients are often exposed to gluten and some continue having mainly gastrointestinal symptoms, and increased health risks. As a result, several research lines are opened to develop complementary and/or alternative therapies to the gluten-free diet.
       In addition to gluten and the individual genetic background, other factors such as the type of feeding-practices or the incidence of infections during childhood which influence the microbiota and the development of the immune system, could also be implicated in the risk of developing CD. In recent years, it has been shown that CD patients have alterations in their intestinal microbiota in comparison with healthy controls. The aim of this thesis has been to progress in the characterization of alterations in the intestinal microbiota composition associated primary or secondary with CD, and to establish the possible role of relevant microbial groups in the pathogenesis of the disease. The study of the functions of specific components of the microbiota in the pathogenesis and risk of the disease may facilitate the further development of nutritional intervention strategies based on probiotics and/or prebiotics to improve the health status of these patients and reduce the risk of developing the disease.
       In the first chapter (studies 1 and 2) fecal and duodenal bacterial populations of CD children were analyzed and compared with controls by using denaturing gradient gel electrophoresis (DGGE). Using this technique, the studied childrenīs groups showed differences in diversity and species composition.
       Next chapter was focused on characterization of relevant bacterial groups for CD, either by the detection of quantitative differences regarding controls or because of their possible action as opportunistic pathogens, using cultured-dependent techniques. Clones of enterobacteria, staphylococci and bacteroides were isolated from stools of CD patients and control subjects and were identified at species level. Dominant species were characterized to determine the presence/absence of virulence factors. In these studies (studies 3, 4 and 5) it was observed that celiac subjects showed a greater abundance of Escherichia coli, Staphylococcus epidermidis and Bacteroides fragilis, and, generally, these isolates harbored a higher number of genes encoding virulence factors. Furthermore, cultivable-associated bacteria were isolated and identified from the duodenal CD mucosa and from control biopsy specimens in order to investigate differences. In this study (study 6) it was observed that patients with CD showed lower abundance of species of the family Streptococcaceae and a greater abundance of species of the families Enterobacteriaceae and Staphylococcaceae, in particular Klebsiella oxytoca, S. epidermidis and Staphylococcus pasteuri.
       The last chapter (study 7) focused on the characterization of the intestinal colonization process of Bacteroides spp. in infants with risk of developing CD, and to determine its relation to environmental and genetic factors. For this purpose, Bacteroides spp. from healthy infants with high genetic risk of developing CD were monitored. The results indicated that the HLA-DQ genotype of healthy infants can influence in the colonization pattern of Bacteorides spp. and, probably, the risk of developing CD.