ABSTRACT:
      The interest on probiotic microorganisms has increased considerably during the last decades. They are mainly intestinal isolates obtained from healthy individuals that when orally administered contribute to the intestinal homeostasis maintenance. Their beneficial effect depends on the interactions they carry out in the gastrointestinal tract with cells of the epithelium, the mucosa associated lymphoid tissue and other microbiota members. The adhesion capacity to gastrointestinal mucosa components would prolong their persistence and increase the possibility that these interactions take place. These reasons make it one of the most employed criterium for the selection of potential probiotic strains. 
      The adhesion mechanisms have been characterized mainly in bacterial pathogens, where they play a key role in the invasion and colonization of the host. It is considered that the probiotic bacteria would share some of the mechanisms present in pathogens, but the availability of information about the process and the molecules implicated is still limited for this group of microorganisms. This work aimed to deepen in the study of the lactobacilli adhesive properties and in the characterization of the determinants implicated. 
      Lactobacillus casei species is naturally present in the gastrointestinal and reproductive tracts of humans and animals, it has been traditionally used to obtain fermented foods and it encompasses multiple strains considered as probiotics. The research carried out in this thesis evidenced the absence of clear adhesion profiles depending on the strain origin and a great intra-specific variability. The protease treatment suggested a major contribution of surface exposed proteic factors in the adhesion to extracellular matrix (ECM) proteins. 
      We employed the L. casei BL23 strain due to its probiotic character, its genetic manipulation susceptibility and the availability of its genome sequence. 
      The identification of possible proteic adhesins was carried out by three complementary strategies: in silico analysis, the search on a phage display gene library and the direct identification using cell wall protein extracts. We determined the adhesive properties of the purified proteins and the effect of mutations of their codifying genes on the adhesive properties of whole cells. 
      The variety of proteins able to interact with ECM components identified reinforces the idea that the adhesion proccess is multifactorial. They possibly have lectin-like characteristics since the mucus and ECM proteins are characterized by being highly modified, mainly by glycosylation. Part of the potential adhesins identified have been previously characterized as intracellular proteins with a main function not related to adhesion (glicolytic enzymes, stress response proteins, etc.) Therefore, they represent moonlighting proteins that would not follow the classic pathways of secretion or anchoring to the cell surface. 
      This work also aimed to determine the possible contribution of sortase-dependent surface proteins to the persistence in the gastrointestinal tract. We found substrates implicated in mucosal carbohydrate utilization and mucosal adhesion. Four sortases were identified in L. casei BL23, two class A (srtA1 and srtA2) and two class C sortases (srtC1 and srtC2) clustered with their potential substrates. The studies of adhesion properties, sortase dependent substrate anchoring and gene expression pointed to srtA1 as the main sortase responsible of anchoring the majority of substrates with sortase cleavage motif, while srtA2 may compensate the absence of srtA1 activity. srtC1 y srtC2 clusters are, respectively, homologous to the spaFED and spaCBA clusters of L. rhamnosus GG responsible for the formation of pili with intestinal mucus adhesive capacity. This suggests that L. casei would be a bacterium with this kind of proteic appendixes that may contribute to the mucus adhesion. 
      This thesis represents an increase in the knowledge of the L. casei adhesive properties to components of the gastrointestinal mucosa and of the proteic factors implicated in the host/probiotic interaction.