The Salmonella genus includes over 2,500 known serotypes distributed betweenthe two species: enterica and bongori. These serotypes differ greatly in terms of pathogenicity and host specificity. Two Salmonella enterica serotypes are of significant relevance: serotypes Gallinarum and Enteritidis.S. Gallinarum has a host range restricted to birds and causes a severe systemicdisease called fowl typhoid, which causes majar economic losses in poultry production in several parts of the world. S. Enteritidis on the other hand, infects a broad range of hosts including humans, mice and avian species. In contrast to S. Gallinarum, S. Enteritidis generates a subclinical infection in poultry, and infected hens can become chronic carriers laying Salmonella contaminated eggs. Human consumption of contaminated poultry or egg products results in an acute self-limiting gastroenteritis, being responsible for -61% of the estimated 1.5 million human salmonellosis casesreported between 1995 and 2008 (WHO Global Foodborne lnfections Network Country Databank).There is little work done on the molecular mechanisms behind the differential host-adaptation and clinical outcomes of infections caused by serotypes Enteritidis and Gallinarum in their susceptible hosts, including birds, but recent evidence suggests thatthese serotypes might possess undescribed virulence factors that may account forthese differences. lnteraction between bacteria and hosts is guided by acommunication/signaling interplay which aims to influence the host response. Among the tools used by bacteria to influence the host response, secretion machines that deliver proteins and toxins into the environment and within eukaryotic target cells are crucial for bacterial virulence and survival.The Type VI Secretion System (T6SS) is a newly described mechanism for protein translocation that exists in most Gram-negative bacteria that come into clasecontact with eukaryotic cells, including plant and animal pathogens. The precise role and mode of action of T6SS is still unknown, but it is clear that plays an important role in bacterial virulence.In Salmonella enterica, only one T6SS encoded in Salmonella Pathogenicitylsland 6 (SPI-6) has been described. In this thesis, through bioinformatics andcomparative genomic analyzes it was determined that the genus Salmonella encodes 5 T6SS loci, differentially distributed among different serotypes and with distinct phylogenetic histories. The novel T6SS loci were identified in genomic islands designated SPI-19, SPI-20, SPI-21 and SPI-22. In addition of the identification of these T6SS loci, a novel "evolved" VgrG protein with a S-Type Pyocin containing-domain, wasidentified in SPI-21. The presence of this protein domain suggested for the first time a role for T6SSs in bacterial killing opening a new chapter in the study of T6SS and its role in inter-bacterial relationships.The SPI-19 T6SS was of significant relevance dueto its wide distribution among virulent Salmonella serotypes and because bioinformatics analyzes showed that while Gallinarum encodes a complete T6SS, serotype Enteritidis only encodes for remnants of this system. Despite being closely related, serotypes Gallinarum and Enteritidis present profound differences in their host-range and pathogenicity. Therefore, it istempting to speculate that the presence of an active T6SS is somehow related to the host-adaptation and pathogenicity differences presented by these serotypes. To resolve this hypothesis and assess the contribution of SPI-19 to Salmonella pathogenicity, the objective of this thesis was to determine whether the SPI-19 genomic island encodes a functional T6SS contributing to the pathogenicity of Gallinarum and Enteritidis in the avian host.In order to characterize the SPI-19 T6SS, gene and operen fusions were constructed and expression, production and secretion of T6SS components were evaluated under different in vitro growth conditions. The analysis showed that most T6SS components remain repressed under the conditions tested. lnfection of murine macrophages with a Gallinarum strain harboring the structural/secreted T6SS component VgrG fused to the GFP reporter showed that T6SS components are preferentially produced inside infected cells. Non-polar deletion mutants of the whole SPl-19 and specific T6SS core components revealed that this T6SS was necessary forSalmonella Gallinarum survival within macrophages at late time points after infection. Furthermore, the SPI-19 T6SS function could not be linked to Salmonella-induced cytotoxicity or cell death of infected macrophages.To determine the contribution of SPI-19 T6SS to Salmonella pathogenesis,T6SS mutants were tested in competitive infection assays against the wild-typeGallinarum parental strain. Oral infection of four-day-old White Leghorn chicks revealed that T6SS mutants colonized the ileum, ceca, liver and spleen poorly compared to the wild-type strain. Restitution of SPI-19 to the LlSPI-19 mutant, using VEX-Capture,complemented this colonization defect. Altogether, the data indicate that SPI-19 and the T6SS encoded therein contributes to macrophage intracellular survival and colonization of chicks infected by S. Gallinarum. To assess the impact of carrying a complete T6SS locus on the ability of S. Enteritidis to colonize the avian host, the SPI-19 from Gallinarum was transferred to Enteritidis. In vivo experiments showed that presence ofa complete SPI-19 significantly increased the ability of Enteritidis to colonize the ileum, liver and spleen of infected chicks by day 1 post-infection. This colonizatlon advantage was not lasting however, as this strain presented a strong colonization defect for eachorgan analyzed from day 3 post infection to the conclusion of the experiment. These results suggest that transfer of SPI-19 from S. Gallinarum has a negative impact on the ability of S. Enteritidis to colonize the avian host. In this context is tempting to speculatethat loss of the SPI-19 T6SS corresponds to a pathoadaptative event during S.Enteritidis evolution. In addition, this is the first report of the use of Vex-Capture method to assess the effect of Genomic lsland transfer in an animal model of bacterial infection.The recent discovery of Type VI Secretion Systems (T6SS) has opened a new chapter in the study of Salmonella host and environmental adaptation. WhetherSalmonella encodes T6SSs and whether they could be considered as quantum leapevolution events are sorne of the questions that the discovery of T6SS in bacterial genomes generated. In this thesis, we have expanded the current knowledge on bacterial T6SSs and Salmonella virulence potential by: i) the identification and description of 4 novel Salmonella Pathogenicity lslands (SPI-19, SPI-20, SPI-21 and SPI-22) encoding phylogenetically distinct T6SS loci, ii) the discovery of a novel "evolved" VgrG protein, which suggested for the first time a role for T6SSs in interbacterialrelationships, iii) identifying that the SPI-19 T6SS contributes to Salmonellaintracellular survival in macrophages and iv) determining that the SPI-19 T6SScontributes to chicken colonization by S. Gallinarum.PFCHA-BecasDoctor en Bioquímica1377.PFCHA-BecasTERMINADA