Strains, Plasmids and primers.

<div> <i>Salmonella enterica</i> serovar Typhimurium causes acute diarrhea upon oral infection in humans. The harsh and proteolytic environment found in the gastrointestinal tract is the first obstacle that these bacteria face after infection. However, the mechanisms that allow <i>Salmonella</i> to survive the hostile conditions of the gut are poorly understood. The <i>ecotin</i> gene is found in an extensive range of known phyla of bacteria and it encodes a protein that has been shown to inhibit serine proteases. Thus, in the present work we studied the role of <i>ecotin</i> of <i>Salmonella</i> Typhimurium in host-pathogen interactions. We found that <i>Salmonella</i> Typhimurium ∆<i>ecotin</i> strain exhibited lower inflammation in a murine model of <i>Salmonella</i> induced colitis. The ∆<i>ecotin</i> mutant was more susceptible to the action of pancreatin and purified pancreatic elastase. In addition, the lack of <i>ecotin</i> led to impaired adhesion to Caco-2 and HT-29 cell lines, related to the proteolytic activity of brush border enzymes. Besides, ∆<i>ecotin</i> showed higher susceptibility to lysosomal proteolytic content and intracellular replication defects in macrophages. In addition, we found Ecotin to have a crucial role in <i>Salmonella</i> against the microbicide action of granules released and neutrophil extracellular traps from human polymorphonuclear leukocytes. Thus, the work presented here highlights the importance of <i>ecotin</i> in <i>Salmonella</i> as countermeasures against the host proteolytic defense system. </div>