ABSTRACTThe complex chemical environment of the intestine is defined largely by the metabolic products of the resident microbiota. Enteric pathogens, elegantly evolved to thrive in the gut, use these chemical products as signals to recognize specific niches and to promote their survival and virulence. Our previous work has shown that a specific class of quorum-sensing molecules found within the gut, termed diffusible signal factors (DSF), signals the repression ofSalmonellatissue invasion, thus defining a means by which this pathogen recognizes its location and modulates virulence to optimize its survival. Here, we determined whether the recombinant production of a DSF could reduceSalmonellavirulenceinvitroandinvivo. We found that the most potent repressor ofSalmonellainvasion,cis-2-hexadecenoic acid (c2-HDA), could be recombinantly produced inE.coliby the addition of a single exogenous gene encoding a fatty acid enoyl-CoA dehydratase/thioesterase and that co-culture of the recombinant strain withSalmonellapotently inhibited tissue invasion by repressingSalmonellagenes required for this essential virulence function. Using the well characterizedE.coliNissle 1917 strain and a chicken infection model, we found that the recombinant DSF-producing strain could be stably maintained in the large intestine. Further, challenge studies demonstrated that this recombinant organism could significantly reduceSalmonellacolonization of the cecum, the site of carriage in this animal species. These findings thus describe a plausible means by whichSalmonellavirulence may be affected in animals byinsituchemical manipulation of functions essential for colonization and virulence.