All cells need to export waste and toxic products. Gram-negative bacteria evolved multi-subunit, tripartite efflux pumps that export molecules from the periplasm through the outer membrane. The Resistance-Nodulation-Division (RND) family of efflux pumps are best known for expelling antibiotics, but they are also critical for infection in the absence of antibiotic treatment. Why bacteria need RND efflux pumps during infection is not clear, and understanding their roles in virulence could facilitate the development of therapeutic inhibitors of efflux pumps. We therefore propose to establish mechanisms by which the major Enterobacteriaceae RND efflux pump, AcrAB-TolC, contributes to virulence. Across Enterobacteriaceae, AcrAB-TolC is needed for infection of animals. In Salmonella enterica serovar Typhimurium (STm), AcrAB-TolC is also required for proliferation in macrophages and for growth in media that mimic the macrophage phagosome microenvironment in which STm resides. We propose to establish whether AcrAB-TolC is required for infection to enable physiological responses to 1) nitrosative stress and 2) host fatty acids.