Ferrous Iron Uptake Is Required for Salmonella to Persist within Vacuoles of Host Cells

Iron is an essential oligoelement that incorporates into proteins as a biocatalyst or electron carrier. The intracellular pathogen Salmonella enterica serovar Typhimurium ( Typhimurium) takes iron as free reduced ferrous cation or as oxidized ferric cation complexed to siderophores or ferrichromes. Deficiencies in ferrous or ferric iron uptake attenuate Typhimurium virulence, but how the uptake systems are used in the intracellular environment remains poorly understood. Here, using Typhimurium mutants deficient in multiple iron uptake systems, we show that SitABCD and FeoABC, involved in ferrous iron uptake, are central for this pathogen to persist within vacuoles of fibroblasts. Assays at the protein level showed that components of these two uptake systems, SitD and FeoB, are produced at high levels by intravacuolar bacteria. Despite not being essential for viability inside the vacuole, intracellular bacteria also upregulate transporters involved in ferric iron uptake such as IroN, FepA, and CirA. In addition, an unprecedented cleavage at the N-terminal region of FepA was observed as a distinctive feature of nonproliferating intravacuolar bacteria. Collectively, our findings indicate that SitABCD and FeoABC contribute to Typhimurium virulence by promoting iron acquisition within the vacuolar compartment.