The bacterial siderophore enterobactin confers survival advantage to Salmonella in macrophages

Enterobactin (Ent), a prototypical bacterial siderophore known for its unparalleled affinity for iron, is widely conserved among members of the Enterobacteriaceae family of Gram-negative bacteria. In this study, we demonstrated that, aside from mediating iron acquisition, Ent also dampened the macrophages (M phi s) antimicrobial responses against intracellular infection by Salmonella enterica serovar Typhimurium. Accordingly, the loss of Ent expression (Delta entB) in Salmonella demoted their survivability against M phi s. Addition of exogenous Ent not only rescued the survival of Delta entB Salmonella, but also augmented WT Salmonella to better withstand the microbicidal activity of M phi s. The protection conferred to WT Salmonella was observed only when Ent was administered as iron-free, thus indicating the requirement of iron chelation in this context. In contrast, the exogenous iron-bound Ent retained its ability to promote the survival of Delta entB Salmonella, albeit modestly. Assessment on M phi s labile iron pool (LIP) revealed that iron-free Ent is able to permeate into M phi s, chelate the intracellular LIP, and regulate the expression of several key iron-regulatory proteins, i.e., divalent metal transporter 1, ferroportin, and hepcidin. Chelation of iron by Ent was also observed to promote the M phi s towards M2 polarization. Collectively, our findings demonstrated that Ent not only facilitates bacterial iron uptake but also disrupts M phi s iron homeostasis and M1/M2 polarization to safeguard intracellular bacteria against the anti-bacterial effects of their host.