Commensal iron acquisition and nutritional immunity during Salmonella infection Original paper

What was studied?

This study interrogates commensal iron acquisition and nutritional immunity during Salmonella infection, focusing on how the gut commensal Bacteroides thetaiotaomicron captures xenosiderophores to sustain fitness and inadvertently modulates host defenses against pathogens. The authors identify XusB, a secreted, surface-anchored lipoprotein that binds ferric enterobactin with high affinity and is enriched in outer membrane vesicles. They combine structural biology, biochemistry, bacterial genetics, and murine infection models to show that XusB-bound enterobactin resists sequestration by host lipocalin 2, can be re-acquired by Salmonella via FepA or IroN, and thereby alters the effective pool of iron chelators available under inflammatory iron restriction. The graphical abstract on page 2 schematically depicts enterobactin crossfeeding, OMV packaging of XusB, and reduced lipocalin 2 access, framing commensal iron acquisition as a third axis in nutritional immunity beyond host and pathogen.

Who was studied?

Experiments used B. thetaiotaomicron VPI-5482 and isogenic mutants lacking xusB or harboring an enterobactin-binding deficient XusB, along with Salmonella enterica serovar Typhimurium strains, including wild type, entB deficient, ΔfepA iroN double mutant, and ΔiroB lacking salmochelin synthesis. Escherichia coli BW25113 mutants from the Keio collection informed receptor requirements for XusB-bound siderophore uptake. In vivo work involved antibiotic-pretreated conventional C57BL/6 mice for competitive infection and colonization studies, and gnotobiotic Swiss Webster mice to measure XusB in luminal OMV fractions without confounding microbes. Figures 4 and 7 detail competitive indices for Salmonella in mice colonized with B. thetaiotaomicron variants or administered OMVs, while supplementary analyses confirm comparable inflammation readouts across conditions.

Most important findings

XusB is a surface-exposed, OMV-enriched lipoprotein required for B. thetaiotaomicron utilization of catecholate xenosiderophores. Structural work shows XusB forms a seven-bladed beta propeller with a positively charged central calyx that accommodates Fe-enterobactin; docking and mutagenesis identify key contact residues, and binding affinity is in the low-nanomolar range near 148 nM. In vitro, OMV-associated or recombinant XusB preloaded with Fe-enterobactin fuels growth of B. thetaiotaomicron under iron chelation, elevates cellular iron by ICP-MS, and supports growth of Salmonella entB mutants and E. coli via TonB-dependent enterobactin receptors FepA and IroN. In species with distinct XusB homologs, XusB-bound siderophores can be “selfish,” restricting access to closely related Bacteroides while remaining accessible to Enterobacteriaceae.

Crucially, XusB changes the competitive landscape in the presence of host lipocalin 2. When lipocalin 2 is added with an iron chelator, XusB increases the competitive index of wild-type Salmonella over the ΔfepA iroN mutant, indicating that XusB-bound enterobactin constitutes an exploitable pool despite host sequestration of free enterobactin. In mice, XusB confers a resilience advantage to B. thetaiotaomicron during Salmonella colitis and increases Salmonella fitness when animals are colonized with XusB-positive B. thetaiotaomicron or given XusB-bearing OMVs. Page 34 visualizes the XusB architecture and calyx electrostatics, pages 39 to 41 depict in vitro and in vivo competition assays that demonstrate altered siderophore accessibility and pathogen advantage.

Key implications

For microbiome signatures, B. thetaiotaomicron emerges as a context-dependent facilitator of pathogen iron access under inflammation, linking a commensal siderophore-binding system to Salmonella expansion. B. thetaiotaomicron with XusB-mediated xenosiderophore capture and Salmonella Typhimurium with enterobactin or salmochelin utilization. Clinically, strategies that modulate iron availability or deploy siderophore-binding therapeutics must account for commensal OMV cargo that can buffer host sequestration and inadvertently aid pathogens. Targeting XusB-like systems may represent a translational lever to restore nutritional immunity without broadly disrupting commensals.

Citation

Spiga L, Fansler RT, Perera YR, et al. Iron acquisition by a commensal bacterium modifies host nutritional immunity during Salmonella infection. Cell Host Microbe. 2023;31(10):1639-1654.e10. doi:10.1016/j.chom.2023.08.018.