Interplay between enterobactin myeloperoxidase and lipocalin 2 in the inflamed gut Original paper

What was studied?

This study investigated the interplay between enterobactin myeloperoxidase and lipocalin 2 as a determinant of Escherichia coli (E. coli) survival within the inflamed gut. The authors tested whether the catecholate siderophore enterobactin, particularly in its iron-free form, directly inactivates the neutrophil peroxidase system that generates hypohalous acids, and whether host lipocalin 2 reverses this effect. Spectral kinetics using lactoperoxidase as a model readout showed that enterobactin and its monomer 2,3-dihydroxybenzoic acid rapidly drive compound I back to the ferric resting state, thereby aborting oxidant formation. Figures and spectra on page 3 document the immediate reversion of the Soret peak to 412 nm, consistent with suicide substrate behavior.

Who was studied?

The work combined in vitro enzyme assays, bacterial killing assays, and murine models of colitis. Nonpathogenic E. coli K-12 and isogenic siderophore mutants were used, including DfepA (overproduces enterobactin), DaroB and DentC (enterobactin deficient), and DaroB/DfepA. Recombinant human or murine lipocalin 2 was applied to test host countermeasures. In vivo, streptomycin-pretreated Salmonella-induced gastroenteritis and dextran sulfate sodium colitis models quantified cecal and colonic myeloperoxidase activity and E. coli colonization. A schematic on page 8 summarizes the experimental mechanism linking hypoferremia, enterobactin release, myeloperoxidase inactivation, and lipocalin 2 rescue.

Most important findings

Enterobactin, only in its aferric form, potently inhibited myeloperoxidase and lactoperoxidase in a dose- and time-dependent manner, outperforming the reference inhibitor 4-aminobenzoic acid hydrazide. DHBA showed similar but weaker effects. Iron-loaded enterobactin and DHBA failed to inhibit, establishing iron-free specificity. Glycosylated or non-catecholate siderophores, including salmochelin, yersiniabactin, and ferrichrome, did not inhibit at much higher concentrations, implying a catecholate-dependent mechanism. In vivo, DfepA reduced mucosal myeloperoxidase activity and achieved higher fecal and tissue burdens than enterobactin-null mutants in both colitis models, indicating a survival and colonization advantage under inflammation. Preincubation of enterobactin or DHBA with human or mouse lipocalin 2 abolished enzyme inhibition, restoring peroxidase activity. The spectral plots on page 3 and bactericidal assays on pages 5–6 show rapid conversion of compound I to Fe(III) and protection from myeloperoxidase–H2O2 killing, while figure panels on page 7 demonstrate lipocalin 2 reversal. Collectively, these data position enterobactin as a dual-use molecule for iron acquisition and immune evasion, with lipocalin 2 as the host countermeasure.

Key implications

Enterobacteriaceae, particularly E. coli, exploit siderophore chemistry to neutralize neutrophil peroxidase-mediated killing during inflammation. Enterobactin emerges as a mechanistic driver of E. coli blooms in inflammatory bowel disease (IBD) by disabling a key oxidative effector, while lipocalin 2 functions as a host rebuttal that can restore peroxidase activity. The failure of salmochelin to inhibit myeloperoxidase, despite its ability to evade lipocalin 2, suggests an evolutionary trade-off that may tune siderophore portfolios across pathotypes and disease niches. Clinically, lipocalin 2 levels, siderophore profiles, and myeloperoxidase activity could serve as coupled biomarkers that stratify inflammatory risk and E. coli overgrowth. Therapeutically, strategies that stabilize peroxidase function, enhance lipocalin 2 binding to catecholate siderophores, or limit aferric enterobactin bioavailability may interrupt the survival advantage documented in this work. The mechanism diagram on page 8 provides a concise translational blueprint for these interventions.

Citation

Singh V, Yeoh BS, Xiao X, et al. Interplay between enterobactin, myeloperoxidase and lipocalin 2 regulates E. coli survival in the inflamed gut. Nat Commun. 2015;6:7113. doi:10.1038/ncomms8113.