Self-poisoning of Mycobacterium tuberculosis by interrupting siderophore recycling

Abstract

Siderophores are small iron-binding molecules secreted by bacteria to scavenge iron. Mycobacterium tuberculosis (Mtb), the etiologic agent of tuberculosis, produces the siderophores mycobactin and carboxymycobactin. Complexes of the mycobacterial membrane proteins MmpS4 and MmpS5 with the transporters MmpL4 and MmpL5 are required for siderophore export and virulence in Mtb.Here we show that, surprisingly, mycobactin or carboxymycobactin did not rescue the low-iron growth defect of the export mutant but severely impaired growth. Exogenous siderophores were taken up by the export mutant, and siderophore-delivered iron was used, but the deferrated siderophores accumulated intracellularly, indicating a blockade of siderophore recycling. This hypothesis was confirmed by the observation that radiolabeled carboxymycobactin was taken up and secreted again by Mtb. Addition of iron salts to an Mtb siderophore biosynthesis mutant stimulated more growth in the presence of a limiting amount of siderophores than iron-loaded siderophores alone. Thus, recycling enables Mtb to acquire iron at lower metabolic cost because Mtb cannot use iron salts without siderophores. Exogenous siderophores were bactericidal for the export mutant in submicromolar quantities. High-resolution mass spectrometry revealed that endogenous carboxymycobactin also accumulated in the export mutant. Toxic siderophore accumulation is prevented by a drug that inhibits siderophore biosynthesis. Intracellular accumulation of siderophores was toxic despite the use of an alternative iron source such as hemin, suggesting an additional inhibitory mechanism independent of iron availability. This study indicates that targeting siderophore export/recycling would deliver a one-two punch to Mtb: restricting access to iron and causing toxic intracellular siderophore accumulation.

Resistance to Thiacetazone Derivatives Active against Mycobacterium abscessus Involves Mutations in the MmpL5 Transcriptional Repressor MAB_4384

“…Of note, inactivation of mmpL5 in M. tuberculosis severely compromised the ability of the mutant to multiply in mouse lungs (52), and more recently, Wells et al established that MmpS4/L4 and MmpS5/L5 form independent and nonredundant siderophore transporter systems (53). These transport systems not only are required for the export of de novo synthesized mycobactins but also are involved in the recycling of exogenous siderophores (54). Whether the MmpS5-MmpL5 couple identified in this study contributes to the export of siderophores and pathogenicity of M. abscessus remains to be addressed.…”

Iron Homeostasis in Mycobacterium tuberculosis: Mechanistic Insights into Siderophore-Mediated Iron Uptake

“…They developed specific knockout mutants and showed that MmpS4 and MmpS5, with their respective MmpL4 and MmpL5 transport proteins, formed a novel siderophore export system for mycobactin and carboxymycobactin. Interestingly, in a subsequent study (59), it was demonstrated that the addition of exogenous siderophore to the ⌬mmpS4 ⌬mmpS5 double KO mutant inhibited its growth due to the toxicity of the accumulated desferrisiderophore. While the mutant strain was able to take up the ferric forms of carboxymycobactin and mycobactin and utilize the iron, it was unable to export the desferric form that clearly implicated that recycling of the siderophore was taking place in M. tuberculosis.…”

Genome-wide screen identifies host colonization determinants in a bacterial gut symbiont

“…S2) is indicative of its importance, particularly for iron bound in soluble complexes, the dominant form in aerobic environments. Three iron importers, for heme-bound Fe 2+ and siderophore-bound Fe 3+ , and associated genes (tonB, exeBD, and fepA), were necessary in vivo, as were the exporters barAB and tolC to avoid toxic buildup of siderophores (28). Siderophore synthesis, however, was not essential, presumably because mutants can "cheat" and take up siderophores secreted by nonmutants in the community.…”