Comprehensive analysis of iron utilization by Mycobacterium tuberculosis.

Lei Zhang,Michael Niederweis,R Curtis Hendrickson,Elliot J Lefkowitz,Thomas R Ioerger,Virginia Meikle,Helena Ingrid Boshoff

doi:10.1371/journal.ppat.1008337

Lei Zhang, Michael Niederweis + Show 5 more

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https://doi.org/10.1371/journal.ppat.1008337

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Abstract

Iron is essential for nearly all bacterial pathogens, including Mycobacterium tuberculosis (Mtb), but is severely limited in the human host. To meet its iron needs, Mtb secretes siderophores, small molecules with high affinity for iron, and takes up iron-loaded mycobactins (MBT) and carboxymycobactins (cMBT), from the environment. Mtb is also capable of utilizing heme and hemoglobin which contain more than 70% of the iron in the human body. However, many components of these iron acquisition pathways are still unknown. In this study, a high-density transposon mutagenesis coupled with deep sequencing (TnSeq) showed that Mtb exhibits nearly opposite requirements for 165 genes in the presence of heme and hemoglobin versus MBT and cMBT as iron sources. The ESX-3 secretion system was assessed as essential for siderophore-mediated iron uptake and, surprisingly, also for heme utilization by Mtb. Predictions derived from the TnSeq analysis were validated by growth experiments with isogenic Mtb mutants. These results showed that (i) the efflux pump MmpL5 plays a dominant role in siderophore secretion, (ii) the Rv2047c protein is essential for growth of Mtb in the presence of mycobactin, and (iii) the transcriptional repressor Zur is required for heme utilization by Mtb. The novel genetic determinants of iron utilization revealed in this study will stimulate further experiments in this important area of Mtb physiology.

Highlights

Iron is an essential nutrient for almost all organisms because of its vital role as a redox cofactor of proteins required for cellular processes ranging from respiration to DNA replication [1,2]
We identified 165 genes involved in iron utilization by Mycobacterium tuberculosis (Mtb), 66 of which were classified as essential or as required for optimal growth of M. tuberculosis in the presence of different iron sources
In order to establish a solid basis for identifying genetic requirements for iron utilization by Mtb, we first wanted to establish which iron sources Mtb can utilize

Summary

Introduction

Iron is an essential nutrient for almost all organisms because of its vital role as a redox cofactor of proteins required for cellular processes ranging from respiration to DNA replication [1,2]. Mtb produces small molecules with extremely high affinity for iron (III) called mycobactins (MBT) and carboxymycobactins (cMBT) [6,7] These siderophores share a common core structure determining their iron (III) chelating properties, but the secreted cMBTs have significantly shorter fatty acid chains and are, more hydrophilic than the MBTs which are completely insoluble in water and are, membrane associated [6,8]. These siderophores are capable of removing iron from host proteins such as transferrin and lactoferrin [9]. It is unclear how siderophores are secreted and taken up across the outer membrane [28] and whether Mtb possesses a TonB-like system which is essential for siderophore and heme uptake in gram-negative bacteria [29,30]

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