Differential Expression of Iron Acquisition Genes by Brucella melitensis and Brucella canis during Macrophage Infection

Linda Eskra,Jeremy Glasner,Gary Splitter,Jill Covert,Jean-Pierre Gorvel

doi:10.1371/journal.pone.0031747

Linda Eskra, Jeremy Glasner + Show 3 more

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https://doi.org/10.1371/journal.pone.0031747

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Abstract

Brucella spp. cause chronic zoonotic disease often affecting individuals and animals in impoverished economic or public health conditions; however, these bacteria do not have obvious virulence factors. Restriction of iron availability to pathogens is an effective strategy of host defense. For brucellae, virulence depends on the ability to survive and replicate within the host cell where iron is an essential nutrient for the growth and survival of both mammalian and bacterial cells. Iron is a particularly scarce nutrient for bacteria with an intracellular lifestyle. Brucella melitensis and Brucella canis share ∼99% of their genomes but differ in intracellular lifestyles. To identify differences, gene transcription of these two pathogens was examined during infection of murine macrophages and compared to broth grown bacteria. Transcriptome analysis of B. melitensis and B. canis revealed differences of genes involved in iron transport. Gene transcription of the TonB, enterobactin, and ferric anguibactin transport systems was increased in B. canis but not B. melitensis during infection of macrophages. The data suggest differences in iron requirements that may contribute to differences observed in the lifestyles of these closely related pathogens. The initial importance of iron for B. canis but not for B. melitensis helps elucidate differing intracellular survival strategies for two closely related bacteria and provides insight for controlling these pathogens.

Highlights

Iron is a required micronutrient for most organisms as it is involved in a wide variety of essential metabolic processes
To determine the differences in bacterial uptake of B. melitensis and B. canis, RAW cells were infected for 5 or 24 h with B. melitensis or B. canis transformed with pBBR1MCS/GFPuv
To determine the intracellular survival and replication of B. melitensis and B. canis, RAW cells were infected with bacteria and CFU were determined at 2, 8, 24 and 48 h (Figure 1C)

Summary

Introduction

Iron is a required micronutrient for most organisms as it is involved in a wide variety of essential metabolic processes. In particular, require efficient iron acquisition mechanisms to enable successful competition for iron in the highly iron-restricted environment of mammalian cells. When the intracellular iron concentration drops below a critical threshold, bacteria obtain iron by the direct uptake of heme or from iron-binding proteins by secreting small high affinity iron chelating compounds termed siderophores. Iron uptake systems are typically composed of an outer membrane transporter, a periplasmic binding protein, and a cytoplasmic ATP-dependent transmembrane transport system [3]. In Gram-negative bacteria, the high affinity iron uptake complex, tonB/exbB/exbD, provides energy for the transport of iron complexes across the outer membrane to the periplasm [4]. Transport across the inner membrane requires a periplasmic binding protein and an ABC transporter [4]

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