Date Published: February 13, 2014
Publisher: Public Library of Science
Author(s): Diego Segond, Elise Abi Khalil, Christophe Buisson, Nadine Daou, Mireille Kallassy, Didier Lereclus, Paolo Arosio, Fadi Bou-Abdallah, Christina Nielsen Le Roux, Andreas Peschel.
In host-pathogen interactions, the struggle for iron may have major consequences on the outcome of the disease. To overcome the low solubility and bio-availability of iron, bacteria have evolved multiple systems to acquire iron from various sources such as heme, hemoglobin and ferritin. The molecular basis of iron acquisition from heme and hemoglobin have been extensively studied; however, very little is known about iron acquisition from host ferritin, a 24-mer nanocage protein able to store thousands of iron atoms within its cavity. In the human opportunistic pathogen Bacillus cereus, a surface protein named IlsA (Iron-regulated leucine rich surface protein type A) binds heme, hemoglobin and ferritin in vitro and is involved in virulence. Here, we demonstrate that IlsA acts as a ferritin receptor causing ferritin aggregation on the bacterial surface. Isothermal titration calorimetry data indicate that IlsA binds several types of ferritins through direct interaction with the shell subunits. UV-vis kinetic data show a significant enhancement of iron release from ferritin in the presence of IlsA indicating for the first time that a bacterial protein might alter the stability of the ferritin iron core. Disruption of the siderophore bacillibactin production drastically reduces the ability of B. cereus to utilize ferritin for growth and results in attenuated bacterial virulence in insects. We propose a new model of iron acquisition in B. cereus that involves the binding of IlsA to host ferritin followed by siderophore assisted iron uptake. Our results highlight a possible interplay between a surface protein and a siderophore and provide new insights into host adaptation of B. cereus and general bacterial pathogenesis.
Iron is an essential nutrient for most forms of life. Owing to the high variability of the Fe3+/Fe2+ redox potential, this transition metal fulfills a large number of biological processes including respiration and DNA synthesis. However, because of its low solubility and propensity to generate highly reactive hydroxyl radicals, iron is a double-edged element and its homeostasis must be finely tuned . Given that most microorganisms require micromolar iron concentrations for growth and multiplication , the ability to obtain iron is thus an important adaptation factor requiring intricately sophisticated iron uptake systems . Upon infection, the host sets up a form of nutritional immunity aimed at depriving the invader of nutritional iron through iron redistribution in the organism and scavenging of certain microbial siderophores , . The importance of this strategy is evidenced by the effects of iron homeostasis disorders on both innate and acquired immune responses , . In this battle for iron, and to circumvent host-iron withholding, pathogenic bacteria are able to acquire iron via siderophore-based systems or by surface and membrane anchored proteins which interfere with host iron-containing proteins such as transferrins, hemoproteins or ferritins . Most of these iron acquisition systems are under the control of the global regulator Fur (Ferric uptake regulator) which also regulates the expression of multiple virulence factors . Over the past 10 years, our understanding of iron import into bacteria has been greatly improved , . The most impressive advances concerned heme acquisition in Gram-positive bacteria. One major discovery has been made with the characterization of the Isd (Iron-regulated surface determinant) system in Staphylococcus aureus. Heme or hemoglobin interaction with this system relies on cell wall–anchored proteins that act as hemoprotein receptors by means of their NEAT (NEAr iron Transporter) domains. Since then, a growing number of studies have emphasized the role of NEAT domains in heme binding in several Gram-positive bacteria including S. aureus, Streptococcus pyogenes, Bacillus anthracis (for review, see ) or Bacillus cereus (Abi Khalil et al., unpublished data). Although most iron is bound to hemoglobin in vertebrates, ferritin can represent another important source of iron for microbes , , , , , , .
For pathogens, the ability to cope with the low iron environment encountered in the host is essential for tissue colonization. Thus, the production of efficient iron acquisition systems represents key factors. Because ferritin is the major iron storage protein found in living organisms, pathogens have developed efficient mechanisms to use this iron source and gain rapid access to sufficient quantities of iron. However, studies of the microbial determinants involved in host ferritin iron theft remain scarce. The present study showed that the bacterial surface protein IlsA interacts directly with the ferritin shell perhaps altering its structural integrity and leading to an amplification of iron release from the nanocage.