Iron Acquisition in Bacillus cereus: The Roles of IlsA and Bacillibactin in Exogenous Ferritin Iron Mobilization

Abstract

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.

Identification of FerLCH, isolation of ferritin and functional analysis related to interaction with pathogens in Eri‐silkworm, Samia cynthia ricini

“…Previous studies have shown that several Gram‐positive bacteria, including Bacillus cereus (Segond et al, 2014), B. anthracis, Streptococcus pyogenes , and S. aureus , can bind to heme (Nobles & Maresso, 2011). Therefore, a bacterial binding assay was performed to analyze the interaction between ferritin and bacteria.…”

Heme interplay between IlsA and IsdC: Two structurally different surface proteins from Bacillus cereus

“…In silico analysis showed a high similarity between the isd operons of B. anthracis and B. cereus. IsdC and IsdX1 from B. anthracis share 98% sequence identity with Bc4549 and Bc4548 respectively compared to 86% for IsdX2 and Bc4547 [21,22].…”

“…Interestingly, and despite relative homology in the iron acquisition systems between B. cereus and B. anthracis, our study highlights significant differences in growth. For instance, the B. anthracis ΔisdC mutant exhibited a small growth defect when grown in an iron-free medium supplemented with hemin [21] whereas the newly identified B. anthracis Hal protein (a protein similar to IlsA with a NEAT domain and several LRRs) had an apparent stronger role than isd compounds in growth with hemin and hemoglobin as sole iron sources [23]. In fact, the current study suggests that IlsA does not have a strong role in heme iron uptake as reported earlier [8] presumably because tetracycline was used as a selection marker in the growth medium in our earlier study.…”

Iron and zinc exploitation during bacterial pathogenesis

“…Bacillus cereus also uses ferritin as an iron source. In this pathogen, the surface protein IlsA (iron-regulated leucine rich surface protein type A) recognizes and binds ferritins, leading to the destabilization and subsequently release of ferric iron ions, which are chelated by the bacterial siderophore bacillibactin and transported via the cognate membrane transporter FeuABC (ferric bacillibactin uptake protein components ABC) 103 . Thus, it appears that when labile iron in circulation is not available, bacteria can prospect into deep host reserves such as ferritin to satisfy their requirement for this metal.…”