Identification and characterization of a Streptococcus pyogenes operon involved in binding of hemoproteins and acquisition of iron

Induction of a quorum sensing pathway by environmental signals enhances group A streptococcal resistance to lysozyme

The human-restricted pathogen Streptococcus pyogenes (Group A Streptococcus, GAS) is responsible for wide-ranging pathologies at numerous sites in the body but has the proclivity to proliferate in individuals asymptomatically. The ability to survive in diverse tissues is undoubtedly benefited by sensory pathways that recognize environmental cues corresponding to stress and nutrient availability and thereby trigger adaptive responses. We investigated the impact that environmental signals contribute to cell-to-cell chemical communication [quorum sensing (QS)] by monitoring activity of the Rgg2/Rgg3 and SHP-pheromone system in GAS. We identified metal limitation and the alternate carbon source mannose as two environmental indicators likely to be encountered by GAS in the host that significantly induced the Rgg-SHP system. Disruption of the metal regulator MtsR partially accounted for the response to metal depletion, whereas ptsABCD was primarily responsible for QS induction due to mannose, but each sensory system induced Rgg-SHP signaling apparently by different mechanisms. Significantly, we found that induction of QS, regardless of the GAS serotype tested, led to enhanced resistance to the antimicrobial agent lysozyme. These results indicate the benefits for GAS to integrate environmental signals with intercellular communication pathways in protection from host defenses.

Mechanisms of group A Streptococcus resistance to reactive oxygen species

Streptococcus pyogenes, also known as group A Streptococcus (GAS), is an exclusively human Gram-positive bacterial pathogen ranked among the ‘top 10’ causes of infection-related deaths worldwide. GAS commonly causes benign and self-limiting epithelial infections (pharyngitis and impetigo), and less frequent severe invasive diseases (bacteremia, toxic shock syndrome and necrotizing fasciitis). Annually, GAS causes 700 million infections, including 1.8 million invasive infections with a mortality rate of 25%. In order to establish an infection, GAS must counteract the oxidative stress conditions generated by the release of reactive oxygen species (ROS) at the infection site by host immune cells such as neutrophils and monocytes. ROS are the highly reactive and toxic byproducts of oxygen metabolism, including hydrogen peroxide (H₂O₂), superoxide anion (O₂•⁻), hydroxyl radicals (OH•) and singlet oxygen (O₂*), which can damage bacterial nucleic acids, proteins and cell membranes. This review summarizes the enzymatic and regulatory mechanisms utilized by GAS to thwart ROS and survive under conditions of oxidative stress.

This review discusses the mechanisms utilized by the bacterial pathogen group A Streptococcus to detoxify reactive oxygen species and survive in the human host under conditions of oxidative stress.

Iron acquisition and regulation systems in Streptococcus species

Gram-positive Streptococcus species are responsible for millions of cases of meningitis, bacterial pneumonia, endocarditis, erysipelas and necrotizing fasciitis. Iron is essential for the growth and survival of Streptococcus in the host environment. Streptococcus species have developed various mechanisms to uptake iron from an environment with limited available iron. Streptococcus can directly extract iron from host iron-containing proteins such as ferritin, transferrin, lactoferrin and hemoproteins, or indirectly by relying on the employment of specialized secreted hemophores (heme chelators) and small siderophore molecules (high affinity ferric chelators). This review presents the most recent discoveries in the iron acquisition system of Streptococcus species - the transporters as well as the regulators.

Utilization of host iron sources by Corynebacterium diphtheriae: multiple hemoglobin-binding proteins are essential for the use of iron from the hemoglobin-haptoglobin complex

The use of hemin iron by Corynebacterium diphtheriae requires the DtxR- and iron-regulated ABC hemin transporter HmuTUV and the secreted Hb-binding protein HtaA. We recently described two surface anchored proteins, ChtA and ChtC, which also bind hemin and Hb. ChtA and ChtC share structural similarities to HtaA; however, a function for ChtA and ChtC was not determined. In this study, we identified additional host iron sources that are utilized by C. diphtheriae. We show that several C. diphtheriae strains use the hemoglobin-haptoglobin (Hb-Hp) complex as an iron source. We report that an htaA deletion mutant of C. diphtheriae strain 1737 is unable to use the Hb-Hp complex as an iron source, and we further demonstrate that a chtA-chtC double mutant is also unable to use Hb-Hp iron. Single-deletion mutants of chtA or chtC use Hb-Hp iron in a manner similar to that of the wild type. These findings suggest that both HtaA and either ChtA or ChtC are essential for the use of Hb-Hp iron. Enzyme-linked immunosorbent assay (ELISA) studies show that HtaA binds the Hb-Hp complex, and the substitution of a conserved tyrosine (Y361) for alanine in HtaA results in significantly reduced binding. C. diphtheriae was also able to use human serum albumin (HSA) and myoglobin (Mb) but not hemopexin as iron sources. These studies identify a biological function for the ChtA and ChtC proteins and demonstrate that the use of the Hb-Hp complex as an iron source by C. diphtheriae requires multiple iron-regulated surface components.

The crimson conundrum: heme toxicity and tolerance in GAS

The massive erythrocyte lysis caused by the Group A Streptococcus (GAS) suggests that the β-hemolytic pathogen is likely to encounter free heme during the course of infection. In this study, we investigated GAS mechanisms for heme sensing and tolerance. We compared the minimal inhibitory concentration of heme among several isolates and established that excess heme is bacteriostatic and exposure to sub-lethal concentrations of heme resulted in noticeable damage to membrane lipids and proteins. Pre-exposure of the bacteria to 0.1 μM heme shortened the extended lag period that is otherwise observed when naive cells are inoculated into heme-containing medium, implying that GAS is able to adapt. The global response to heme exposure was determined using microarray analysis revealing a significant transcriptome shift that included 79 up regulated and 84 down regulated genes. Among other changes, the induction of stress-related chaperones and proteases, including groEL/ES (8x), the stress regulators spxA2 (5x) and ctsR (3x), as well as redox active enzymes were prominent. The heme stimulon also encompassed a number of regulatory proteins and two-component systems that are important for virulence. A three-gene cluster that is homologous to the pefRCD system of the Group B Streptococcus was also induced by heme. PefR, a MarR-like regulator, specifically binds heme with stoichiometry of 1:2 and protoporphyrin IX (PPIX) with stoichiometry of 1:1, implicating it is one of the GAS mediators to heme response. In summary, here we provide evidence that heme induces a broad stress response in GAS, and that its success as a pathogen relies on mechanisms for heme sensing, detoxification, and repair.

Molecular and evolutionary analysis of NEAr-iron Transporter (NEAT) domains

Iron is essential for bacterial survival, being required for numerous biological processes. NEAr-iron Transporter (NEAT) domains have been studied in pathogenic Gram-positive bacteria to understand how their proteins obtain heme as an iron source during infection. While a 2002 study initially discovered and annotated the NEAT domain encoded by the genomes of several Gram-positive bacteria, there remains a scarcity of information regarding the conservation and distribution of NEAT domains throughout the bacterial kingdom, and whether these domains are restricted to pathogenic bacteria. This study aims to expand upon initial bioinformatics analysis of predicted NEAT domains, by exploring their evolution and conserved function. This information was used to identify new candidate domains in both pathogenic and nonpathogenic organisms. We also searched metagenomic datasets, specifically sequence from the Human Microbiome Project. Here, we report a comprehensive phylogenetic analysis of 343 NEAT domains, encoded by Gram-positive bacteria, mostly within the phylum Firmicutes, with the exception of Eggerthella sp. (Actinobacteria) and an unclassified Mollicutes bacterium (Tenericutes). No new NEAT sequences were identified in the HMP dataset. We detected specific groups of NEAT domains based on phylogeny of protein sequences, including a cluster of novel clostridial NEAT domains. We also identified environmental and soil organisms that encode putative NEAT proteins. Biochemical analysis of heme binding by a NEAT domain from a protein encoded by the soil-dwelling organism Paenibacillus polymyxa demonstrated that the domain is homologous in function to NEAT domains encoded by pathogenic bacteria. Together, this study provides the first global bioinformatics analysis and phylogenetic evidence that NEAT domains have a strong conservation of function, despite group-specific differences at the amino acid level. These findings will provide information useful for future projects concerning the structure and function of NEAT domains, particularly in pathogens where they have yet to be studied.

Non-heme-binding domains and segments of the Staphylococcus aureus IsdB protein critically contribute to the kinetics and equilibrium of heme acquisition from methemoglobin

The hemoglobin receptor IsdB rapidly acquires heme from methemoglobin (metHb) in the heme acquisition pathway of Staphylococcus aureus. IsdB consists of N-terminal segment (NS), NEAT1 (N1), middle (MD), and heme binding NEAT2 (N2) domains, and C-terminal segment (CS). This study aims to elucidate the roles of these domains or segments in the metHb/IsdB reaction. Deletion of CS does not alter the kinetics and equilibrium of the reaction. Sequential deletions of NS and N1 in NS-N1-MD-N2 progressively reduce heme transfer rates and change the kinetic pattern from one to two phases, but have no effect on the equilibrium of the heme transfer reaction, whereas further deletion of MD reduces the percentage of transferred metHb heme. MD-N2 has higher affinity for heme than N2. MD in trans reduces rates of heme dissociation from holo-N2 and increases the percentage of metHb heme captured by N2 by 4.5 fold. NS-N1-MD and N2, but not NS-N1, MD, and N2, reconstitute the rapid metHb/IsdB reaction. NS-N1-MD-NIsdC, a fusion protein of NS-N1-MD and the NEAT domain of IsdC, slowly acquires heme from metHb by itself but together with N2 results in rapid heme loss from metHb. Thus, NS-N1 and MD domains specifically and critically contribute to the kinetics and equilibrium of the metHb/IsdB reaction, respectively. These findings support a mechanism of direct heme acquisition by IsdB in which MD enhances the affinity of N2 for heme to thermodynamically drive heme transfer from metHb to IsdB and in which NS is required for the rapid and single phase kinetics of the metHb/IsdB reaction.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

A comprehensive analysis of the Streptococcus pyogenes and human plasma protein interaction network

Streptococcus pyogenes is a major human bacterial pathogen responsible for severe and invasive disease associated with high mortality rates. The bacterium interacts with several human blood plasma proteins and clarifying these interactions and their biological consequences will help to explain the progression from mild to severe infections. In this study, we used a combination of mass spectrometry (MS) based techniques to comprehensively quantify the components of the S. pyogenes-plasma protein interaction network. From an initial list of 181 interacting human plasma proteins defined using liquid chromatography (LC)-MS/MS analysis we further subdivided the interacting protein list using selected reaction monitoring (SRM) depending on the level of enrichment and protein concentration on the bacterial surface. The combination of MS methods revealed several previously characterized interactions between the S. pyogenes surface and human plasma along with many more, so far uncharacterised, possible plasma protein interactions with S. pyogenes. In follow-up experiments, the combination of MS techniques was applied to study differences in protein binding to a S. pyogenes wild type strain and an isogenic mutant lacking several important virulence factors, and a unique pair of invasive and non-invasive S. pyogenes isolates from the same patient. Comparing the plasma protein-binding properties of the wild type and the mutant and the invasive and non-invasive S. pyogenes bacteria revealed considerable differences, underlining the significance of these protein interactions. The results also demonstrate the power of the developed mass spectrometry method to investigate host-microbial relationships with a large proteomics depth and high quantitative accuracy.

Shared and distinct mechanisms of iron acquisition by bacterial and fungal pathogens of humans

Iron is the most abundant transition metal in the human body and its bioavailability is stringently controlled. In particular, iron is tightly bound to host proteins such as transferrin to maintain homeostasis, to limit potential damage caused by iron toxicity under physiological conditions and to restrict access by pathogens. Therefore, iron acquisition during infection of a human host is a challenge that must be surmounted by every successful pathogenic microorganism. Iron is essential for bacterial and fungal physiological processes such as DNA replication, transcription, metabolism, and energy generation via respiration. Hence, pathogenic bacteria and fungi have developed sophisticated strategies to gain access to iron from host sources. Indeed, siderophore production and transport, iron acquisition from heme and host iron-containing proteins such as hemoglobin and transferrin, and reduction of ferric to ferrous iron with subsequent transport are all strategies found in bacterial and fungal pathogens of humans. This review focuses on a comparison of these strategies between bacterial and fungal pathogens in the context of virulence and the iron limitation that occurs in the human body as a mechanism of innate nutritional defense.

Axial ligand replacement mechanism in heme transfer from streptococcal heme-binding protein Shp to HtsA of the HtsABC transporter

The heme-binding protein Shp of Group A Streptococcus rapidly transfers its heme to HtsA, the lipoprotein component of the HtsABC transporter, in a concerted two-step process with one kinetic phase. Heme axial residue-to-alanine replacement mutant proteins of Shp and HtsA (Shp(M66A), Shp(M153A), HtsA(M79A), and HtsA(H229A)) were used to probe the axial displacement mechanism of this heme transfer reaction. Ferric Shp(M66A) at high pH and Shp(M153A) have a pentacoordinate heme iron complex with a methionine axial ligand. ApoHtsA(M79A) efficiently acquires heme from ferric Shp but alters the reaction mechanism to two kinetic phases from a single phase in the wild-type protein reactions. In contrast, apoHtsA(H229A) cannot assimilate heme from ferric Shp. The conversion of pentacoordinate holoShp(M66A) into pentacoordinate holoHtsA(H229A) involves an intermediate, whereas holoHtsA(H229A) is directly formed from pentacoordinate holoShp(M153A). Conversely, apoHtsA(M79A) reacts with holoShp(M66A) and holoShp(M153A) in mechanisms with one and two kinetic phases, respectively. These results imply that the Met79 and His229 residues of HtsA displace the Met66 and Met153 residues of Shp, respectively. Structural docking analysis supports this mechanism of the specific axial residue displacement. Furthermore, the rates of the cleavage of the axial bond in Shp in the presence of a replacing HtsA axial residue are greater than that in the absence of a replacing HtsA axial residue. These findings reveal a novel heme transfer mechanism of the specific displacement of the Shp axial residues with the HtsA axial residues and the involvement of the HtsA axial residues in the displacement.

Kinetics of heme transfer by the Shr NEAT domains of Group A Streptococcus

The hemolytic Group A Streptococcus (GAS) is a notorious human pathogen. Shr protein of GAS participates in iron acquisition by obtaining heme from host hemoglobin and delivering it to the adjacent receptor on the surface, Shp. Heme is then conveyed to the SiaABC proteins for transport across the membrane. Using rapid kinetic studies, we investigated the role of the two heme binding NEAT modules of Shr. Stopped-flow analysis showed that holoNEAT1 quickly delivered heme to apoShp. HoloNEAT2 did not exhibit such activity; only little and slow transfer of heme from NEAT2 to apoShp was seen, suggesting that Shr NEAT domains have distinctive roles in heme transport. HoloNEAT1 also provided heme to apoNEAT2, by a fast and reversible process. To the best of our knowledge this is the first transfer observed between isolated NEAT domains of the same receptor. Sequence alignment revealed that Shr NEAT domains belong to two families of NEAT domains that are conserved in Shr orthologs from several species. Based on the heme transfer kinetics, we propose that Shr proteins modulate heme uptake according to heme availability by a mechanism where NEAT1 facilitates fast heme delivery to Shp, whereas NEAT2 serves as a temporary storage for heme on the bacterial surface.

Sequestration and scavenging of iron in infection

The proliferative capability of many invasive pathogens is limited by the bioavailability of iron. Pathogens have thus developed strategies to obtain iron from their host organisms. In turn, host defense strategies have evolved to sequester iron from invasive pathogens. This review explores the mechanisms employed by bacterial pathogens to gain access to host iron sources, the role of iron in bacterial virulence, and iron-related genes required for the establishment or maintenance of infection. Host defenses to limit iron availability for bacterial growth during the acute-phase response and the consequences of iron overload conditions on susceptibility to bacterial infection are also examined. The evidence summarized herein demonstrates the importance of iron bioavailability in influencing the risk of infection and the ability of the host to clear the pathogen.

Environmental heme utilization by heme-auxotrophic bacteria

Heme, an iron-containing porphyrin, is the prosthetic group for numerous key cellular enzymatic and regulatory processes. Many bacteria encode the biosynthetic enzymes needed for autonomous heme production. Remarkably, however, numerous other bacteria lack a complete heme biosynthesis pathway, yet encode heme-requiring functions. For such heme-auxotrophic bacteria (HAB), heme or porphyrins must be captured from the environment. Functional studies, aided by genomic analyses, provide insight into the HAB lifestyle, how they acquire and manage heme, and the uses of heme that make it worthwhile, and sometimes necessary, to capture this bioactive molecule.

Heme uptake and metabolism in bacteria

All but a few bacterial species have an absolute need for heme, and most are able to synthesize it via a pathway that is highly conserved among all life domains. Because heme is a rich source for iron, many pathogenic bacteria have also evolved processes for sequestering heme from their hosts. The heme biosynthesis pathways are well understood at the genetic and structural biology levels. In comparison, much less is known about the heme acquisition, trafficking, and degradation processes in bacteria. Gram-positive and Gram-negative bacteria have evolved similar strategies but different tactics for importing and degrading heme, likely as a consequence of their different cellular architectures. The differences are manifested in distinct structures for molecules that perform similar functions. Consequently, the aim of this chapter is to provide an overview of the structural biology of proteins and protein-protein interactions that enable Gram-positive and Gram-negative bacteria to sequester heme from the extracellular milieu, import it to the cytosol, and degrade it to mine iron.

Heme proteins in lactic acid bacteria

Lactic acid bacteria (LAB) are of profound importance in food production and infection medicine. LAB do not rely on heme (protoheme IX) for growth and are unable to synthesize this cofactor but are generally able to assemble a small repertoire of heme-containing proteins if heme is provided from an exogenous source. These features are in contrast to other bacteria, which synthesize their heme or depend on heme for growth. We here present the cellular function of heme proteins so far identified in LAB and discuss their biogenesis as well as applications of the extraordinary heme physiology of LAB.

The streptococcal hemoprotein receptor: a moonlighting protein or a virulence factor?

The β-hemolytic group A streptococcus (GAS) is a major pathogen that readily uses hemoglobin to satisfy its requirements for iron. The streptococcal hemoprotein receptor in GAS plays a central role in heme utilization and binds fibronectin and laminin in vitro. Shr inactivation attenuates the virulent M1T1 GAS strain in two murine infection models and reduces bacterial growth in blood and binding to laminin. Shr impact on the globally disseminated M1T1 strain underscores the importance of heme uptake in GAS pathogenesis and raises the possibility of targeting heme-uptake proteins in the development of new methods to combat GAS infections.

Study of streptococcal hemoprotein receptor (Shr) in iron acquisition and virulence of M1T1 group A streptococcus

Streptococcus pyogenes (group A streptococcus, GAS) is a human bacterial pathogen of global significance, causing severe invasive diseases associated with serious morbidity and mortality. To survive within the host and establish an infection, GAS requires essential nutrients, including iron. The streptococcal hemoprotein receptor (Shr) is a surface-localized GAS protein that binds heme-containing proteins and extracellular matrix components. In this study, we employ targeted allelic exchange mutagenesis to investigate the role of Shr in the pathogenesis of the globally disseminated serotype M1T1 GAS. The shr mutant exhibited a growth defect in iron-restricted medium supplemented with ferric chloride, but no significant differences were observed in neutrophil survival, antimicrobial peptide resistance, cell surface charge, fibronectin-binding or adherence to human epithelial cells and keratinocytes, compared with wild-type. However, the shr mutant displayed a reduction in human blood proliferation, laminin-binding capacity and was attenuated for virulence in in vivo models of skin and systemic infection. We conclude that Shr augments GAS adherence to laminin, an important extracellular matrix attachment component. Furthermore, Shr-mediated iron uptake contributes to GAS growth in human blood, and is required for full virulence of serotype M1T1 GAS in mouse models of invasive disease.

The role of ATP-binding cassette transporters in bacterial pathogenicity

The ATP-binding cassette transporter superfamily is present in all three domains of life. This ubiquitous class of integral membrane proteins have diverse biological functions, but their fundamental role involves the unidirectional translocation of compounds across cellular membranes in an ATP coupled process. The importance of this class of proteins in eukaryotic systems is well established as typified by their association with genetic diseases and roles in the multi-drug resistance of cancer. In stark contrast, the ABC transporters of prokaryotes have not been exhaustively investigated due to the sheer number of different roles and organisms in which they function. In this review, we examine the breadth of functions associated with microbial ABC transporters in the context of their contribution to bacterial pathogenicity and virulence.

The influence of iron availability on human salivary microbial community composition

It is a well-recognized fact that the composition of human salivary microbial community is greatly affected by its nutritional environment. However, most studies are currently focused on major carbon or nitrogen sources with limited attention to trace elements like essential mineral ions. In this study, we examined the effect of iron availability on the bacterial profiles of an in vitro human salivary microbial community as iron is an essential trace element for the survival and proliferation of virtually all microorganisms. Analysis via a combination of PCR with denaturing gradient gel electrophoresis demonstrated a drastic change in species composition of an in vitro human salivary microbiota when iron was scavenged from the culture medium by addition of the iron chelator 2,2'-bipyridyl. This shift in community profile was prevented by the presence of excessive ferrous iron (Fe(2+)). Most interestingly, under iron deficiency, the in vitro grown salivary microbial community became dominated by several hemolytic bacterial species, including Streptococcus spp., Gemella spp., and Granulicatella spp. all of which have been implicated in infective endocarditis. These data provide evidence that iron availability can modulate host-associated oral microbial communities, resulting in a microbiota with potential clinical impact.

Direct heme transfer reactions in the Group A Streptococcus heme acquisition pathway

The heme acquisition machinery in Group A Streptococcus (GAS) consists of the surface proteins Shr and Shp and ATP-binding cassette transporter HtsABC. Shp cannot directly acquire heme from methemoglobin (metHb) but directly transfers its heme to HtsA. It has not been previously determined whether Shr directly relays heme from metHb to Shp. Thus, the complete pathway for heme acquisition from metHb by the GAS heme acquisition machinery has remained unclear. In this study, the metHb-to-Shr and Shr-to-Shp heme transfer reactions were characterized by spectroscopy, kinetics and protein-protein interaction analyses. Heme is efficiently transferred from the β and α subunits of metHb to Shr with rates that are 7 and 60 times greater than those of the passive heme release from metHb, indicating that Shr directly acquires heme from metHb. The rapid heme transfer from Shr to Shp involves an initial heme donor/acceptor complex and a spectrally and kinetically detectable transfer intermediate, implying that heme is directly channeled from Shr to Shp. The present results show that Shr speeds up heme transfer from metHb to Shp, whereas Shp speeds up heme transfer from Shr to HtsA. Furthermore, the findings demonstrate that Shr can interact with metHb and Shp but not HtsA. Taken together with our published results on the Shp/HtsA reaction, these findings establish a model of the heme acquisition pathway in GAS in which Shr directly extracts heme from metHb and Shp relays it from Shr to HtsA.

Identification of iron-regulated genes of Bifidobacterium breve UCC2003 as a basis for controlled gene expression

Iron is an essential growth factor for virtually all organisms. However, iron is not readily available in most environments and microorganisms have evolved specialized mechanisms, such as the use of siderophores and high-affinity transport systems, to acquire iron when confronted with iron-limiting conditions. In general these systems are tightly regulated to prevent iron-induced toxicity and because they are quite costly to the microbe. Because of this tight regulation we chose to explore the response of Bifidobacterium breve UCC2003 to iron limitation. Through microarray and complementation analyses we identified and characterized a presumed ferrous iron uptake system, encoded by bfeUOB, from B. breve UCC2003 and exploited its regulated transcription to develop an inducible expression system for use in bifidobacteria.

Group a streptococcal vaccine candidates: potential for the development of a human vaccine

Currently there is no commercial Group A Streptococcus (GAS; S. pyogenes) vaccine available. The development of safe GAS vaccines is challenging, researchers are confronted with obstacles such as the occurrence of many unique serotypes (there are greater than 150 M types), antigenic variation within the same serotype, large variations in the geographical distribution of serotypes, and the production of antibodies cross-reactive with human tissue which can lead to host auto-immune disease. Cell wall anchored, cell membrane associated, secreted and anchorless proteins have all been targeted as GAS vaccine candidates. As GAS is an exclusively human pathogen, the quest for an efficacious vaccine is further complicated by the lack of an animal model which mimics human disease and can be consistently and reproducibly colonized by multiple GAS strains.

Extracellular heme uptake and the challenges of bacterial cell membranes

In bacteria, the fine balance of maintaining adequate iron levels while preventing the deleterious effects of excess iron has led to the evolution of sophisticated cellular mechanisms to obtain, store, and regulate iron. Iron uptake provides a significant challenge given its limited bioavailability and need to be transported across the bacterial cell wall and membranes. Pathogenic bacteria have circumvented the iron-availability issue by utilizing the hosts' heme-containing proteins as a source of iron. Once internalized, iron is liberated from the porphyrin enzymatically for cellular processes within the bacterial cell. Heme, a lipophilic and toxic molecule, poses a significant challenge in terms of transport given its chemical reactivity. As such, pathogenic bacteria have evolved sophisticated membrane transporters to coordinate, sequester, and transport heme. Recent advances in the biochemical and structural characterization of the membrane-bound heme transport proteins are discussed in the context of ligand coordination, protein-protein interaction, and heme transfer.

Structural basis for hemoglobin capture by Staphylococcus aureus cell-surface protein, IsdH

Pathogens must steal iron from their hosts to establish infection. In mammals, hemoglobin (Hb) represents the largest reservoir of iron, and pathogens express Hb-binding proteins to access this source. Here, we show how one of the commonest and most significant human pathogens, Staphylococcus aureus, captures Hb as the first step of an iron-scavenging pathway. The x-ray crystal structure of Hb bound to a domain from the Isd (iron-regulated surface determinant) protein, IsdH, is the first structure of a Hb capture complex to be determined. Surface mutations in Hb that reduce binding to the Hb-receptor limit the capacity of S. aureus to utilize Hb as an iron source, suggesting that Hb sequence is a factor in host susceptibility to infection. The demonstration that pathogens make highly specific recognition complexes with Hb raises the possibility of developing inhibitors of Hb binding as antibacterial agents.

Peroxide stimulon and role of PerR in group A Streptococcus

We have characterized group A Streptococcus (GAS) genome-wide responses to hydrogen peroxide and assessed the role of the peroxide response regulator (PerR) in GAS under oxidative stress. Comparison of transcriptome changes elicited by peroxide in wild-type bacteria with those in a perR deletion mutant showed that 76 out of 237 peroxide-regulated genes are PerR dependent. Unlike the PerR-mediated upregulation of peroxidases and other peroxide stress defense mechanisms previously reported in gram-positive species, PerR-dependent genes in GAS were almost exclusively downregulated and encoded proteins involved in purine and deoxyribonucleotide biosynthesis, heme uptake, and amino acid/peptide transport, but they also included a strongly activated putative transcriptional regulator (SPy1198). Of the 161 PerR-independent loci, repressed genes (86 of 161) encoded proteins with functions similar to those coordinated by PerR, in contrast to upregulated loci that encoded proteins that function in DNA damage repair, cofactor metabolism, reactive oxygen species detoxification, pilus biosynthesis, and hypothetical proteins. Complementation of the perR deletion mutant with wild-type PerR restored PerR-dependent regulation, whereas complementation with either one of two PerR variants carrying single mutations in two predicted metal-binding sites did not rescue the mutant phenotype. Metal content analyses of the recombinant wild type and respective PerR mutants, in addition to regulation studies in metal-supplemented and iron-depleted media, showed binding of zinc and iron by PerR and an iron requirement for optimal responses to peroxide. Our findings reveal a novel physiological contribution of PerR in coordinating DNA and protein metabolic functions in peroxide and identify GAS adaptive responses that may serve to enhance oxidative stress resistance and virulence in the host.

Defense from the Group A Streptococcus by active and passive vaccination with the streptococcal hemoprotein receptor

BACKGROUND

The worldwide burden of the Group A Streptococcus (GAS) primary infection and sequelae is considerable, although immunization programs with broad coverage of the hyper variable GAS are still missing. We evaluate the streptococcal hemoprotein receptor (Shr), a conserved streptococcal protein, as a vaccine candidate against GAS infection.

METHODS

Mice were immunized intraperitoneally with purified Shr or intranasally with Shr-expressing Lactococcus lactis. The resulting humoral response in serum and secretions was determined. We evaluated protection from GAS infection in mice after active or passive vaccination with Shr, and Shr antiserum was tested for bactericidal activity.

RESULTS

A robust Shr-specific immunoglobulin (Ig) G response was observed in mouse serum after intraperitoneal vaccination with Shr. Intranasal immunization elicited both a strong IgG reaction in the serum and a specific IgA reaction in secretions. Shr immunization in both models allowed enhanced protection from systemic GAS challenge. Rabbit Shr antiserum was opsonizing, and mice that were administrated with Shr antiserum prior to the infection demonstrated a significantly higher survival rate than did mice treated with normal rabbit serum.

CONCLUSIONS

Shr is a promising vaccine candidate that is capable of eliciting bactericidal antibody response and conferring immunity against systemic GAS infection in both passive and active vaccination models.

Streptolysin S-like virulence factors: the continuing sagA

Streptolysin S (SLS) is a potent cytolytic toxin and virulence factor that is produced by nearly all Streptococcus pyogenes strains. Despite a 100-year history of research on this toxin, it has only recently been established that SLS is just one of an extended family of post-translationally modified virulence factors (the SLS-like peptides) that are produced by some streptococci and other Gram-positive pathogens, such as Listeria monocytogenes and Clostridium botulinum. In this Review, we describe the identification, genetics, biochemistry and various functions of SLS. We also discuss the shared features of the virulence-associated SLS-like peptides, as well as their place within the rapidly expanding family of thiazole/oxazole-modified microcins (TOMMs).

Novel hemin binding domains in the Corynebacterium diphtheriae HtaA protein interact with hemoglobin and are critical for heme iron utilization by HtaA

The human pathogen Corynebacterium diphtheriae utilizes hemin and hemoglobin as iron sources for growth in iron-depleted environments. The use of hemin iron in C. diphtheriae involves the dtxR- and iron-regulated hmu hemin uptake locus, which encodes an ABC hemin transporter, and the surface-anchored hemin binding proteins HtaA and HtaB. Sequence analysis of HtaA and HtaB identified a conserved region (CR) of approximately 150 amino acids that is duplicated in HtaA and present in a single copy in HtaB. The two conserved regions in HtaA, designated CR1 and CR2, were used to construct glutathione S-transferase (GST) fusion proteins (GST-CR1 and GST-CR2) to assess hemin binding by UV-visual spectroscopy. These studies showed that both domains were able to bind hemin, suggesting that the conserved sequences are responsible for the hemin binding property previously ascribed to HtaA. HtaA and the CR2 domain were also shown to be able to bind hemoglobin (Hb) by the use of an enzyme-linked immunosorbent assay (ELISA) method in which Hb was immobilized on a microtiter plate. The CR1 domain exhibited a weak interaction with Hb in the ELISA system, while HtaB showed no significant binding to Hb. Competitive binding studies demonstrated that soluble hemin and Hb were able to inhibit the binding of HtaA and the CR domains to immobilized Hb. Moreover, HtaA was unable to bind to Hb from which the hemin had been chemically removed. Alignment of the amino acid sequences of CR domains from various Corynebacterium species revealed several conserved residues, including two highly conserved tyrosine (Y) residues and one histidine (H) residue. Site-directed mutagenesis studies showed that Y361 and H412 were critical for the binding to hemin and Hb by the CR2 domain. Biological assays showed that Y361 was essential for the hemin iron utilization function of HtaA. Hemin transfer experiments demonstrated that HtaA was able to acquire hemin from Hb and that hemin bound to HtaA could be transferred to HtaB. These findings are consistent with a proposed mechanism of hemin uptake in C. diphtheriae in which hemin is initially obtained from Hb by HtaA and then transferred between surface-anchored proteins, with hemin ultimately transported into the cytosol by an ABC transporter.

The theft of host heme by Gram-positive pathogenic bacteria

The element iron is essential for bacteria and plays a key role in the virulence and pathology of bacterial diseases. The largest reservoir of iron within the human body is in protoporphyrin IX, the compound commonly referred to as heme and bound by hemoglobin. For many years, the study of heme uptake in bacteria was restricted to Gram-negative organisms. However, recent studies have shed light on how bacteria containing a thick peptidoglycan, such as Gram-positive bacteria, acquire and transport heme. This review summarizes old and new research covering the acquisition, transport, and utilization of heme in Gram-positive bacterial pathogens.

Control of heme homeostasis in Corynebacterium glutamicum by the two-component system HrrSA

The response regulator HrrA of the HrrSA two-component system (previously named CgtSR11) was recently found to be repressed by the global iron-dependent regulator DtxR in Corynebacterium glutamicum. Here, we provide evidence that HrrA mediates heme-dependent gene regulation in this nonpathogenic soil bacterium. Growth experiments and DNA microarray analysis revealed that C. glutamicum is able to use hemin as an alternative iron source and emphasize the involvement of the putative hemin ABC transporter HmuTUV and heme oxygenase (HmuO) in heme utilization. As a central part of this study, we investigated the regulon of the response regulator HrrA via comparative transcriptome analysis of an hrrA deletion mutant and C. glutamicum wild-type strain in combination with DNA-protein interaction studies with purified HrrA protein. Our data provide evidence for a heme-dependent transcriptional activation of heme oxygenase. Based on our results, it can be furthermore deduced that HrrA activates the expression of heme-containing components of the respiratory chain, namely, ctaD and the ctaE-qcrCAB operon encoding subunits I and III of cytochrome aa(3) oxidase and three subunits of the cytochrome bc(1) complex. In addition, HrrA was found to repress almost all genes involved in heme biosynthesis, including those for glutamyl-tRNA reductase (hemA), uroporphyrinogen decarboxylase (hemE), and ferrochelatase (hemH). Growth experiments with an hrrA deletion mutant showed that this strain is significantly impaired in heme utilization. In summary, our results provide evidence for a central role of the HrrSA system in the control of heme homeostasis in C. glutamicum.

Shr of group A streptococcus is a new type of composite NEAT protein involved in sequestering haem from methaemoglobin

A growing body of evidence suggests that surface or secreted proteins with NEAr Transporter (NEAT) domains play a central role in haem acquisition and trafficking across the cell envelope of Gram-positive bacteria. Group A streptococcus (GAS), a β-haemolytic human pathogen, expresses a NEAT protein, Shr, which binds several haemoproteins and extracellular matrix (ECM) components. Shr is a complex, membrane-anchored protein, with a unique N-terminal domain (NTD) and two NEAT domains separated by a central leucine-rich repeat region. In this study we have carried out an analysis of the functional domains in Shr. We show that Shr obtains haem in solution and furthermore reduces the haem iron; this is the first report of haem reduction by a NEAT protein. More specifically, we demonstrate that both of the constituent NEAT domains of Shr are responsible for binding haem, although they are missing a critical tyrosine residue found in the ligand-binding pocket of other haem-binding NEAT domains. Further investigations show that a previously undescribed region within the Shr NTD interacts with methaemoglobin. Shr NEAT domains, however, do not contribute significantly to the binding of methaemoglobin but mediate binding to the ECM components fibronectin and laminin. A protein fragment containing the NTD plus the first NEAT domain was found to be sufficient to sequester haem directly from methaemoglobin. Correlating these in vitro findings to in vivo biological function, mutants analysis establishes the role of Shr in GAS growth with methaemoglobin as a sole source of iron, and indicates that at least one NEAT domain is necessary for the utilization of methaemoglobin. We suggest that Shr is the prototype of a new group of NEAT composite proteins involved in haem uptake found in pyogenic streptococci and Clostridium novyi.

Haptoglobin polymorphism and infection

The haptoglobin gene is highly polymorphic in humans with strong evidence of functionally distinct biochemical phenotypes. In all human populations, three major haptoglobin phenotypes Hp 1-1, Hp 2-1, and Hp 2-2 are present, but additional phenotypes have been identified. Haptoglobin polymorphism has important biological and clinical significance. In this review, we examine the putative role of haptoglobin polymorphism in parasitic, bacterial, and viral infections. Despite many striking effects of haptoglobin polymorphism in infectious conditions, the effects of haptoglobin genetic variation upon infections are not always predictable due to the multifunctional character of the plasma protein (e.g., antibody-like properties, immunomodulation, iron metabolism). More studies on the interplay of haptoglobin polymorphism, vaccination, and susceptibility or resistance to common infections seem warranted.

Characterization of the haem-uptake system of the equine pathogen Streptococcus equi subsp. equi

Streptococcus equi possesses a haem-uptake system homologous to that of Streptococcus pyogenes and Streptococcus zooepidemicus. The system consists of two ligand-binding proteins (Shr and Shp) and proteins (HtsA–C) with homology to an ABC transporter. The haem-uptake system of S. equi differs from that of S. pyogenes and S. zooepidemicus in that Shr is truncated by two-thirds. This study focused on the SeShr, SeShp and SeHtsA proteins of S. equi. Analysis of shr, shp and shphtsA knockout mutants showed that all three proteins were expressed in vitro and that expression was upregulated under conditions of iron limitation. SeShr possesses no membrane-/cell wall-spanning sequences and was shown to be secreted. Both SeShp and SeHtsA were confirmed to be envelope-associated. Recombinant SeShp and SeHtsA proteins have been previously shown to bind haem and SeHtsA could capture haem from SeShp. This report extends these studies and shows that both SeShp and SeHtsA can sequester haem from haemoglobin but not from haemoglobin–haptoglobin complexes. Like full-length Shr, SeShr possesses haemoglobin and haemoglobin–haptoglobin binding ability but unlike full-length Shr, it lacks haem- or fibronectin-binding capabilities. Analysis of SeShr truncates showed that residues within and upstream of the near transporter (NEAT) domain are required for this ligand binding. Structural predictions suggest that truncation of NEAT1 in SeShr accounts for its impaired ability to bind haem. Haem and haemoglobin restored to almost normal the impaired growth rates of wild-type S. equi cultured under iron-limiting conditions. However, no difference in the growth rates of wild-type and mutants could be detected under the in vitro growth conditions tested.

MtsR is a dual regulator that controls virulence genes and metabolic functions in addition to metal homeostasis in the group A streptococcus

MtsR is a metal-dependent regulator in the group A streptococcus (GAS) that directly represses the transcription of genes involved in haem and metal uptake. While MtsR has been implicated in GAS virulence, the DNA recognition and full regulatory scope exerted by the protein are unknown. In this study we identified the shr promoter (P(shr)) and mapped MtsR binding to a 69 bp segment in P(shr) that overlaps the core promoter elements. A global transcriptional analysis demonstrated that MtsR modulates the expression of 64 genes in GAS, 44 of which were upregulated and 20 were downregulated in the mtsR mutant. MtsR controls genes with diverse functions including metal homeostasis, nucleic acid and amino acid metabolism, and protein fate. Importantly, the MtsR regulon includes mga, emm49 and ska, which are central for GAS pathogenesis. MtsR binding to the promoter region of both negatively and positively regulated genes demonstrates that it functions as a dual regulator. MtsR footprints are large (47-130 bp) and vary between target promoters. A 16 bp motif that consists of an interrupted palindrome is implicated in the DNA recognition by the metalloregulator. In conclusion, we report here that MtsR is a global regulator in GAS that shapes the expression of vital virulence factors and genes involved in metabolic functions and metal transport, and we discuss the implications for the GAS disease process.

Spectroscopic identification of heme axial ligands in HtsA that are involved in heme acquisition by Streptococcus pyogenes

The heme-binding proteins Shp and HtsA of Streptococcus pyogenes are part of the heme acquisition machinery in which Shp directly transfers its heme to HtsA. Mutagenesis and spectroscopic analyses were performed to identify the heme axial ligands in HtsA and to characterize axial mutants of HtsA. Replacements of the M79 and H229 residues, not the other methionine and histidine residues, with alanine convert UV-vis spectra of HtsA with a low-spin, hexacoordinate heme iron into spectra of high-spin heme complexes. Ferrous M79A and H229A HtsA mutants possess magnetic circular dichroism (MCD) spectra that are similar with those of proteins with pentacoordinate heme iron. Ferric M79A HtsA displays UV-vis, MCD, and resonance Raman (RR) spectra that are typical of a hexacoordinate heme iron with histidine and water ligands. In contrast, ferric H229A HtsA has UV-vis, MCD, and RR spectra that represent a pentacoordinate heme iron complex with a methionine axial ligand. Imidazole readily forms a low-spin hexacoordinate adduct with M79A HtsA with a K(d) of 40.9 muM but not with H229A HtsA, and cyanide binds to M79A and H229A with K(d) of 0.5 and 19.1 microM, respectively. The ferrous mutants rapidly bind CO and form simple CO complexes. These results establish the H229 and M79 residues as the axial ligands of the HtsA heme iron, indicate that the M79 side is more accessible to the solvent than the H229 side of the bound heme in HtsA, and provide unique spectral features for a protein with pentacoordinate, methionine-ligated heme iron. These findings will facilitate elucidation of the molecular mechanism and structural basis for rapid and direct heme transfer from Shp to HtsA.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

The metal homeostasis protein, Lsp, of Streptococcus pyogenes is necessary for acquisition of zinc and virulence

"Cluster 9" family lipoproteins function as ligand-binding subunits of ABC-type transporters in maintaining transition metal homeostasis and have been implicated in the virulence of several bacteria. While these proteins share high similarity, the specific metal that they recognize and whether their role in virulence directly involves metal homeostasis cannot be reliably predicted. We examined the cluster 9 protein Lsp of Streptococcus pyogenes and found that specific deletion of lsp produced mutants highly attenuated in a murine model of soft tissue infection. Under standard in vitro conditions, growth of the Lsp(-) mutant was indistinguishable from that of the wild type, but growth was defective under zinc-limited conditions. The growth defect could be complemented by plasmids expressing wild-type Lsp but not Lsp engineered to lack its putative lipidation residue. Furthermore, Zn(2+) but not Mn(2+) rescued Lsp(-) growth, implicating Zn(2+) as the physiological ligand for Lsp. Mutation of residues in the putative Zn(2+)-binding pocket generated variants both hypo- and hyper-resistant to zinc starvation, and both mutant classes displayed attenuated virulence. Together, these data suggest that Lsp is a ligand-binding component of an ABC-type zinc permease and that perturbation of zinc homeostasis inhibits the ability of S. pyogenes to cause disease in a zinc-limited host milieu.

HtaA is an iron-regulated hemin binding protein involved in the utilization of heme iron in Corynebacterium diphtheriae

Many human pathogens, including Corynebacterium diphtheriae, the causative agent of diphtheria, use host compounds such as heme and hemoglobin as essential iron sources. In this study, we examined the Corynebacterium hmu hemin transport region, a genetic cluster that contains the hmuTUV genes encoding a previously described ABC-type hemin transporter and three additional genes, which we have designated htaA, htaB, and htaC. The hmu gene cluster is composed of three distinct transcriptional units. The htaA gene appears to be part of an iron- and DtxR-regulated operon that includes hmuTUV, while htaB and htaC are transcribed from unique DtxR-regulated promoters. Nonpolar deletion of either htaA or the hmuTUV genes resulted in a reduced ability to use hemin as an iron source, while deletion of htaB had no effect on hemin iron utilization in C. diphtheriae. A comparison of the predicted amino acid sequences of HtaA and HtaB showed that they share some sequence similarity, and both proteins contain leader sequences and putative C-terminal transmembrane regions. Protein localization studies with C. diphtheriae showed that HtaA is associated predominantly with the cell envelope when the organism is grown in minimal medium but is secreted during growth in nutrient-rich broth. HtaB and HmuT were detected primarily in the cytoplasmic membrane fraction regardless of the growth medium. Hemin binding studies demonstrated that HtaA and HtaB are able to bind hemin, suggesting that these proteins may function as cell surface hemin receptors in C. diphtheriae.

Bacterial heme-transport proteins and their heme-coordination modes

Efficient iron acquisition is critical for an invading microbe's survival and virulence. Most of the iron in mammals is incorporated into heme, which can be plundered by certain bacterial pathogens as a nutritional iron source. Utilization of exogenous heme by bacteria involves the binding of heme or hemoproteins to the cell surface receptors, followed by the transport of heme into cells. Once taken into the cytosol, heme is presented to heme oxygenases where the tetrapyrrole ring is cleaved in order to release the iron. Some Gram-negative bacteria also secrete extracellular heme-binding proteins called hemophores, which function to sequester heme from the environment. The heme-transport genes are often genetically linked as gene clusters under Fur (ferric uptake regulator) regulation. This review discusses the gene clusters and proteins involved in bacterial heme acquisition, transport and processing processes, with special focus on the heme-coordination, protein structures and mechanisms underlying heme-transport.

Streptococcus pyogenes CovRS mediates growth in iron starvation and in the presence of the human cationic antimicrobial peptide LL-37

We found that the global regulatory two-component signal transduction system CovRS mediates the ability of group A streptococcus (GAS) to grow under two stresses encountered during infection: iron starvation and the presence of LL-37. We also showed that CovRS regulates transcription of the multimetal transporter operon that is important for GAS growth in a low concentration of iron.

Regulation of sagA, siaA and scpC by SilCR, a putative signaling peptide of Streptococcus pyogenes

SilCR, a 17 amino acid putative signaling peptide, was proposed to modulate gene expression in Streptococcus pyogenes. We showed that SilCR added exogenously to an M1 serotype strain lacking the sil locus upregulates the in vitro expression of sagA, siaA, and scpC, genes associated with S. pyogenes pathogenesis. Interestingly, only sagA and siaA were upregulated by SilCR in vivo, whereas the expression of scpC remained unaltered. A previous report indicated that exogenously added SilCR protects mice to some degree from developing necrotic lesions caused by an invasive strain of S. pyogenes. In contrast to this report, we found that SilCR did not reduce lesion formation in a subcutaneous murine model of S. pyogenes infection but rather appeared to delay wound healing.

Characterization of Bacillus anthracis iron-regulated surface determinant (Isd) proteins containing NEAT domains

Three iron-regulated surface determinant (Isd) proteins, containing NEAr Transporter (NEAT) domains (GBAA4789-7), constitute part of an eight-member Bacillus anthracis operon. GBAA4789 (IsdC), previously characterized by others as a haem-binding protein, and two novel Isd proteins characterized in this study, GBAA4788 (IsdJ) and GBAA4787 (IsdK) proteins, can be translated from two alternative overlapping transcriptional units. The three NEAT-containing Isd proteins are shown to be expressed in vivo during B. anthracis infection. Expression in vitro is regulated by iron ions independent of the virulence plasmids pXO1 and pXO2, yet their presence affects the range of response to iron ion concentration. The expression of IsdC, J and K is strongly repressed under high CO(2) tension, conditions that are optimal for B. anthracis toxin and capsule expression, suggesting that these Isd proteins are elements of a B. anthracis'air-regulon'. Deletion mutants of isdC, isdK or the entire isdCJK locus are as virulent and pathogenic to guinea pigs as the fully virulent wild-type Vollum strain. The isdC-deleted mutant is defective in sequestration of haemin, consistent with previous biochemical observations, while the DeltaisdK mutant is defective in haemoglobin uptake. Studies with recombinant IsdK demonstrate specific binding to haemoglobin.