Metal Homeostasis in Pathogenic Streptococci

Streptococcus spp. are an important genus of Gram-positive bacteria, many of which are opportunistic pathogens that are capable of causing invasive disease in a wide range of populations. Metals, especially transition metal ions, are an essential nutrient for all organisms. Therefore, to survive across dynamic host environments, Streptococci have evolved complex systems to withstand metal stress and maintain metal homeostasis, especially during colonization and infection. There are many different types of transport systems that are used by bacteria to import or export metals that can be highly specific or promiscuous. Focusing on the most well studied transition metals of zinc, manganese, iron, nickel, and copper, this review aims to summarize the current knowledge of metal homeostasis in pathogenic Streptococci, and their role in virulence.

Heme Uptake in Lactobacillus sakei Evidenced by a New Energy Coupling Factor (ECF)-Like Transport System

Lactobacillus sakei is a nonpathogenic lactic acid bacterium and a natural inhabitant of meat ecosystems. Although red meat is a heme-rich environment, L. sakei does not need iron or heme for growth, although it possesses a heme-dependent catalase. Iron incorporation into L. sakei from myoglobin and hemoglobin was previously shown by microscopy and the L. sakei genome reveals the complete equipment for iron and heme transport. Here, we report the characterization of a five-gene cluster (from lsa1836 to lsa1840 [lsa1836-1840]) encoding a putative metal iron ABC transporter. Interestingly, this cluster, together with a heme-dependent catalase gene, is also conserved in other species from the meat ecosystem. Our bioinformatic analyses revealed that the locus might correspond to a complete machinery of an energy coupling factor (ECF) transport system. We quantified in vitro the intracellular heme in the wild type (WT) and in our Δlsa1836-1840 deletion mutant using an intracellular heme sensor and inductively coupled plasma mass spectrometry for quantifying incorporated 57Fe heme. We showed that in the WT L. sakei, heme accumulation occurs rapidly and massively in the presence of hemin, while the deletion mutant was impaired in heme uptake; this ability was restored by in trans complementation. Our results establish the main role of the L. sakei Lsa1836-1840 ECF-like system in heme uptake. Therefore, this research outcome sheds new light on other possible functions of ECF-like systems.IMPORTANCELactobacillus sakei is a nonpathogenic bacterial species exhibiting high fitness in heme-rich environments such as meat products, although it does not need iron or heme for growth. Heme capture and utilization capacities are often associated with pathogenic species and are considered virulence-associated factors in the infected hosts. For these reasons, iron acquisition systems have been deeply studied in such species, while for nonpathogenic bacteria the information is scarce. Genomic data revealed that several putative iron transporters are present in the genome of the lactic acid bacterium L. sakei In this study, we demonstrate that one of them is an ECF-like ABC transporter with a functional role in heme transport. Such evidence has not yet been brought for an ECF; therefore, our study reveals a new class of heme transport system.

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.

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.

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.

The five near-iron transporter (NEAT) domain anthrax hemophore, IsdX2, scavenges heme from hemoglobin and transfers heme to the surface protein IsdC

Pathogenic bacteria require iron to replicate inside mammalian hosts. Recent studies indicate that heme acquisition in Gram-positive bacteria is mediated by proteins containing one or more near-iron transporter (NEAT) domains. Bacillus anthracis is a spore-forming, Gram-positive pathogen and the causative agent of anthrax disease. The rapid, extensive, and efficient replication of B. anthracis in host tissues makes this pathogen an excellent model organism for the study of bacterial heme acquisition. B. anthracis secretes two NEAT hemophores, IsdX1 and IsdX2. IsdX1 contains a single NEAT domain, whereas IsdX2 has five, a novel property among hemophores. To understand the functional significance of harboring multiple, non-identical NEAT domains, we purified each individual NEAT domain of IsdX2 as a GST fusion and analyzed the specific function of each domain as it relates to heme acquisition and transport. NEAT domains 1, 3, 4, and 5 all bind heme, with domain 5 having the highest affinity. All NEATs associate with hemoglobin, but only NEAT1 and -5 can extract heme from hemoglobin, seemingly by a specific and active process. NEAT1, -3, and -4 transfer heme to IsdC, a cell wall-anchored anthrax NEAT protein. These results indicate that IsdX2 has all the features required to acquire heme from the host and transport heme to the bacterial cell wall. Additionally, these results suggest that IsdX2 may accelerate iron import rates by acting as a "heme sponge" that enhances B. anthracis replication in iron-starved environments.

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.