Iron content of Streptococcus suis and evidence for a dpr homologue

Dps Is a Universally Conserved Dual-Action DNA-Binding and Ferritin Protein

The DNA-binding protein from starved cells, Dps, is a universally conserved prokaryotic ferritin that, in many species, also binds DNA. Dps homologs have been identified in the vast majority of bacterial species and several archaea. Dps also may play a role in the global regulation of gene expression, likely through chromatin reorganization. Dps has been shown to use both its ferritin and DNA-binding functions to respond to a variety of environmental pressures, including oxidative stress. One mechanism that allows Dps to achieve this is through a global nucleoid restructuring event during stationary phase, resulting in a compact, hexacrystalline nucleoprotein complex called the biocrystal that occludes damaging agents from DNA. Due to its small size, hollow spherical structure, and high stability, Dps is being developed for applications in biotechnology.

PmtA functions as a ferrous iron and cobalt efflux pump in Streptococcus suis

Transition metals are nutrients essential for life. However, an excess of metals can be toxic to cells, and host-imposed metal toxicity is an important mechanism for controlling bacterial infection. Accordingly, bacteria have evolved metal efflux systems to maintain metal homeostasis. Here, we established that PmtA functions as a ferrous iron [Fe(II)] and cobalt [Co(II)] efflux pump in Streptococcus suis, an emerging zoonotic pathogen responsible for severe infections in both humans and pigs. pmtA expression is induced by Fe(II), Co(II), and nickel [Ni(II)], whereas PmtA protects S. suis against Fe(II) and ferric iron [Fe(III)]-induced bactericidal effect, as well as Co(II) and zinc [Zn(II)]-induced bacteriostatic effect. In the presence of elevated concentrations of Fe(II) and Co(II), ΔpmtA accumulates high levels of intracellular iron and cobalt, respectively. ΔpmtA is also more sensitive to streptonigrin, a Fe(II)-activated antibiotic. Furthermore, growth defects of ΔpmtA under Fe(II) or Co(II) excess conditions can be alleviated by manganese [Mn(II)] supplementation. Finally, PmtA plays a role in tolerance to H₂O₂-induced oxidative stress, yet is not involved in the virulence of S. suis in mice. Together, these data demonstrate that S. suis PmtA acts as a Fe(II) and Co(II) efflux pump, and contributes to oxidative stress resistance.

Effect of nonheme iron-containing ferritin Dpr in the stress response and virulence of pneumococci

Streptococcus pneumoniae (pneumococcus) produces hydrogen peroxide as a by-product of metabolism and provides a competitive advantage against cocolonizing bacteria. As pneumococci do not produce catalase or an inducible regulator of hydrogen peroxide, the mechanism of resistance to hydrogen peroxide is unclear. A gene responsible for resistance to hydrogen peroxide and iron in other streptococci is that encoding nonheme iron-containing ferritin, dpr, but previous attempts to study this gene in pneumococcus by generating a dpr mutant were unsuccessful. In the current study, we found that dpr is in an operon with the downstream genes dhfr and clpX. We generated a dpr deletion mutant which displayed normal early-log-phase and mid-log-phase growth in bacteriologic medium but survived less well at stationary phase; the addition of catalase partially rescued the growth defect. We showed that the dpr mutant is significantly more sensitive to pH, heat, iron concentration, and oxidative stress due to hydrogen peroxide. Using a mouse model of colonization, we also showed that the dpr mutant displays a reduced ability to colonize and is more rapidly cleared from the nasopharynx. Our results thus suggest that Dpr is important for pneumococcal resistance to stress and for nasopharyngeal colonization.

Virulence factors involved in the pathogenesis of the infection caused by the swine pathogen and zoonotic agent Streptococcus suis

Streptococcus suis is a major swine pathogen responsible for important economic losses to the swine industry worldwide. It is also an emerging zoonotic agent of meningitis and streptococcal toxic shock-like syndrome. Since the recent recognition of the high prevalence of S. suis human disease in southeast and east Asia, the interest of the scientific community in this pathogen has significantly increased. In the last few years, as a direct consequence of these intensified research efforts, large amounts of data on putative virulence factors have appeared in the literature. Although the presence of some proposed virulence factors does not necessarily define a S. suis strain as being virulent, several cell-associated or secreted factors are clearly important for the pathogenesis of the S. suis infection. In order to cause disease, S. suis must colonize the host, breach epithelial barriers, reach and survive in the bloodstream, invade different organs, and cause exaggerated inflammation. In this review, we discuss the potential contribution of different described S. suis virulence factors at each step of the pathogenesis of the infection. Finally, we briefly discuss other described virulence factors, virulence factor candidates and virulence markers for which a precise role at specific steps of the pathogenesis of the S. suis infection has not yet been clearly established.

The rggC locus, with a frameshift mutation, is involved in oxidative stress response by Streptococcus thermophilus

In Streptococcus thermophilus, the locus rggC contains a frameshift mutation and thus consists of two open reading frames (ORFs), rggC (1) and rggC (2), which encode proteins exhibiting similarity with the Rgg transcriptional regulator family. In this work, mutants showing a partial deletion of rggC (1) and rggC (2 )were constructed and their response to menadione, a superoxide-generating compound, was analysed. These mutants exhibited different behaviour to this oxidative stress compared with the wild-type strain. Analysis of this locus among 21 strains of S. thermophilus showed a polythymine tract length variability and a strain-dependant adenine residue could be found upstream of this repeat. This interstrain polymorphism supports evidence for the hypothesis that the rggC locus is phase variable.

Dps/Dpr ferritin-like protein: insights into the mechanism of iron incorporation and evidence for a central role in cellular iron homeostasis in Streptococcus suis

The Dps family members constitute a distinct group of multimeric and ferritin-like iron binding proteins (up to 500 iron atoms/12-mer) that are widespread in eubacteria and archaea and implicated in oxidative stress resistance and virulence. Despite the wealth of structural knowledge, the mechanism of iron incorporation has remained elusive. Here, we provide evidence on Dpr of the swine and human pathogen Streptococcus suis that: (i) iron incorporation proceeds by Fe(II) binding, Fe(II) oxidation and subsequent storage as Fe(III); (ii) Fe(II) atoms enter the 12-mer cavity through four hydrophilic pores; and (iii) Fe(II) atoms are oxidized inside the 12-mer cavity at 12 identical inter-subunit sites, which are structurally different but functionally equivalent to the ferroxidase centres of classical ferritins. We also provide evidence, by deleting and ectopically overexpressing Dpr, that Dpr affects cellular iron homeostasis. The key residues responsible for iron incorporation in S. suis Dpr are well conserved throughout the Dps family. A model for the iron incorporation mechanism of the Dps/Dpr ferritin-like protein is proposed.

Characterization of MtsR, a new metal regulator in group A streptococcus, involved in iron acquisition and virulence

Group A streptococcus (GAS) is a common pathogen of the human skin and mucosal surfaces capable of producing a variety of diseases. In this study, we investigated regulation of iron uptake in GAS and the role of a putative transcriptional regulator named MtsR (for Mts repressor) with homology to the DtxR family of metal-dependent regulatory proteins. An mtsR mutant was constructed in NZ131 (M49 serotype) and analyzed. Western blot and RNA analysis showed that mtsR inactivation results in constitutive transcription of the sia (streptococcal iron acquisition) operon, which was negatively regulated by iron in the parent strain. A recombinant MtsR with C-terminal His(6) tag fusion (rMtsR) was cloned and purified. Electrophoretic mobility gel shift assays demonstrated that rMtsR specifically binds to the sia promoter region in an iron- and manganese-dependent manner. Together, these observations indicate that MtsR directly represses the sia operon during cell growth under conditions of high metal levels. Consistent with deregulation of iron uptake, the mtsR mutant is hypersensitive to streptonigrin and hydrogen peroxide, and (55)Fe uptake assays demonstrate that it accumulates 80% +/- 22.5% more iron than the wild-type strain during growth in complete medium. Studies with a zebrafish infection model revealed that the mtsR mutant is attenuated for virulence in both the intramuscular and the intraperitoneal routes. In conclusion, MtsR, a new regulatory protein in GAS, controls iron homeostasis and has a role in disease production.

The physiological role of ferritin-like compounds in bacteria

Iron, as the ferrous or ferric ion, is essential for the life processes of all eukaryotes and most prokaryotes; however, the element is toxic when in excess of that needed for cellular homeostasis. Ferrous ions can react with metabolically generated hydrogen peroxide to yield toxic hydroxyl radicals that in turn degrade lipids, DNA, and other cellular biomolecules. Mechanisms have evolved in living systems for iron detoxification and for the removal of excess ferrous ions from the cytosol. These detoxification mechanisms involve the oxidation of excess ferrous ions to the ferric state and storage of the ferric ions in ferritin-like proteins. There are at least three types of ferritin-like proteins in bacteria: bacterial ferritin, bacterioferritin, and dodecameric ferritin. These bacterial proteins are related to the ferritins found in eukaryotes. The structure and physical characteristics of the ferritin-like compounds have been elucidated in several bacteria. Unfortunately, the physiological roles of the bacterial ferritin-like compounds have been less thoroughly studied. A few studies conducted with mutants indicated that ferritin-like compounds can protect bacterial cells from iron overload, serve as an iron source when iron is limited, protect the bacterial cells against oxidative stress and/or protect DNA against enzymatic or oxidative attack. There is very little information available concerning the roles that ferritin-like compounds might play in the survival of bacteria in food, water, soil, or eukaryotic host environments.

Characterisation of oxidative stress-resistant mutants of Streptococcus thermophilus CNRZ368

During industrial processes, the dairy organism Streptococcus thermophilus is exposed to stress conditions. Its ability to survive and grow in an aerobic environment indicates that it must possess defensive mechanisms against reactive oxygen species. To identify the genes involved in oxidative stress defence, a collection of mutants was generated by random insertional mutagenesis and screened for menadione sensitivity and resistance. Results obtained for resistant clones allowed the identification of eight loci. The insertions affected genes whose homologues in other bacteria were previously identified as being involved in stress response(deoB, gst) or transcription regulation (rggC) and five ORFs of unknown function. The tolerance of the eight mutants to air-exposure, methyl viologen and H2O2 was studied. Real-time quantitative PCR was used to analyse the transcript level of mutated genes and revealed that most were down-regulated during oxidative stress.

Crystal Structure of Streptococcus suis Dps-like Peroxide Resistance Protein Dpr: Implications for Iron Incorporation

The Dps-like peroxide resistance protein (Dpr) is an aerotolerance and hydrogen peroxide resistance agent found in the meningitis-associated pathogen Streptococcus suis. Dpr is believed to act by binding free intracellular iron to prevent Fenton chemistry-catalysed formation of toxic hydroxyl radicals. The crystal structure of Dpr has been determined to 1.95 A resolution. The final model has an Rcyst value of 18.5% (Rfree = 22.4%) and consists of 12 identical monomers (each of them comprising a four alpha-helix bundle) that form a hollow sphere obeying 23 symmetry. Structural features show that Dpr belongs to the Dps family of bacterial proteins. Twelve putative ferroxidase centers, each formed at the interface of neighboring monomer pairs, were identified in the Dpr structure with structural similarities to those found in other Dps family members. Dpr was crystallized in the absence of iron, hence no bound iron was found in the structure in contrast to other Dps family members. A novel metal-binding site approximately 6A from the ferroxidase centre was identified and assigned to a bound calcium ion. Two residues from the ferroxidase centre (Asp63 and Asp74) were found to be involved in calcium binding. Structural comparison with other family members revealed that Asp63 and Asp74 adopt different conformation in the Dpr structure. The structure of Dpr presented here shows potential local conformational changes that may occur during iron incorporation. A role for the metal-binding site in iron uptake is proposed.

Molecular basis of H2O2 resistance mediated by Streptococcal Dpr. Demonstration of the functional involvement of the putative ferroxidase center by site-directed mutagenesis in Streptococcus suis

H(2)O(2) is an unavoidable cytotoxic by-product of aerobic life. Dpr, a recently discovered member of the Dps protein family, provides a means for catalase-negative bacteria to tolerate H(2)O(2). Potentially, Dpr could bind free intracellular iron and thus inhibit the Fenton chemistry-catalyzed formation of toxic hydroxyl radicals (H(2)O(2) + Fe(2+) --> (.)OH + (-)OH + Fe(3+)). We explored the in vivo function of Dpr in the catalase- and NADH peroxidase-negative pig and human pathogen Streptococcus suis. We show that: (i) a Dpr allelic exchange knockout mutant was hypersensitive ( approximately 10(6)-fold) to H(2)O(2), (ii) Dpr incorporated iron in vivo, (iii) a putative ferroxidase center was present in Dpr, (iv) single amino acid substitutions D74A or E78A to the putative ferroxidase center abolished the in vivo iron incorporation, and (v) the H(2)O(2) hypersensitive phenotype was complemented by wild-type Dpr or by a membrane-permeating iron chelator, but not by the site-mutated forms of Dpr. These results demonstrate that the putative ferroxidase center of Dpr is functionally active in iron incorporation and that the H(2)O(2) resistance is mediated by Dpr in vivo by its iron binding activity.

Development of a multilocus sequence typing scheme for the pig pathogen Streptococcus suis: identification of virulent clones and potential capsular serotype exchange

Streptococcus suis is an important pathogen of pigs and occasionally causes serious human disease. However, little is known about the S. suis population structure, the clonal relationships between strains, the potential of particular clones to cause disease, and the relevance of serotype as a marker for epidemiology. Here we describe a multilocus sequence typing (MLST) scheme for S. suis developed in order to begin to address these issues. Seven housekeeping gene fragments from each of 294 S. suis isolates obtained from various S. suis diseases and from asymptomatic carriage representing 28 serotypes and nine distinct countries of origin were sequenced. Between 32 and 46 alleles per locus were identified, giving the ability to distinguish >1.6 x 10(11) sequence types (STs). However only 92 STs were identified in this study. Of the 92 STs 18 contained multiple isolates, the most common of which, ST1, was identified on 141 occasions from six countries. Assignment of the STs to lineages resulted in 37 being identified as unique and unrelated STs while the remaining 55 were assigned to 10 complexes. ST complexes ST1, ST27, and ST87 dominate the population; while the ST1 complex was strongly associated with isolates from septicemia, meningitis, and arthritis, the ST87 and ST27 complexes were found to contain significantly higher numbers of lung isolates. In agreement with the observed distribution of disease-causing isolates of S. suis, most isolates previously characterized as of high virulence in porcine infection models belong to ST1, while isolates belonging to other STs appear to be less virulent in general. Finally nine STs were found to contain isolates of multiple serotypes, and many isolates belonging to the same serotypes were found to have very disparate genetic backgrounds. As well as highlighting that the serotype can often be a poor indicator of genetic relatedness between S. suis isolates, these findings suggest that capsular genes may be moving horizontally through the S. suis population.