Role of the dpr product in oxygen tolerance in Streptococcus mutans

Multiple factors regulate the expression of sufCDSUB in Streptococcus mutans

Introduction

The sufCDSUB gene cluster, encoding the sole iron-sulfur (Fe-S) cluster assembly system in S. mutans, was recently shown to be up-regulated in response to oxidative stressors and Fe limitation.

Methods

In this study, luciferase reporter fusion assays, electrophoretic gel mobility shift assays (EMSA) and in vitro transcription assays (IVT) were used to dissect the cis- and trans-acting factors that regulate the expression of sufCDSUB.

Results and discussion

Results showed deletion of perR, for the only Fur-family transcriptional regulator in S. mutans, resulted in >5-fold increases in luciferase activity under the control of the sufCDSUB promoter (P<0.01), as compared to the parent strain, UA159 when the reporter strains were grown in medium with no supplemental iron. Site-directed mutagenesis of a PerR-box in the promoter region led to elevation of the reporter activity by >1.6-fold (P<0.01). In an EMSA, recombinant PerR (rPerR) was shown to bind to the cognate sufCDSUB promoter leading to mobility retardation. On the other hand, the reporter activity was increased by >84-fold (P<0.001) in response to the addition of cysteine at 4 mM to the culture medium. Deletion of cysR, for a LysR-type of transcriptional regulator, led to reduction of the reporter activity by >11.6-fold (P<0.001). Addition of recombinant CysR (rCysR) to an EMSA caused mobility shift of the sufCDSUB promoter probe, indicative of rCysR-promoter interaction, and rCysR was shown to enhance sufC transcription under the direction of sufCDSUB promoter in vitro. These results suggest that multiple factors are involved in the regulation of sufCDSUB expression in response to environmental cues, including cysteine and Fe availability, consistent with the important role of sufCDSUB in S. mutans physiology.

Oxidative stress response: a critical factor affecting the ecological competitiveness of Streptococcus mutans

Oral microecological balance is closely associated with the development of dental caries. Oxidative stress is one of the important factors regulating the composition and structure of the oral microbial community. Streptococcus mutans is linked to the occurrence and development of dental caries. The ability of S. mutans to withstand oxidative stress affects its survival competitiveness in biofilms. The oxidative stress regulatory mechanisms of S. mutans include synthesis of reductase, regulation of metal ions uptake, regulator PerR, transcription regulator Spx, extracellular uptake of glutathione, and other related signal transduction systems. Here, we provide an overview of how S. mutans adapts to oxidative stress and its influence on oral microecology, which may offer novel options to investigate the cariogenic mechanisms of S. mutans in the oral microenvironment, and new targets for the ecological prevention and treatment of dental caries.

Anti-Bacterial and Anti-Biofilm Activities of Anandamide against the Cariogenic Streptococcus mutans

Streptococcus mutans is a cariogenic bacterium in the oral cavity involved in plaque formation and dental caries. The endocannabinoid anandamide (AEA), a naturally occurring bioactive lipid, has been shown to have anti-bacterial and anti-biofilm activities against Staphylococcus aureus. We aimed here to study its effects on S. mutans viability, biofilm formation and extracellular polysaccharide substance (EPS) production. S. mutans were cultivated in the absence or presence of various concentrations of AEA, and the planktonic growth was followed by changes in optical density (OD) and colony-forming units (CFU). The resulting biofilms were examined by MTT metabolic assay, Crystal Violet (CV) staining, spinning disk confocal microscopy (SDCM) and high-resolution scanning electron microscopy (HR-SEM). The EPS production was determined by Congo Red and fluorescent dextran staining. Membrane potential and membrane permeability were determined by diethyloxacarbocyanine iodide (DiOC2(3)) and SYTO 9/propidium iodide (PI) staining, respectively, using flow cytometry. We observed that AEA was bactericidal to S. mutans at 12.5 µg/mL and prevented biofilm formation at the same concentration. AEA reduced the biofilm thickness and biomass with concomitant reduction in total EPS production, although there was a net increase in EPS per bacterium. Preformed biofilms were significantly affected at 50 µg/mL AEA. We further show that AEA increased the membrane permeability and induced membrane hyperpolarization of these bacteria. AEA caused S. mutans to become elongated at the minimum inhibitory concentration (MIC). Gene expression studies showed a significant increase in the cell division gene ftsZ. The concentrations of AEA needed for the anti-bacterial effects were below the cytotoxic concentration for normal Vero epithelial cells. Altogether, our data show that AEA has anti-bacterial and anti-biofilm activities against S. mutans and may have a potential role in preventing biofilms as a therapeutic measure.

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.

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.

Pilot study on selective antimicrobial effect of a halitosis mouthrinse: monospecies and saliva-derived microbiome in an in vitro model system

Background

Halitosis refers to malodor emanating from the oral cavity. Several mouthrinses with halitosis-reduction exist on the market, but their effect on the oral microbiome is largely unknown. In this study, we used an efficient in vitro model system to investigate a test mouthrinse's impact on the oral microbiome.

Methods

Single halitosis-associated species and other common oral microorganism cultures were exposed to the test mouthrinse over time, and their viability was determined by culture-based selective plating. Next, the saliva-derived microbiome from healthy and halitosis-associated individuals was cultured in the presence of the test mouthrinse over time using the previously developed in vitro model system. The microbiome composition was assessed with 16S rRNA gene sequencing and downstream bioinformatics analyses.

Results

The test mouthrinse displayed antimicrobial activity against known anaerobic bacterial species producing halitosis-related compounds such as Fusobacterium nucleatum, F. periodonticum, and Prevotella intermedia but not against other common oral microorganisms. In the multispecies, saliva-derived cultures, mouthrinse exposure decreased the relative abundance of the Fusobacterium and Prevotella genera while not affecting overall diversity.

Conclusions

The test mouthrinse had promising anti-halitosis characteristics at the microbiome level, as demonstrated by the reduction in the relative abundance of halitosis-associated taxa while maintaining microbial diversity.

Regulatory involvement of the PerR and SloR metalloregulators in the Streptococcus mutans oxidative stress response

Streptococcus mutans is a commensal of the human oral microbiome that can promote dental caries under conditions of dysbiosis. This study investigates metalloregulators and their involvement in the S. mutans oxidative stress response. Oxidative stress in the human mouth can derive from temporal increases in reactive oxygen species (ROS) after meal consumption and from endogenous bacterial ROS-producers that colonize the dentition. We hypothesize that the S. mutans PerR (SMU.593) and SloR (SMU.186) metalloregulatory proteins contribute to the regulation of oxidative stress genes and their products. Expression assays with S. mutans UA159 wild type cultures exposed to H₂O₂ reveal that H₂O₂ upregulates perR, and that PerR represses sloR transcription upon binding directly to Fur and PerR consensus sequences within the sloR operator. In addition, the results of Western blot experiments implicate the Clp proteolytic system in SloR degradation under conditions of H₂O₂-stress. To reveal a potential role for SloR in the H₂O₂-resistant phenotype of S. mutans GMS802 (a perR-deficient strain), we generated a sloR/perR double knockout mutant, GMS1386, where we observed upregulation of the tpx and dpr antioxidant genes. These results are consistent with GMS802 H₂O₂ resistance and with a role for PerR as a transcriptional repressor. Cumulatively, these findings support a reciprocal relationship between PerR and SloR during the S. mutans oxidative stress response and begin to elucidate the fitness strategies that evolved to foster S. mutans persistence in the transient environments of the human oral cavity.IMPORTANCEIn 2020, untreated dental caries, especially in the permanent dentition, ranked among the most prevalent infectious diseases worldwide, disproportionately impacting individuals of low socioeconomic status. Untreated caries can lead to systemic health problems and has been associated with extended school and work absences, inappropriate use of emergency departments, and an inability for military forces to deploy. Together with public health policy, research aimed at alleviating S. mutans -induced tooth decay is important because it can improve oral health (and overall health), especially in underserved populations. This research, focused on S. mutans metalloregulatory proteins and their gene targets, is significant because it can promote virulence gene control in an important oral pathogen, and contribute to the development of an anti-caries therapeutic that can reduce tooth decay.

Manganese Depletion Leads to Multisystem Changes in the Transcriptome of the Opportunistic Pathogen Streptococcus sanguinis

Streptococcus sanguinis is a primary colonizer of teeth and is typically considered beneficial due to its antagonistic relationship with the cariogenic pathogen Streptococcus mutans. However, S. sanguinis can also act as an opportunistic pathogen should it enter the bloodstream and colonize a damaged heart valve, leading to infective endocarditis. Studies have implicated manganese acquisition as an important virulence determinant in streptococcal endocarditis. A knockout mutant lacking the primary manganese import system in S. sanguinis, SsaACB, is severely attenuated for virulence in an in vivo rabbit model. Manganese is a known cofactor for several important enzymes in S. sanguinis, including superoxide dismutase, SodA, and the aerobic ribonucleotide reductase, NrdEF. To determine the effect of manganese depletion on S. sanguinis, we performed transcriptomic analysis on a ΔssaACB mutant grown in aerobic fermentor conditions after the addition of the metal chelator EDTA. Despite the broad specificity of EDTA, analysis of cellular metal content revealed a decrease in manganese, but not in other metals, that coincided with a drop in growth rate. Subsequent supplementation with manganese, but not iron, zinc, or magnesium, restored growth in the fermentor post-EDTA. Reduced activity of Mn-dependent SodA and NrdEF likely contributed to the decreased growth rate post-EDTA, but did not appear entirely responsible. With the exception of the Dps-like peroxide resistance gene, dpr, manganese depletion did not induce stress response systems. By comparing the transcriptome of ΔssaACB cells pre- and post-EDTA, we determined that manganese deprivation led to altered expression of diverse systems. Manganese depletion also led to an apparent induction of carbon catabolite repression in a glucose-independent manner. The combined results suggest that manganese limitation produces effects in S. sanguinis that are diverse and complex, with no single protein or system appearing entirely responsible for the observed growth rate decrease. This study provides further evidence for the importance of this trace element in streptococcal biology. Future studies will focus on determining mechanisms for regulation, as the multitude of changes observed in this study indicate that multiple regulators may respond to manganese levels.

Association between the gut and oral microbiome with obesity

In recent decades, obesity has become one of the most common lifestyle-associated disorders. Obesity is a major contributing factor for several other lifestyles associated disorders such as type 2 diabetes mellitus, hypertension, and cardiovascular disease. Although genetics and lifestyle have been directly implicated in the onset and progression of obesity, recent studies have established that gut microbiome plays a crucial role in obesity progression. A higher proportion of Firmicutes and a skewed Firmicutes/Bacteroidetes ratio may contribute to gut dysbiosis and subsequent disturbances in the overall body metabolisms. Like gut microbiome, the oral cavity of humans also harbors a characteristic microbial population called "oral microbiome". The oral microbiome has also been implicated in the development of obesity due to its modulating effects on the gut microbiome. Due to its critical role in obesity, alteration in the gut microbiome has been suggested as one of the therapeutic strategies to manage obesity itself. For example, fecal microbiome transfer, or the use of probiotics and prebiotics have been suggested. These therapies not only restore the gut microbiome to the "pre-obese stage" but also ameliorate many functional aspects of the metabolic syndrome such as systemic inflammation, insulin resistance, and fat accumulation. However, the efficacy and safety of some of the methods have not been tested for their long-term implications, and further research in this area is warranted to understand the molecular mechanisms involved in this process completely.

NADH peroxidase plays a crucial role in consuming H2O2 in Lactobacillus casei IGM394

The facultative anaerobic bacterium Lactobacillus casei IGM394 is used as a host for drug delivery systems, and it exhibits the same growth rate under aerobic and anaerobic conditions. L. casei strains carry several genes that facilitate oxygen and reactive oxygen species (ROS) tolerance in their genomes, but their complete functions have not been uncovered. To clarify the oxygen and ROS tolerance mechanisms of L. casei IGM394, we constructed 23 deficient mutants targeting genes that confer oxidative stress resistance. Significantly decreased growth and high H₂O₂ accumulation were observed in the NADH peroxidase gene-mutated strain (Δnpr) compared with the findings in the wild type. The H₂O₂ degradation capacity of Δnpr revealed that NADH peroxidase is a major H₂O₂-degrading enzyme in L. casei IGM394. Interestingly, ΔohrR, a mutant deficient in the organic hydroperoxide (OhrA) repressor, exhibited higher H₂O₂ resistance than the wild-type strain. Increased Npr expression and H₂O₂ degradation ability were observed in ΔohrR, further supporting the importance of OhrA to ROS tolerance mechanisms. The other mutants did not exhibit altered growth rates, although some mutants had higher growth in the presence of oxygen. From these results, it is presumed that L. casei IGM394 has multiple oxygen tolerance mechanisms and that the loss of a single gene does not alter the growth rate because of the presence of complementary mechanisms. Contrarily, the H₂O₂ tolerance mechanism is solely dependent on NADH peroxidase in L. casei IGM394.

Disruption of l-Rhamnose Biosynthesis Results in Severe Growth Defects in Streptococcus mutans

The rhamnose-glucose cell wall polysaccharide (RGP) of Streptococcus mutans plays a significant role in cell division, virulence, and stress protection. Prior studies examined function of the RGP using strains carrying deletions in the machinery involved in RGP assembly. In this study, we explored loss of the substrate for RGP, l-rhamnose, via deletion of rmlD (encoding the protein responsible for the terminal step in l-rhamnose biosynthesis). We demonstrate that loss of rhamnose biosynthesis causes a phenotype similar to strains with disrupted RGP assembly (ΔrgpG and ΔrgpF strains). Deletion of rmlD not only caused a severe growth defect under nonstress growth conditions but also elevated susceptibility of the strain to acid and oxidative stress, common conditions found in the oral cavity. A genetic complement of the ΔrmlD strain completely restored wild-type levels of growth, whereas addition of exogenous rhamnose did not. The loss of rhamnose production also significantly disrupted biofilm formation, an important aspect of S. mutans growth in the oral cavity. Further, we demonstrate that loss of either rmlD or rgpG results in ablation of rhamnose content in the S. mutans cell wall. Taken together, these results highlight the importance of rhamnose production in both the fitness and the ability of S. mutans to overcome environmental stresses.IMPORTANCE Streptococcus mutans is a pathogenic bacterium that is the primary etiologic agent of dental caries, a disease that affects billions yearly. Rhamnose biosynthesis is conserved not only in streptococcal species but in other Gram-positive, as well as Gram-negative, organisms. This study highlights the importance of rhamnose biosynthesis in RGP production for protection of the organism against acid and oxidative stresses, the two major stressors that the organism encounters in the oral cavity. Loss of RGP also severely impacts biofilm formation, the first step in the onset of dental caries. The high conservation of the rhamnose synthesis enzymes, as well as their importance in S. mutans and other organisms, makes them favorable antibiotic targets for the treatment of disease.

Antibiotic and Antibiofilm Activities of Salvadora persica L. Essential Oils against Streptococcus mutans: A Detailed Comparative Study with Chlorhexidine Digluconate

The use of organic components from plants as an alternative antimicrobial agent is becoming popular due to the development of drug-resistance in various pathogens. Essential oils from fresh (MF-1) and dried (MD-1) roots of Salvadora persica L. were extracted and benzyl isothiocynate was determined as their chief constituent using GC-MS and GC-FID. The antibiofilm and antimicrobial activities of MD-1 and MF-1 against Streptococcus mutans a dental caries causing bacteria were determined using multiple assays. These activities were compared with chlorhexidine digluconate (CHX) and clove oil, well known antimicrobial agents for oral hygiene. Essential oils demonstrated IC₅₀ values (10–11 µg/mL) comparable to that of CHX, showed a significant reduction (82 ± 7–87 ± 6%) of the biofilm formation at a very low concentration. These results were supported by RT-PCR studies showing change in the expression levels of AtlE, gtfB, ymcA and sodA genes involved in autolysis, biofilm formation and oxidative stress, respectively. The results presented in this study show the robust bactericidal and antibiofilm activity of MD-1 and MF-1 against S. mutans which is comparable to Chlorhexidine digluconate. Our results suggest that these essential oils can be as effective as CHX and hence can serve as a good alternative antimicrobial agent for oral hygiene.

Live and let die: Hydrogen peroxide production by the commensal flora and its role in maintaining a symbiotic microbiome

The majority of commensal oral streptococci are able to generate hydrogen peroxide (H₂ O₂ ) during aerobic growth, which can diffuse through the cell membrane and inhibit competing species in close proximity. Competing H₂ O₂ production is mainly dependent upon the pyruvate oxidase SpxB, and to a lesser extent the lactate oxidase LctO, both of which are important for energy generation in aerobic environments. Several studies point to a broad impact of H₂ O₂ production in the oral environment, including a potential role in biofilm homeostasis, signaling, and interspecies interactions. Here, we summarize the current research regarding oral streptococcal H₂ O₂ generation, resistance mechanisms, and the ecological impact of H₂ O₂ production. We also discuss the potential therapeutic utility of H₂ O₂ for the prevention/treatment of dysbiotic diseases as well as its potential role as a biomarker of oral health.

Disruption of a Novel Iron Transport System Reverses Oxidative Stress Phenotypes of a dpr Mutant Strain of Streptococcus mutans

The Dps-like peroxide resistance protein (Dpr) is essential for H₂O₂ stress tolerance and aerobic growth of the oral pathogen Streptococcus mutans Dpr accumulates during oxidative stress, protecting the cell by sequestering iron ions and thereby preventing the generation of toxic hydroxyl radicals that result from the interaction of iron with H₂O₂ Previously, we reported that the SpxA1 and SpxA2 regulators positively regulate expression of dpr in S. mutans Using an antibody raised against S. mutans Dpr, we confirmed at the protein level the central and cooperative nature of SpxA1 and SpxA2 regulation in Dpr production. During phenotypic characterization of the S. mutans Δdpr strain, we observed the appearance of distinct colony variants, which sometimes lost the oxidative stress sensitivity typical of Δdpr strains. Whole-genome sequencing of these phenotypically distinct Δdpr isolates revealed that a putative iron transporter operon, smu995-smu998, was a genomic hot spot with multiple single nucleotide polymorphisms identified within the different isolates. Deletion of smu995 or the entire smu995-smu998 operon in the Δdpr background strain completely reversed the oxidative stress-sensitive phenotypes associated with dpr inactivation. Conversely, inactivation of genes encoding the ferrous iron transport system FeoABC did not alleviate phenotypes of the Δdpr strain. Preliminary characterization of strains lacking smu995-smu998, feoABC, and the iron/manganese transporter gene sloABC revealed the interactive nature of these three systems in iron transport but also indicated that there may be additional iron uptake systems in S. mutansIMPORTANCE The dental caries-associated pathogen Streptococcus mutans routinely encounters oxidative stress within the human plaque biofilm. Previous studies revealed that the iron-binding protein Dpr confers protection toward oxidative stress by limiting free iron availability, which is associated with the generation of toxic hydroxyl radicals. Here, we report the identification of spontaneously occurring mutations within Δdpr strains. Several of those mutations were mapped to the operon smu995-smu998, revealing a previously uncharacterized system that appears to be important in iron acquisition. Disruption of the smu995-smu998 operon resulted in reversion of the stress-sensitive phenotype typical of a Δdpr strain. Our data suggest that the Smu995-Smu998 system works along with other known metal transport systems of S. mutans, i.e., FeoABC and SloABC, to coordinate iron uptake.

Thymol and carvacrol induce autolysis, stress, growth inhibition and reduce the biofilm formation by Streptococcus mutans

Organic compounds from plants are an attractive alternative to conventional antimicrobial agents. Therefore, two compounds namely M-1 and M-2 were purified from Origanum vulgare L. and were identified as carvacrol and thymol, respectively. Antimicrobial and antibiofilm activities of these compounds along with chlorhexidine digluconate using various assays was determined against dental caries causing bacteria Streptococcus mutans. The IC₅₀ values of carvacrol (M-1) and thymol (M-2) against S. mutans were 65 and 54 µg/ml, respectively. Live and dead staining and the MTT assays reveal that a concentration of 100 µg/ml of these compounds reduced the viability and the metabolic activity of S. mutans by more than 50%. Biofilm formation on the surface of polystyrene plates was significantly reduced by M-1 and M-2 at 100 µg/ml as observed under scanning electron microscope and by colorimetric assay. These results were in agreement with RT-PCR studies. Wherein exposure to 25 µg/ml of M-1 and M-2 showed a 2.2 and 2.4-fold increase in Autolysin gene (AtlE) expression level, respectively. While an increase of 1.3 and 1.4 fold was observed in the super oxide dismutase gene (sodA) activity with the same concentrations of M-1 and M-2, respectively. An increase in the ymcA gene and a decrease in the gtfB gene expression levels was observed following the treatment with M-1 and M-2. These results strongly suggest that carvacrol and thymol isolated from O. vulgare L. exhibit good bactericidal and antibiofilm activity against S. mutans and can be used as a green alternative to control dental caries.

Oxidative Stressors Modify the Response of Streptococcus mutans to Its Competence Signal Peptides

The dental caries pathogen Streptococcus mutans is continually exposed to several types of stress in the oral biofilm environment. Oxidative stress generated by reactive oxygen species has a major impact on the establishment, persistence, and virulence of S. mutans Here, we combined fluorescent reporter-promoter fusions with single-cell imaging to study the effects of reactive oxygen species on activation of genetic competence in S. mutans Exposure to paraquat, which generates superoxide anion, produced a qualitatively different effect on activation of expression of the gene for the master competence regulator, ComX, than did treatment with hydrogen peroxide (H₂O₂), which can yield hydroxyl radical. Paraquat suppressed peptide-mediated induction of comX in a progressive and cumulative fashion, whereas the response to H₂O₂ displayed a strong threshold behavior. Low concentrations of H₂O₂ had little effect on induction of comX or the bacteriocin gene cipB, but expression of these genes declined sharply if extracellular H₂O₂ exceeded a threshold concentration. These effects were not due to decreased reporter gene fluorescence. Two different threshold concentrations were observed in the response to H₂O₂, depending on the gene promoter that was analyzed and the pathway by which the competence regulon was stimulated. The results show that paraquat and H₂O₂ affect the S. mutans competence signaling pathway differently, and that some portions of the competence signaling pathway are more sensitive to oxidative stress than others.IMPORTANCEStreptococcus mutans inhabits the oral biofilm, where it plays an important role in the development of dental caries. Environmental stresses such as oxidative stress influence the growth of S. mutans and its important virulence-associated behaviors, such as genetic competence. S. mutans competence development is a complex behavior that involves two different signaling peptides and can exhibit cell-to-cell heterogeneity. Although oxidative stress is known to influence S. mutans competence, it is not understood how oxidative stress interacts with the peptide signaling or affects heterogeneity. In this study, we used fluorescent reporters to probe the effect of reactive oxygen species on competence signaling at the single-cell level. Our data show that different reactive oxygen species have different effects on S. mutans competence, and that some portions of the signaling pathway are more acutely sensitive to oxidative stress than others.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

Transcriptional profile of glucose-shocked and acid-adapted strains of Streptococcus mutans

The aciduricity of Streptococcus mutans is an important virulence factor of the organism, required to both out-compete commensal oral microorganisms and cause dental caries. In this study, we monitored transcriptional changes that occurred as a continuous culture of either an acid-tolerant strain (UA159) or an acid-sensitive strain (fabM::Erm) moved from steady-state growth at neutral pH, experienced glucose-shock and acidification of the culture, and transitioned to steady-state growth at low pH. Hence, the timing of elements of the acid tolerance response (ATR) could be observed and categorized as acute vs. adaptive ATR mechanisms. Modulation of branched chain amino acid biosynthesis, DNA/protein repair mechanisms, reactive oxygen species metabolizers and phosphoenolpyruvate:phosphotransferase systems occurred in the initial acute phase, immediately following glucose-shock, while upregulation of F1 F0 -ATPase did not occur until the adaptive phase, after steady-state growth had been re-established. In addition to the archetypal ATR pathways mentioned above, glucose-shock led to differential expression of genes suggesting a re-routing of resources away from the synthesis of fatty acids and proteins, and towards synthesis of purines, pyrimidines and amino acids. These adjustments were largely transient, as upon establishment of steady-state growth at acidic pH, transcripts returned to basal expression levels. During growth at steady-state pH 7, fabM::Erm had a transcriptional profile analogous to that of UA159 during glucose-shock, indicating that even during growth in rich media at neutral pH, the cells were stressed. These results, coupled with a recently established collection of deletion strains, provide a starting point for elucidation of the acid tolerance response in S. mutans.

The copYAZ Operon Functions in Copper Efflux, Biofilm Formation, Genetic Transformation, and Stress Tolerance in Streptococcus mutans

In bacteria, copper homeostasis is closely monitored to ensure proper cellular functions while avoiding cell damage. Most Gram-positive bacteria utilize the copYABZ operon for copper homeostasis, where copA and copB encode copper-transporting P-type ATPases, whereas copY and copZ regulate the expression of the cop operon. Streptococcus mutans is a biofilm-forming oral pathogen that harbors a putative copper-transporting copYAZ operon. Here, we characterized the role of copYAZ operon in the physiology of S. mutans and delineated the mechanisms of copper-induced toxicity in this bacterium. We observed that copper induced toxicity in S. mutans cells by generating oxidative stress and disrupting their membrane potential. Deletion of the copYAZ operon in S. mutans strain UA159 resulted in reduced cell viability under copper, acid, and oxidative stress relative to the viability of the wild type under these conditions. Furthermore, the ability of S. mutans to form biofilms and develop genetic competence was impaired under copper stress. Briefly, copper stress significantly reduced cell adherence and total biofilm biomass, concomitantly repressing the transcription of the gtfB, gtfC, gtfD, gbpB, and gbpC genes, whose products have roles in maintaining the structural and/or functional integrity of the S. mutans biofilm. Furthermore, supplementation with copper or loss of copYAZ resulted in significant reductions in transformability and in the transcription of competence-associated genes. Copper transport assays revealed that the ΔcopYAZ strain accrued significantly large amounts of intracellular copper compared with the amount of copper accumulation in the wild-type strain, thereby demonstrating a role for CopYAZ in the copper efflux of S. mutans. The complementation of the CopYAZ system restored copper expulsion, membrane potential, and stress tolerance in the copYAZ-null mutant. Taking these results collectively, we have established the function of the S. mutans CopYAZ system in copper export and have further expanded knowledge on the importance of copper homeostasis and the CopYAZ system in modulating streptococcal physiology, including stress tolerance, membrane potential, genetic competence, and biofilm formation.

IMPORTANCE

S. mutans is best known for its role in the initiation and progression of human dental caries, one of the most common chronic diseases worldwide. S. mutans is also implicated in bacterial endocarditis, a life-threatening inflammation of the heart valve. The core virulence factors of S. mutans include its ability to produce and sustain acidic conditions and to form a polysaccharide-encased biofilm that provides protection against environmental insults. Here, we demonstrate that the addition of copper and/or deletion of copYAZ (the copper homeostasis system) have serious implications in modulating biofilm formation, stress tolerance, and genetic transformation in S. mutans. Manipulating the pathways affected by copper and the copYAZ system may help to develop potential therapeutics to prevent S. mutans infection in and beyond the oral cavity.

Transcriptional and Phenotypic Characterization of Novel Spx-Regulated Genes in Streptococcus mutans

In oral biofilms, two of the major environmental challenges encountered by the dental pathogen Streptococcus mutans are acid and oxidative stresses. Previously, we showed that the S. mutans transcriptional regulators SpxA1 and SpxA2 (formerly SpxA and SpxB, respectively) are involved in stress survival by activating the expression of classic oxidative stress genes such as dpr, nox, sodA and tpx. We reasoned that some of the uncharacterized genes under SpxA1/A2 control are potentially involved in oxidative stress management. Therefore, the goal of this study was to use Spx-regulated genes as a tool to identify novel oxidative stress genes in S. mutans. Quantitative real-time PCR was used to evaluate the responses of ten Spx-regulated genes during H₂O₂ stress in the parent and Δspx strains. Transcription activation of the H₂O₂-induced genes (8 out of 10) was strongly dependent on SpxA1 and, to a lesser extent, SpxA2. In vitro transcription assays revealed that one or both Spx proteins directly regulate three of these genes. The gene encoding the FeoB ferrous permease was slightly repressed by H₂O₂ but constitutively induced in strains lacking SpxA1. Nine genes were selected for downstream mutational analysis but inactivation of smu127, encoding a subunit of the acetoin dehydrogenase was apparently lethal. In vitro and in vivo characterization of the viable mutants indicated that, in addition to the transcriptional activation of reducing and antioxidant pathways, Spx performs an important role in iron homeostasis by regulating the intracellular availability of free iron. In particular, inactivation of the genes encoding the Fe-S biogenesis SUF system and the previously characterized iron-binding protein Dpr resulted in impaired growth under different oxidative stress conditions, increased sensitivity to iron and lower infectivity in rats. These results serve as an entryway into the characterization of novel genes and pathways that allow S. mutans to cope with oxidative stress.

Proteome analysis identifies the Dpr protein of Streptococcus mutans as an important factor in the presence of early streptococcal colonizers of tooth surfaces

Oral streptococci are primary colonizers of tooth surfaces and Streptococcus mutans is the principal causative agent of dental caries in humans. A number of proteins are involved in the formation of monospecies biofilms by S. mutans. This study analyzed the protein expression profiles of S. mutans biofilms formed in the presence or absence of S. gordonii, a pioneer colonizer of the tooth surface, by two-dimensional gel electrophoresis (2-DE). After identifying S. mutans proteins by Mass spectrometric analysis, their expression in the presence of S. gordonii was analyzed. S. mutans was inoculated with or without S. gordonii DL1. The two species were compartmentalized using 0.2-μl Anopore membranes. The biofilms on polystyrene plates were harvested, and the solubilized proteins were separated by 2-DE. When S. mutans biofilms were formed in the presence of S. gordonii, the peroxide resistance protein Dpr of the former showed 4.3-fold increased expression compared to biofilms that developed in the absence of the pioneer colonizer. In addition, we performed a competition assay using S. mutans antioxidant protein mutants together with S. gordonii and other initial colonizers. Growth of the dpr-knockout S. mutans mutant was significantly inhibited by S. gordonii, as well as by S. sanguinis. Furthermore, a cell viability assay revealed that the viability of the dpr-defective mutant was significantly attenuated compared to the wild-type strain when co-cultured with S. gordonii. Therefore, these results suggest that Dpr might be one of the essential proteins for S. mutans survival on teeth in the presence of early colonizing oral streptococci.

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.

Comparing the cariogenic species Streptococcus sobrinus and S. mutans on whole genome level

Background

Two closely related species of mutans streptococci, namely Streptococcus mutans and Streptococcus sobrinus, are associated with dental caries in humans. Their acidogenic and aciduric capacity is directly associated with the cariogenic potential of these bacteria. To survive acidic and temporarily harsh conditions in the human oral cavity with hundreds of other microbial co-colonizers as competitors, both species have developed numerous mechanisms for adaptation.

Objectives

The recently published novel genome information for both species is used to elucidate genetic similarities but especially differences and to discuss the impact on cariogenicity of the corresponding phenotypic properties including adhesion, carbohydrate uptake and fermentation, acid tolerance, signaling by two component systems, competence, and oxidative stress resistance.

Conclusions

S. sobrinus can down-regulate the SpaA-mediated adherence to the pellicle. It has a smaller number of two-component signaling systems and bacteriocin-related genes than S. mutans, but all or even more immunity proteins. It lacks the central competence genes comC, comS, and comR. There are more genes coding for glucosyltransferases and a novel energy production pathway formed by lactate oxidase, which is not found in S. mutans. Both species show considerable differences in the regulation of fructan catabolism. However, both S. mutans and S. sobrinus share most of these traits and should therefore be considered as equally virulent with regard to dental caries.

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.

The relationship of the lipoprotein SsaB, manganese and superoxide dismutase in Streptococcus sanguinis virulence for endocarditis

Streptococcus sanguinis colonizes teeth and is an important cause of infective endocarditis. Our prior work showed that the lipoprotein SsaB is critical for S. sanguinis virulence for endocarditis and belongs to the LraI family of conserved metal transporters. In this study, we demonstrated that an ssaB mutant accumulates less manganese and iron than its parent. A mutant lacking the manganese-dependent superoxide dismutase, SodA, was significantly less virulent than wild-type in a rabbit model of endocarditis, but significantly more virulent than the ssaB mutant. Neither the ssaB nor the sodA mutation affected sensitivity to phagocytic killing or efficiency of heart valve colonization. Animal virulence results for all strains could be reproduced by growing bacteria in serum under physiological levels of O(2). SodA activity was reduced, but not eliminated in the ssaB mutant in serum and in rabbits. Growth of the ssaB mutant in serum was restored upon addition of Mn(2+) or removal of O(2). Antioxidant supplementation experiments suggested that superoxide and hydroxyl radicals were together responsible for the ssaB mutant's growth defect. We conclude that manganese accumulation mediated by the SsaB transport system imparts virulence by enabling cell growth in oxygen through SodA-dependent and independent mechanisms.

Adaptive response of Amphibacillus xylanus to normal aerobic and forced oxidative stress conditions

Amphibacillus xylanus grows at the same rate and with the same cell yield under aerobic and anaerobic conditions. Under aerobic conditions, it exhibits vigorous oxygen consumption in spite of lacking a respiratory system and haem catalase. To understand the adaptive response of A. xylanus to oxidative stresses, a genomic analysis of A. xylanus was conducted. The analysis showed that A. xylanus has the genes of four metabolic systems: two pyruvate metabolic pathways, a glycolytic metabolic pathway and an NADH oxidase (Nox)-AhpC (Prx) system. A transcriptional study confirmed that A. xylanus has these metabolic systems. Moreover, genomic analysis revealed the presence of two genes for NADH oxidase (nox1 and nox2), both of which were identified in the transcriptional analysis. The nox1 gene in A. xylanus was highly expressed under normal aerobic conditions but that of nox2 was not. A purification study of NADH oxidases indicated that the gene product of nox1 is a primary metabolic enzyme responsible for metabolism of both oxygen and reactive oxygen species. A. xylanus was successfully grown under forced oxidative stress conditions such as 0.1 mM H2O2, 0.3 mM paraquat and 80 % oxygen. Proteomic analysis revealed that manganese SOD, Prx, pyruvate dehydrogenase complex E1 and E3 components, and riboflavin synthase β-chain are induced under normal aerobic conditions, and the other proteins except the five aerobically induced proteins were not induced under forced oxidative stress conditions. Taken together, the present findings indicate that A. xylanus has a unique defence system against forced oxidative stress.

Genetic variability of mutans streptococci revealed by wide whole-genome sequencing

Background

Mutans streptococci are a group of bacteria significantly contributing to tooth decay. Their genetic variability is however still not well understood.

Results

Genomes of 6 clinical S. mutans isolates of different origins, one isolate of S. sobrinus (DSM 20742) and one isolate of S. ratti (DSM 20564) were sequenced and comparatively analyzed. Genome alignment revealed a mosaic-like structure of genome arrangement. Genes related to pathogenicity are found to have high variations among the strains, whereas genes for oxidative stress resistance are well conserved, indicating the importance of this trait in the dental biofilm community. Analysis of genome-scale metabolic networks revealed significant differences in 42 pathways. A striking dissimilarity is the unique presence of two lactate oxidases in S. sobrinus DSM 20742, probably indicating an unusual capability of this strain in producing H₂O₂ and expanding its ecological niche. In addition, lactate oxidases may form with other enzymes a novel energetic pathway in S. sobrinus DSM 20742 that can remedy its deficiency in citrate utilization pathway.

Using 67 S. mutans genomes currently available including the strains sequenced in this study, we estimates the theoretical core genome size of S. mutans, and performed modeling of S. mutans pan-genome by applying different fitting models. An “open” pan-genome was inferred.

Conclusions

The comparative genome analyses revealed diversities in the mutans streptococci group, especially with respect to the virulence related genes and metabolic pathways. The results are helpful for better understanding the evolution and adaptive mechanisms of these oral pathogen microorganisms and for combating them.

Binding analysis of ferritin with heme using α-casein and biotinylated-hemin: detection of heme-binding capacity of Dpr derived from heme synthesis-deficient Streptococcus mutans

Bacterial and mammalian ferritins are known to bind heme. The use of α-casein and biotinylated hemin could be applicable to detection of protein-bound heme and of proteins with heme-binding capacity, respectively. Although commercial horse spleen ferritin and purified horse spleen ferritin (L:H subunit ratio=4) bound to an α-casein-coated plate, and this binding could be inhibited by hemin, recombinant iron-binding protein (rDpr), derived from heme-deficient Streptococcus mutans and expressed in Escherichia coli, did not bind to an α-casein-coated plate. Both horse spleen ferritins bound to α-casein-immobilized beads. Commercial horse spleen ferritin and rDpr showed direct binding to hemin-agarose beads. After preincubation of commercial horse spleen ferritin or rDpr with biotinylated hemin, they showed indirect binding to avidin-immobilized beads through biotinylated hemin. These results demonstrate that α-casein is useful for detection of heme-binding ferritin and that both hemin-agarose and the combination of biotinylated hemin and avidin-beads are useful for detection of the heme-binding capacity of ferritin. In addition, this study also revealed that Dpr, a decameric iron-binding protein, from heme-deficient cells binds heme.

Effect ofAchyranthes asperaRoot andTerminalia arjunaBark on Aerotolerant Responses inStreptococcus mutans

In the present study, the effect of hydroalcoholic extract ofAchyranthes asperaroot (AAR) andTerminalia arjunabark (TAB) on the aerotolerant capacities ofStreptococcus mutanswas examined. AAR and TAB extracts were found to inhibit the prime antioxidant enzymes like manganese superoxide dismutase (MnSOD), NADH oxidases, and glutathione peroxidase (GPx), altering the aerotolerant responses inS. mutans. Consequently,S. mutansare unable to withstand the oxidative stress and thus are made more susceptible to the antimicrobial activity.

dpr and sod in Streptococcus mutans are involved in coexistence with S. sanguinis, and PerR is associated with resistance to H2O2

Large numbers of bacteria coexist in the oral cavity. Streptococcus sanguinis, one of the major bacteria in dental plaque, produces hydrogen peroxide (H(2)O(2)), which interferes with the growth of other bacteria. Streptococcus mutans, a cariogenic bacterium, can coexist with S. sanguinis in dental plaque, but to do so, it needs a means of detoxifying the H(2)O(2) produced by S. sanguinis. In this study, we investigated the association of three oxidative stress factors, Dpr, superoxide dismutase (SOD), and AhpCF, with the resistance of S. sanguinis to H(2)O(2). The knockout of dpr and sod significantly increased susceptibility to H(2)O(2), while the knockout of ahpCF had no apparent effect on susceptibility. In particular, dpr inactivation resulted in hypersensitivity to H(2)O(2). Next, we sought to identify the factor(s) involved in the regulation of these oxidative stress genes and found that PerR negatively regulated dpr expression. The knockout of perR caused increased dpr expression levels, resulting in low-level susceptibility to H(2)O(2) compared with the wild type. Furthermore, we evaluated the roles of perR, dpr, and sod when S. mutans was cocultured with S. sanguinis. Culturing of the dpr or sod mutant with S. sanguinis showed a significant decrease in the S. mutans population ratio compared with the wild type, while the perR mutant increased the ratio. Our results suggest that dpr and sod in S. mutans are involved in coexistence with S. sanguinis, and PerR is associated with resistance to H(2)O(2) in regulating the expression of Dpr.

Haemophilus influenzae and oxidative stress

Haemophilus influenzae is a commensal of the human upper respiratory tract. H. influenzae can, however, move out of its commensal niche and cause multiple respiratory tract diseases. Such diseases include otitis media in young children, as well as exacerbations of chronic obstructive pulmonary disease (COPD), sinusitis, conjunctivitis, and bronchitis. During the course of colonization and infection, H. influenzae must withstand oxidative stress generated by multiple reactive oxygen species produced endogenously, by other co-pathogens and by host cells. H. influenzae has, therefore, evolved multiple mechanisms that protect the cell against oxygen-generated stresses. In this review, we will describe these systems relative to the well-described systems in Escherichia coli. Moreover, we will compare how H. influenzae combats the effect of oxidative stress as a necessary phenotype for its roles as both a successful commensal and pathogen.

Persistence of Bartonella spp. stealth pathogens: from subclinical infections to vasoproliferative tumor formation

Bartonella spp. are facultative intracellular bacteria that typically cause a long-lasting intraerythrocytic bacteremia in their mammalian reservoir hosts, thereby favoring transmission by blood-sucking arthropods. In most cases, natural reservoir host infections are subclinical and the relapsing intraerythrocytic bacteremia may last weeks, months, or even years. In this review, we will follow the infection cycle of Bartonella spp. in a reservoir host, which typically starts with an intradermal inoculation of bacteria that are superficially scratched into the skin from arthropod feces and terminates with the pathogen exit by the blood-sucking arthropod. The current knowledge of bacterial countermeasures against mammalian immune response will be presented for each critical step of the pathogenesis. The prevailing models of the still-enigmatic primary niche and the anatomical location where bacteria reside, persist, and are periodically seeded into the bloodstream to cause the typical relapsing Bartonella spp. bacteremia will also be critically discussed. The review will end up with a discussion of the ability of Bartonella spp., namely Bartonella henselae, Bartonella quintana, and Bartonella bacilliformis, to induce tumor-like vascular deformations in humans having compromised immune response such as in patients with AIDS.

Coevolution of aah: a dps-like gene with the host bacterium revealed by comparative genomic analysis

A protein named AAH was isolated from the bacterium Microbacterium arborescens SE14, a gut commensal of the lepidopteran larvae. It showed not only a high sequence similarity to Dps-like proteins (DNA-binding proteins from starved cell) but also reversible hydrolase activity. A comparative genomic analysis was performed to gain more insights into its evolution. The GC profile of the aah gene indicated that it was evolved from a low GC ancestor. Its stop codon usage was also different from the general pattern of Actinobacterial genomes. The phylogeny of dps-like proteins showed strong correlation with the phylogeny of host bacteria. A conserved genomic synteny was identified in some taxonomically related Actinobacteria, suggesting that the ancestor genes had incorporated into the genome before the divergence of Micrococcineae from other families. The aah gene had evolved new function but still retained the typical dodecameric structure.

From meadows to milk to mucosa - adaptation of Streptococcus and Lactococcus species to their nutritional environments

Lactic acid bacteria (LAB) are indigenous to food-related habitats as well as associated with the mucosal surfaces of animals. The LAB family Streptococcaceae consists of the genera Lactococcus and Streptococcus. Members of the family include the industrially important species Lactococcus lactis, which has a long history safe use in the fermentative food industry, and the disease-causing streptococci Streptococcus pneumoniae and Streptococcus pyogenes. The central metabolic pathways of the Streptococcaceae family have been extensively studied because of their relevance in the industrial use of some species, as well as their influence on virulence of others. Recent developments in high-throughput proteomic and DNA-microarray techniques, in in vivo NMR studies, and importantly in whole-genome sequencing have resulted in new insights into the metabolism of the Streptococcaceae family. The development of cost-effective high-throughput sequencing has resulted in the publication of numerous whole-genome sequences of lactococcal and streptococcal species. Comparative genomic analysis of these closely related but environmentally diverse species provides insight into the evolution of this family of LAB and shows that the relatively small genomes of members of the Streptococcaceae family have been largely shaped by the nutritionally rich environments they inhabit.

Peroxide stress elicits adaptive changes in bacterial metal ion homeostasis

Exposure to hydrogen peroxide (H(2)O(2)) and other reactive oxygen species is a universal feature of life in an aerobic environment. Bacteria express enzymes to detoxify H(2)O(2) and to repair the resulting damage, and their synthesis is typically regulated by redox-sensing transcription factors. The best characterized bacterial peroxide-sensors are Escherichia coli OxyR and Bacillus subtilis PerR. Analysis of their regulons has revealed that, in addition to inducible detoxification enzymes, adaptation to H(2)O(2) is mediated by modifications of metal ion homeostasis. Analogous adaptations appear to be present in other bacteria as here reviewed for Deinococcus radiodurans, Neisseria gonorrhoeae, Streptococcus pyogenes, and Bradyrhizobium japonicum. As a general theme, peroxide stress elicits changes in cytosolic metal distribution with the net effect of reducing the damage caused by reactive ferrous iron. Iron levels are reduced by repression of uptake, sequestration in storage proteins, and incorporation into metalloenzymes. In addition, peroxide-inducible transporters elevate cytosolic levels of Mn(II) and/or Zn(II) that can displace ferrous iron from sensitive targets. Although bacteria differ significantly in the detailed mechanisms employed to modulate cytosolic metal levels, a high Mn:Fe ratio has emerged as one key correlate of reactive oxygen species resistance.

Characterization of the Streptococcus sobrinus acid-stress response by interspecies microarrays and proteomics

Streptococcus mutans and Streptococcus sobrinus are considered the primary organisms responsible for human dental caries. The ability to generate acids and to adapt to low pH conditions is directly associated with the cariogenic potential of these bacteria. To survive acidic conditions, both species have been shown to mount an acid-tolerance response (ATR). However, previous characterization of the S. sobrinus ATR identified critical differences in the mechanisms of acid adaptation between S. mutans and S. sobrinus. Here, interspecies microarray and proteomic approaches were used to identify novel, previously unrecognized genes and pathways that participate in the S. sobrinus acid-stress response. The results revealed that, among other things, metabolic alterations that enhance energy generation and upregulation of the malolactic fermentation enzyme activity constitute important acid-resistance properties in S. sobrinus. Some of these acid adaptive traits are shared by S. mutans and might be considered optimal targets for therapeutic treatments designed to control dental caries.

Regulation of the properties of superoxide dismutase from the dental pathogenic microorganism Streptococcus mutans by iron- and manganese-bound co-factor

Streptococcus mutans, the main pathogen involved in the development of dental caries, is an aerotolerant microorganism. The bacterium lacks cytochromes and catalase, but possesses other antioxidant enzymes, such as superoxide dismutase (SmSOD). Previous researches suggested that SmSOD belongs to the 'cambialistic' group, functioning with Fe or Mn in the active site. A recombinant SmSOD (rSmSOD) with a His-tail has been produced and characterised. Studies on metal uptake and exchange proved that rSmSOD binds either Fe or Mn as a metal co-factor, even though with a consistent preference for Fe accommodation. The analysis of several enzyme samples with different values of the Mn/Fe ratio in the active site proved that the type of metal is crucial for the regulation of the activity of rSmSOD. Indeed, differently from the significant preference for Fe displayed by the enzyme in the binding reaction, its Mn-form was 71-fold more active compared to the Fe-form. The rSmSOD was endowed with a significant thermostability, its half-inactivation occurring after 10 min exposure at 71 or 73 degrees C, depending on the bound metal. Moreover, the enthalpic and entropic contribution to the heat inactivation process of rSmSOD were strongly regulated by the Mn content of the enzyme. The effect of typical inhibitors/inactivators has been investigated. rSmSOD was inhibited by sodium azide, and its sensitivity increased in the presence of higher Mn levels. Concerning two physiological inactivators, the enzyme displayed a different behaviour, being quite resistant to hydrogen peroxide and significantly sensitive to sodium peroxynitrite. Furthermore, the Mn co-factor had an amplifying role in the regulation of this different sensitivity. These results confirm the cambialistic nature of SmSOD and prove that its properties are regulated by the different metal content. The adaptative response of S. mutans during its aerobic exposure in the oral cavity could involve a different metal uptake by SmSOD.

Genomic island TnSmu2 of Streptococcus mutans harbors a nonribosomal peptide synthetase-polyketide synthase gene cluster responsible for the biosynthesis of pigments involved in oxygen and H2O2 tolerance

The oral biofilm community consists of >800 microbial species, among which Streptococcus mutans is considered a primary pathogen for dental caries. The genomic island TnSmu2 of S. mutans comprises >2% of the genome. In this study, we demonstrate that TnSmu2 harbors a gene cluster encoding nonribosomal peptide synthetases (NRPS), polyketide synthases (PKS), and accessory proteins and regulators involved in nonribosomal peptide (NRP) and polyketide (PK) biosynthesis. Interestingly, the sequences of these genes and their genomic organizations and locations are highly divergent among different S. mutans strains, yet each TnSmu2 region encodes NRPS/PKS and accessory proteins. Mutagenesis of the structural genes and putative regulatory genes in strains UA159, UA140, and MT4653 resulted in colonies that were devoid of their yellow pigmentation (for strains UA140 and MT4653). In addition, these mutant strains also displayed retarded growth under aerobic conditions and in the presence of H(2)O(2). High-performance liquid chromatography profiling of cell surface extracts identified unique peaks that were missing in the mutant strains, and partial characterization of the purified product from UA159 demonstrated that it is indeed a hybrid NRP/PK, as predicted. A genomic survey of 94 clinical S. mutans isolates suggests that the TnSmu2 gene cluster may be more prevalent than previously recognized.

The multifaceted capacity of Dps proteins to combat bacterial stress conditions: Detoxification of iron and hydrogen peroxide and DNA binding

BACKGROUND

The widely expressed Dps proteins, so named after the DNA-binding properties of the first characterized member of the family in Escherichia coli, are considered major players in the bacterial response to stress.

SCOPE OF REVIEW

The review describes the distinctive features of the "ferritin-like" ferroxidation reaction, which uses hydrogen peroxide as physiological iron oxidant and therefore permits the concomitant removal of the two reactants that give rise to hydroxyl radicals via Fenton chemistry. It also illustrates the structural elements identified to date that render the interaction of some Dps proteins with DNA possible and outlines briefly the significance of Dps-DNA complex formation and of the Dps interaction with other DNA-binding proteins in relation to the organization of the nucleoid and microbial survival.

GENERAL SIGNIFICANCE

Understanding in molecular terms the distinctive role of Dps proteins in bacterial resistance to general and specific stress conditions.

MAJOR CONCLUSIONS

The state of the art is that the response to oxidative and peroxide-mediated stress is mediated directly by Dps proteins via their ferritin-like activity. In contrast, the response to other stress conditions derives from the concerted interplay of diverse interactions that Dps proteins may establish with DNA and with other DNA-binding proteins.

Up-expression of NapA and other oxidative stress proteins is a compensatory response to loss of majorHelicobacter pyloristress resistance factors

Twenty-six Helicobacter pylori targeted mutant strains with deficiencies in oxidative stress combating proteins, including 12 double mutant strains were analyzed via physiological and proteomic approaches to distinguish the major expression changes caused by the mutations. Mutations were introduced into both a Mtz(S) and a Mtz(R) strain background. Most of the mutations caused increased growth sensitivity of the strains to oxygen, and they all exhibited clear compensatory up-expression of oxidative stress resistance proteins enabling survival of the bacterium. The most frequent up-expressed oxidative stress resistance factor (observed in 16 of the mutants) was the iron-sequestering protein NapA, linking iron sequestration with oxidative stress resistance. The up-expression of individual proteins in mutants ranged from 2 to 10 fold that of the wild type strain, even when incubated in a low O(2) environment. For example, a considerably higher level of catalase expression (4 fold of that in the wild-type strain) was observed in ahpC napA and ahpC sodB double mutants. A Fur mutant up-expressed ferritin (Pfr) protein 20-fold. In some mutant strains the bacterial DNA is protected from oxidative stress damage apparently via overexpression of oxidative stress-combating proteins such as NapA, catalase or MdaB (an NADPH quinone reductase). Our results show that H. pylori has a variety of ways to compensate for loss of major oxidative stress combating factors.

An iron-binding protein, Dpr, decreases hydrogen peroxide stress and protects Streptococcus pyogenes against multiple stresses

Streptococcus pyogenes does not produce catalase, but it can grow in aerobic environments and survive in the presence of peroxide. One of the stress proteins of this organism, peroxide resistance protein (Dpr), has been studied to examine its role in resistance to hydrogen peroxide, but the protective mechanism of Dpr is not clear. The aim of this study was to characterize the dpr gene and its role in dealing with different stresses. A dpr deletion mutant was constructed by double-crossover mutagenesis. The dpr mutant was more sensitive to H(2)O(2), and complementation could partially restore the defect in the mutant. Pretreatment with the iron chelator deferoxamine mesylate rescued the survival activity of the mutant under oxidative stress conditions. The dpr mutant also showed a low survival rate in the long-term stationary phase, when it was treated with extreme acids, and under alkaline pH conditions compared to the wild-type strain. The growth of the dpr mutant was slower than that of the wild-type strain in iron-limiting conditions. The dpr mutant showed high sensitivity to iron and zinc but not to manganese, copper, nickel, and calcium. Recombinant Dpr protein was purified and showed iron-binding activity, whereas no DNA-binding activity was found. These data indicate that an iron-binding protein, Dpr, provides protection from hydrogen peroxide stress by preventing the Fenton reaction, and Dpr was identified as a novel stress protein that protects against several stresses in group A streptococci.

Differential role of ferritins in iron metabolism and virulence of the plant-pathogenic bacterium Erwinia chrysanthemi 3937

During infection, the phytopathogenic enterobacterium Erwinia chrysanthemi has to cope with iron-limiting conditions and the production of reactive oxygen species by plant cells. Previous studies have shown that a tight control of the bacterial intracellular iron content is necessary for full virulence. The E. chrysanthemi genome possesses two loci that could be devoted to iron storage: the bfr gene, encoding a heme-containing bacterioferritin, and the ftnA gene, coding for a paradigmatic ferritin. To assess the role of these proteins in the physiology of this pathogen, we constructed ferritin-deficient mutants by reverse genetics. Unlike the bfr mutant, the ftnA mutant had increased sensitivity to iron deficiency and to redox stress conditions. Interestingly, the bfr ftnA mutant displayed an intermediate phenotype for sensitivity to these stresses. Whole-cell analysis by Mössbauer spectroscopy showed that the main iron storage protein is FtnA and that there is an increase in the ferrous iron/ferric iron ratio in the ftnA and bfr ftnA mutants. We found that ftnA gene expression is positively controlled by iron and the transcriptional repressor Fur via the small antisense RNA RyhB. bfr gene expression is induced at the stationary phase of growth. The sigmaS transcriptional factor is necessary for this control. Pathogenicity tests showed that FtnA and the Bfr contribute differentially to the virulence of E. chrysanthemi depending on the host, indicating the importance of a perfect control of iron homeostasis in this bacterial species during infection.

Deficiency of the Rgg regulator promotes H2O2 resistance, AhpCF-mediated H2O2 decomposition, and virulence in Streptococcus pyogenes

Streptococcus pyogenes (group A streptococcus [GAS]), a catalase-negative gram-positive bacterium, is aerotolerant and survives H2O2 exposures that kill many catalase-positive bacteria. The molecular basis of the H2O2 resistance is poorly known. Here, we demonstrate that serotype M49 GAS lacking the Rgg regulator is more resistant to H2O2 and also decomposes more H2O2 than the parental strain. Subgenomic transcriptional profiling and genome-integrated green fluorescent protein reporters showed that a bicistronic operon, a homolog of the Streptococcus mutans ahpCF operon, is transcriptionally up-regulated in the absence of Rgg. Phenotypic assays with ahpCF operon knockouts demonstrated that the gene products decompose H2O2 and protect GAS against peroxide stress. In a murine intraperitoneal-infection model, Rgg deficiency increased the virulence of GAS, although in an ahpCF-independent manner. Rgg-mediated repression of H2O2 resistance is divergent from the previously characterized peroxide resistance repressor PerR. Moreover, Rgg-mediated repression of H2O2 resistance is inducible by cellular stresses of diverse natures--ethanol, organic hydroperoxide, and H2O2. Rgg is thus identified as a novel sensoregulator of streptococcal H2O2 resistance with potential implications for the virulence of the catalase-negative GAS.

Peroxiredoxins in bacterial antioxidant defense

Peroxiredoxins constitute an important component of the bacterial defense against toxic peroxides. These enzymes use reactive cysteine thiols to reduce peroxides with electrons ultimately derived from reduced pyridine dinucleotides. Studies examining the regulation and physiological roles of AhpC, Tpx, Ohr and OsmC reveal the multilayered nature of bacterial peroxide defense. AhpC is localized in the cytoplasm and has a wide substrate range that includes H2O2, organic peroxides and peroxynitrite. This enzyme functions in both the control of endogenous peroxides, as well as in the inducible defense response to exogenous peroxides or general stresses. Ohr, OsmC and Tpx are organic peroxide specific. Tpx is localized to the periplasm and can be involved in either constitutive peroxide defense or participate in oxidative stress inducible responses depending on the organism. Ohr is an organic peroxide specific defense system that is under the control of the organic peroxide sensing repressor OhrR. In some organisms Ohr homologs are regulated in response to general stress. Clear evidence indicates that AhpC, Tpx and Ohr are involved in virulence. The role of OsmC is less clear. Regulation of OsmC expression is not oxidative stress inducible, but is controlled by multiple general stress responsive regulators.

Iron incorporation in Streptococcus suis Dps-like peroxide resistance protein Dpr requires mobility in the ferroxidase center and leads to the formation of a ferrihydrite-like core

The Dps-like peroxide resistance protein (Dpr) is a dodecameric protein that protects the human and swine pathogen Streptococcus suis from hydrogen peroxide by removing free Fe2+ from the cytosol. Subsequent oxidation of iron by Dpr results in the deposition of Fe3+ inside the protein's central cavity. Structural changes that occur in the ferroxidase center were studied by X-ray crystallography after soaking Dpr crystals with Fe2+ in the presence of sodium dithionite. Twelve iron-binding sites were identified with each site formed by residues Asp74 and Glu78 from one subunit, and Asp63, His47 and His59 from a 2-fold symmetry-related subunit. Compared to the iron-free Dpr, Asp74 and Glu78 were found to be the most flexible amino acid residues and able to adopt a variety of conformations in different subunits. The crystal structure of an Asp74Ala Dpr mutant soaked with a Fe2+ -solution revealed variations in the Asp63 position and no iron bound to the ferroxidase center. These results indicate an intrinsic flexibility in the active site that may be important for the catalytic reaction and subsequent nucleation events. Two iron cores with remarkably different features were identified in Dpr using X-ray absorption spectroscopy. Purified Dpr was found to have a small-size iron core with only approximately 16 iron atoms/dodecamer forming a ferritin-like ferrihydrite structure. Because of its size, this core represents the smallest iron core identified so far in ferritins and other Dps-like proteins. A large-size core (approximately 180 iron atoms/dodecamer) formed after incubating the protein with a ferrous solution shows differences in iron coordination compared to the small size core. Characterization of the two iron cores in Dpr could provide insights into nucleation events and the mechanism of iron core growth in the Dps family of proteins.

Overexpression and characterization of an iron storage and DNA-binding Dps protein from Trichodesmium erythraeum

Although the role of iron in marine productivity has received a great deal of attention, no iron storage protein has been isolated from a marine microorganism previously. We describe an Fe-binding protein belonging to the Dps family (DNA binding protein from starved cells) in the N(2)-fixing marine cyanobacterium Trichodesmium erythraeum. A dps gene encoding a protein with significant levels of identity to members of the Dps family was identified in the genome of T. erythraeum. This gene codes for a putative Dps(T. erythraeurm) protein (Dps(tery)) with 69% primary amino acid sequence similarity to Synechococcus DpsA. We expressed and purified Dps(tery), and we found that Dps(tery), like other Dps proteins, is able to bind Fe and DNA and protect DNA from degradation by DNase. We also found that Dps(tery) binds phosphate, like other ferritin family proteins. Fe K near-edge X-ray absorption of Dps(tery) indicated that it has an iron core that resembles that of horse spleen ferritin.

Expression and regulation pattern of ferritin-like DpsA in the archaeon Halobacterium Salinarum

Very recently, an iron-rich protein, DpsA, was isolated from the extreme halophilic euryarchaeon Halobacterium salinarum JW5 and characterized. The amino acid sequence of DpsA is related to Dps proteins which belong structurally to the ferritin superfamily but differ from ferritins in their function and regulation. Employing Northern and Western blot analysis, the expression of DpsA in H. salinarum was examined throughout all growth phases and under a variety of growth conditions (iron deficiency, iron supplied growth, oxidative stress). DpsA shows increasing expression of dpsA mRNA in iron-rich media and under conditions of oxidative stress (H(2)O(2)), whereas under iron-deficient conditions mRNA-levels decrease. This is in contrast to Dps-type proteins the transcription of which is induced under conditions of iron starvation. Northern blot experiments show that the expression pattern of halobacterial DpsA is the same as that found in the few bacterial non-heme ferritin the expression pattern of which has been analyzed so far. Based on Western-blot analysis post-transcriptional regulation, typical of mammalian ferritins, can be excluded. This protein exhibits features of a non-heme type bacterial ferritin although it shares only little sequence similarity with Ftn from E. coli.

Listeria monocytogenes ferritin protects against multiple stresses and is required for virulence

In this study, the role of Listeria monocytogenes ferritin was investigated. The fri gene encoding the ferritin was deleted and the phenotype of the mutant was analyzed demonstrating that ferritin is necessary for optimal growth in minimal medium in both presence and absence of iron, as well as after cold- and heat-shock. We also showed that ferritin provides protection against reactive oxygen species and is essential for full virulence of L. monocytogenes. A comparative proteomic analysis revealed an effect of the fri deletion on the levels of listeriolysin O and several stress proteins. Together, our study demonstrates that fri has multiple roles that contribute to Listeria virulence.

A study ofStreptococcus thermophilus proteome by integrated analytical procedures and differential expression investigations

Streptococcus thermophilus is a Gram-positive bacterium belonging to the group of lactic acid bacteria, among which several genera play an essential role in manufacture of food products. Recently, a genomic consortium sequenced and annotated its entire genome, which has been demonstrated to contain 1900 coding sequences. In this study, we have revealed the expression products of almost 200 different genes using a proteomic strategy combining 2-DE plus MALDI-TOF PMF and differential 1-DE plus muLC-ESI-IT-MS/MS. Thus, a number of cellular pathways related to important physiological processes were described at the proteomic level. Almost 50 genes were related to multiple electrophoretic species, whose heterogeneity was mainly due to variability in pI values. A 2-DE reference map obtained for lactose-grown cells was compared with those obtained after heat, cold, acid, oxidative and starvation stresses. Protein up/down-regulation measurements demonstrated that adaptation to different environmental challenges may involve the contribution of unique as well as combined physiological mechanisms. Common regulatory sites in the promoter region of genes whose expression was induced after stress were identified. These results provide a better comprehension of biochemical processes related to stress resistance in S. thermophilus, allowing defining the molecular bases of adaptative responses or markers for the identification of strains with potential industrial applications.

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.