Genetic characterization of a Streptococcus mutans LraI family operon and role in virulence

When oral health affects overall health: biofilms, dental infections, and emerging antimicrobial strategies

Dental health is a crucial component of overall health, yet it is frequently overlooked in discussions about well-being. This article explores the multifaceted aspects of dental infections, primarily focusing on biofilms formed by pathogenic bacteria such as Streptococcus mutans and Porphyromonas gingivalis. These biofilms contribute to dental caries and periodontal disease, conditions that affect oral health and have systemic consequences. Recent advancements in understanding biofilm formation and interactions have led to novel strategies for prevention and treatment, including using nanoparticles and smart hydrogels designed to disrupt biofilm integrity while promoting biocompatibility with human tissues. Furthermore, the article highlights the potential of natural remedies, including herbal extracts, as adjuncts in maintaining oral hygiene and combating microbial infections. A comprehensive overview of biofilm dynamics, including adhesion, maturation, and dispersion, is presented, alongside discussions on innovative therapeutic approaches addressing the limitations of conventional treatments. Ultimately, this article emphasizes the importance of maintaining dental health in preventing a wide spectrum of health issues, reinforcing that the mouth is a gateway to the body.

Small regulatory RNAs are mediators of the Streptococcus mutans SloR regulon

Dental caries is among the most prevalent chronic diseases worldwide. Streptococcus mutans, the chief causative agent of caries, uses a 25-kDa manganese-dependent SloR protein to coordinate the uptake of essential manganese with the transcription of its virulence attributes. Small non-coding RNAs (sRNAs) can either enhance or repress gene expression, and reports in the literature ascribe an emerging role for sRNAs in the environmental stress response. Herein, we focused our attention on 18-50 nt sRNAs as mediators of the S. mutans SloR and manganese regulons. Specifically, the results of RNA sequencing revealed 19 sRNAs in S. mutans, which were differentially transcribed in the SloR-proficient UA159 and SloR-deficient GMS584 strains, and 10 sRNAs that were differentially expressed in UA159 cells grown in the presence of low vs high manganese. We describe SmsR1532 and SmsR1785 as SloR- and manganese-responsive sRNAs that are processed from large transcripts and that bind SloR directly in their promoter regions. The predicted targets of these sRNAs include regulators of metal ion transport, growth management via a toxin-antitoxin operon, and oxidative stress tolerance. These findings support a role for sRNAs in coordinating intracellular metal ion homeostasis with virulence gene control in an important oral cariogen. IMPORTANCE Small regulatory RNAs (sRNAs) are critical mediators of environmental signaling, particularly in bacterial cells under stress, but their role in Streptococcus mutans is poorly understood. S. mutans, the principal causative agent of dental caries, uses a 25-kDa manganese-dependent protein, called SloR, to coordinate the regulated uptake of essential metal ions with the transcription of its virulence genes. In the present study, we identified and characterized sRNAs that are both SloR and manganese responsive. Taken together, this research can elucidate the details of regulatory networks that engage sRNAs in an important oral pathogen and that can enable the development of an effective anti-caries therapeutic.

Small regulatory RNAs are mediators of the Streptococcus mutans SloR regulon

Dental caries is among the most prevalent chronic infectious diseases worldwide. Streptococcus mutans , the chief causative agent of caries, uses a 25 kDa manganese dependent SloR protein to coordinate the uptake of essential manganese with the transcription of its virulence attributes. Small non-coding RNAs (sRNAs) can either enhance or repress gene expression and reports in the literature ascribe an emerging role for sRNAs in the environmental stress response. Herein, we identify 18-50 nt sRNAs as mediators of the S. mutans SloR and manganese regulons. Specifically, the results of sRNA-seq revealed 56 sRNAs in S. mutans that were differentially transcribed in the SloR-proficient UA159 and SloR-deficient GMS584 strains, and 109 sRNAs that were differentially expressed in UA159 cells grown in the presence of low versus high manganese. We describe SmsR1532 and SmsR1785 as SloR- and/or manganese-responsive sRNAs that are processed from large transcripts, and that bind SloR directly in their promoter regions. The predicted targets of these sRNAs include regulators of metal ion transport, growth management via a toxin-antitoxin operon, and oxidative stress tolerance. These findings support a role for sRNAs in coordinating intracellular metal ion homeostasis with virulence gene control in an important oral cariogen.

IMPORTANCE

Small regulatory RNAs (sRNAs) are critical mediators of environmental signaling, particularly in bacterial cells under stress, but their role in Streptococcus mutans is poorly understood. S. mutans, the principal causative agent of dental caries, uses a 25 kDa manganese-dependent protein, called SloR, to coordinate the regulated uptake of essential metal ions with the transcription of its virulence genes. In the present study, we identified and characterize sRNAs that are both SloR- and manganese-responsive. Taken together, this research can elucidate the details of regulatory networks that engage sRNAs in an important oral pathogen, and that can enable the development of an effective anti-caries therapeutic.

Managing Manganese: The Role of Manganese Homeostasis in Streptococcal Pathogenesis

Pathogenic streptococci require manganese for survival in the host. In response to invading pathogens, the host recruits nutritional immune effectors at infection sites to withhold manganese from the pathogens and control bacterial growth. The manganese scarcity impairs several streptococcal processes including oxidative stress defenses, de novo DNA synthesis, bacterial survival, and virulence. Emerging evidence suggests that pathogens also encounter manganese toxicity during infection and manganese excess impacts streptococcal virulence by manganese mismetallation of non-cognate molecular targets involved in bacterial antioxidant defenses and cell division. To counter host-imposed manganese stress, the streptococcal species employ a sophisticated sensory system that tightly coordinates manganese stress-specific molecular strategies to negate host induced manganese stress and proliferate in the host. Here we review the molecular details of host-streptococcal interactions in the battle for manganese during infection and the significance of streptococcal effectors involved to bacterial pathophysiology.

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.

S-glutathionylation proteome profiling reveals a crucial role of a thioredoxin-like protein in interspecies competition and cariogenecity of Streptococcus mutans

S-glutathionylation is an important post-translational modification (PTM) process that targets protein cysteine thiols by the addition of glutathione (GSH). This modification can prevent proteolysis caused by the excessive oxidation of protein cysteine residues under oxidative or nitrosative stress conditions. Recent studies have suggested that protein S-glutathionylation plays an essential role in the control of cell-signaling pathways by affecting the protein function in bacteria and even humans. In this study, we investigated the effects of S-glutathionylation on physiological regulation within Streptococcus mutans, the primary etiological agent of human dental caries. To determine the S-glutathionylated proteins in bacteria, the Cys reactive isobaric reagent iodoacetyl Tandem Mass Tag (iodoTMT) was used to label the S-glutathionylated Cys site, and an anti-TMT antibody-conjugated resin was used to enrich the modified peptides. Proteome profiling identified a total of 357 glutathionylated cysteine residues on 239 proteins. Functional enrichment analysis indicated that these S-glutathionylated proteins were involved in diverse important biological processes, such as pyruvate metabolism and glycolysis. Furthermore, we studied a thioredoxin-like protein (Tlp) to explore the effect of S-glutathionylation on interspecies competition between oral streptococcal biofilms. Through site mutagenesis, it was proved that glutathionylation on Cys41 residue of Tlp is crucial to protect S. mutans from oxidative stress and compete with S. sanguinis and S. gordonii. An addition rat caries model showed that the loss of S-glutathionylation attenuated the cariogenicity of S. mutans. Taken together, our study provides an insight into the S-glutathionylation of bacterial proteins and the regulation of oxidative stress resistance and interspecies competition.

Manganese Uptake, Mediated by SloABC and MntH, Is Essential for the Fitness of Streptococcus mutans

Early epidemiological studies implicated manganese (Mn) as a possible caries-promoting agent, while laboratory studies have indicated that manganese stimulates the expression of virulence-related factors in the dental pathogen Streptococcus mutans To better understand the importance of manganese homeostasis to S. mutans pathophysiology, we first used RNA sequencing to obtain the global transcriptional profile of S. mutans UA159 grown under Mn-restricted conditions. Among the most highly expressed genes were those of the entire sloABC operon, encoding a dual iron/manganese transporter, and an uncharacterized gene, here mntH, that codes for a protein bearing strong similarity to Nramp-type transporters. While inactivation of sloC, which encodes the lipoprotein receptor of the SloABC system, or of mntH alone had no major consequence for the overall fitness of S. mutans, simultaneous inactivation of sloC and mntH (ΔsloC ΔmntH) impaired growth and survival under Mn-restricted conditions, including in human saliva or in the presence of calprotectin. Further, disruption of Mn transport resulted in diminished stress tolerance and reduced biofilm formation in the presence of sucrose. These phenotypes were markedly improved when cells were provided with excess Mn. Metal quantifications revealed that the single mutant strains contained intracellular levels of Mn similar to those seen with the parent strain, whereas Mn was nearly undetectable in the ΔsloC ΔmntH strain. Collectively, these results reveal that SloABC and MntH work independently and cooperatively to promote cell growth under Mn-restricted conditions and that maintenance of Mn homeostasis is essential for the expression of major virulence attributes in S. mutansIMPORTANCE As transition biometals such as manganese (Mn) are essential for all forms of life, the ability to scavenge biometals in the metal-restricted host environment is an important trait of successful cariogenic pathobionts. Here, we showed that the caries pathogen Streptococcus mutans utilizes two Mn transport systems, namely, SloABC and MntH, to acquire Mn from the environment and that the ability to maintain the cellular levels of Mn is important for the manifestation of characteristics that associate S. mutans with dental caries. Our results indicate that the development of strategies to deprive S. mutans of Mn hold promise in the combat against this important bacterial pathogen.

Corynebacterium diphtheriae: Diphtheria Toxin, the tox Operon, and Its Regulation by Fe2+ Activation of apo-DtxR

Diphtheria is one of the most well studied of all the bacterial infectious diseases. These milestone studies of toxigenic Corynebacterium diphtheriae along with its primary virulence determinant, diphtheria toxin, have established the paradigm for the study of other related bacterial protein toxins. This review highlights those studies that have contributed to our current understanding of the structure-function relationships of diphtheria toxin, the molecular mechanism of its entry into the eukaryotic cell cytosol, the regulation of diphtheria tox expression by holo-DtxR, and the molecular basis of transition metal ion activation of apo-DtxR itself. These seminal studies have laid the foundation for the protein engineering of diphtheria toxin and the development of highly potent eukaryotic cell-surface receptor-targeted fusion protein toxins for the treatment of human diseases that range from T cell malignancies to steroid-resistant graft-versus-host disease to metastatic melanoma. This deeper scientific understanding of diphtheria toxin and the regulation of its expression have metamorphosed the third-most-potent bacterial toxin known into a life-saving targeted protein therapeutic, thereby at least partially fulfilling Paul Erlich's concept of a magic bullet-"a chemical that binds to and specifically kills microbes or tumor cells."

Manganese acquisition is essential for virulence of Enterococcus faecalis

Manganese (Mn) is an essential micronutrient that is not readily available to pathogens during infection due to an active host defense mechanism known as nutritional immunity. To overcome this nutrient restriction, bacteria utilize high-affinity transporters that allow them to compete with host metal-binding proteins. Despite the established role of Mn in bacterial pathogenesis, little is known about the relevance of Mn in the pathophysiology of E. faecalis. Here, we identified and characterized the major Mn acquisition systems of E. faecalis. We discovered that the ABC-type permease EfaCBA and two Nramp-type transporters, named MntH1 and MntH2, work collectively to promote cell growth under Mn-restricted conditions. The simultaneous inactivation of EfaCBA, MntH1 and MntH2 (ΔefaΔmntH1ΔmntH2 strain) led to drastic reductions (>95%) in cellular Mn content, severe growth defects in body fluids (serum and urine) ex vivo, significant loss of virulence in Galleria mellonella, and virtually complete loss of virulence in rabbit endocarditis and murine catheter-associated urinary tract infection (CAUTI) models. Despite the functional redundancy of EfaCBA, MntH1 and MntH2 under in vitro or ex vivo conditions and in the invertebrate model, dual inactivation of efaCBA and mntH2 (ΔefaΔmntH2 strain) was sufficient to prompt maximal sensitivity to calprotectin, a Mn- and Zn-chelating host antimicrobial protein, and for the loss of virulence in mammalian models. Interestingly, EfaCBA appears to play a prominent role during systemic infection, whereas MntH2 was more important during CAUTI. The different roles of EfaCBA and MntH2 in these sites could be attributed, at least in part, to the differential expression of efaA and mntH2 in cells isolated from hearts or from bladders. Collectively, this study demonstrates that Mn acquisition is essential for the pathogenesis of E. faecalis and validates Mn uptake systems as promising targets for the development of new antimicrobials.

Autoregulation of the Streptococcus mutans SloR Metalloregulator Is Constitutive and Driven by an Independent Promoter

Streptococcus mutans, one of ∼600 bacterial species in the human oral cavity, is among the most acidogenic constituents of the plaque biofilm. Considered to be the primary causative agent of dental caries, S. mutans harbors a 25-kDa SloR metalloregulatory protein which controls metal ion transport across the bacterial cell membrane to maintain essential metal ion homeostasis. The expression of SloR derives in part from transcriptional readthrough of the sloABC operon, which encodes a Mn²⁺/Fe²⁺ ABC transport system. Here we describe the details of the sloABC promoter that drives this transcription as well as those for a novel independent promoter in an intergenic region (IGR) that contributes to downstream sloR expression. Reverse transcriptase PCR (RT-PCR) studies support the occurrence of sloR transcription that is independent of sloABC expression, and the results of 5' rapid amplification of cDNA ends (5' RACE) revealed a sloR transcription start site in the IGR, from which the -10 and -35 promoter regions were predicted. The results of gel mobility shift assays support direct SloR binding to the IGR, albeit with a lower affinity than that for SloR binding to the sloABCR promoter. The function of the sloR promoter was validated by semiquantitative real-time PCR (qRT-PCR) experiments. Interestingly, sloR expression was not significantly affected when bacteria were grown in the presence of a high manganese concentration, whereas expression of the sloABC operon was repressed under these conditions. The results of in vitro transcription studies support the occurrence of SloR-mediated transcriptional activation of sloR and repression of sloABC Taken together, these findings implicate SloR as a bifunctional regulator that represses sloABC promoter activity and encourages sloR transcription from an independent promoter.IMPORTANCE Tooth decay is a ubiquitous infectious disease that is especially pervasive in underserved communities worldwide. S. mutans-induced carious lesions cause functional, physical, and/or esthetic impairment in the vast majority of adults and in 60 to 90% of schoolchildren in industrialized countries. Billions of dollars are spent annually on caries treatment, and productivity losses due to absenteeism from the workplace are significant. Research aimed at alleviating S. mutans-induced tooth decay is important because it can address the socioeconomic disparity that is associated with dental cavities and improve overall general health, which is inextricably linked to oral health. Research focused on the S. mutans SloR metalloregulatory protein can guide the development of novel therapeutics and thus alleviate the burden of dental cavities.

Contribution of Streptococcus mutans Strains with Collagen-Binding Proteins in the Presence of Serum to the Pathogenesis of Infective Endocarditis

Streptococcus mutans, a major pathogen of dental caries, is considered one of the causative agents of infective endocarditis (IE). Recently, bacterial DNA encoding 120-kDa cell surface collagen-binding proteins (CBPs) has frequently been detected from S. mutans-positive IE patients. In addition, some of the CBP-positive S. mutans strains lacked a 190-kDa protein antigen (PA), whose absence strengthened the adhesion to and invasion of endothelial cells. The interaction between pathogenic bacteria and serum or plasma is considered an important virulence factor in developing systemic diseases; thus, we decided to analyze the pathogenesis of IE induced by S. mutans strains with different patterns of CBP and PA expression by focusing on the interaction with serum or plasma. CBP-positive (CBP⁺)/PA-negative (PA^-) strains showed prominent aggregation in the presence of human serum or plasma, which was significantly greater than that with CBP⁺/PA-positive (PA⁺) and CBP-negative (CBP^-)/PA+ strains. Aggregation of CBP⁺/PA^- strains was also observed in the presence of a high concentration of type IV collagen, a major extracellular matrix protein in serum. In addition, aggregation of CBP⁺/PA^- strains was drastically reduced when serum complement was inactivated. Furthermore, an ex vivo adherence model and an in vivo rat model of IE showed that extirpated heart valves infected with CBP⁺/PA^- strains displayed prominent bacterial mass formation, which was not observed following infection with CBP⁺/PA⁺ and CBP^-/PA⁺ strains. These results suggest that CBP⁺/PA^-S. mutans strains utilize serum to contribute to their pathogenicity in IE.

Effects of Arginine on Streptococcus mutans Growth, Virulence Gene Expression, and Stress Tolerance

Streptococcus mutans is a common constituent of oral biofilms and a primary etiologic agent of human dental caries. The bacteria associated with dental caries have potent abilities to produce organic acids from dietary carbohydrates and to grow and metabolize in acidic conditions. By contrast, many commensal bacteria produce ammonia through the arginine deiminase system (ADS), which moderates the pH of oral biofilms. Arginine metabolism by the ADS is a significant deterrent to the initiation and progression of dental caries. In this study, we observed how exogenously provided l-arginine affects the growth, the virulence properties, and the tolerance of environmental stresses of S. mutans Supplementation with 1.5% arginine (final concentration) had an inhibitory effect on the growth of S. mutans in complex and chemically defined media, particularly when cells were exposed to acid or oxidative stress. The genes encoding virulence factors required for attachment/accumulation (gtfB and spaP), bacteriocins (nlmA, nlmB, nlmD, and cipB), and the sigma factor required for competence development (comX) were downregulated during growth with 1.5% arginine. Deep sequencing of RNA (RNA-Seq) comparing the transcriptomes of S. mutans growing in chemically defined media with and without 1.5% arginine revealed differential expression of genes encoding ATP-binding cassette transporters, metal transporters, and constituents required for survival, metabolism, and biofilm formation. Therefore, the mechanisms of action by which arginine inhibits dental caries include direct adverse effects on multiple virulence-related properties of the most common human dental caries pathogen.IMPORTANCE Metabolism of the amino acid arginine by the arginine deiminase system (ADS) of certain oral bacteria raises the pH of dental plaque and provides a selective advantage to health-associated bacteria, thereby protecting the host from dental caries (cavities). Here, we examine the effects of arginine on the cavity-causing bacterium Streptococcus mutans We find that arginine negatively impacts the growth, the pathogenic potential, and the tolerance of environmental stresses in a way that is likely to compromise the ability of S. mutans to cause disease. Using genetic and genomic techniques, multiple mechanisms by which arginine exerts its influence on virulence-related properties of S. mutans are discovered. This report demonstrates that a primary mechanism of action by which arginine inhibits the initiation and progression of dental caries may be by reducing the pathogenic potential of S. mutans.

The SloR metalloregulator is involved in the Streptococcus mutans oxidative stress response

SloR, a 25-kDa metalloregulatory protein in Streptococcus mutans modulates the expression of multiple genes, including the sloABC operon that encodes essential Mn²⁺ transport and genes that promote cariogenesis. In this study, we report on SloC- and SloR-deficient strains of S. mutans (GMS284 and GMS584, respectively) that demonstrate compromised survivorship compared with their UA159 wild-type progenitor and their complemented strains (GMS285 and GMS585, respectively), when challenged with streptonigrin and/or in growth competition experiments. The results of streptonigrin assays revealed significantly larger zones of inhibition for GMS584 than for either UA159 or GMS585, indicating weakened S. mutans survivorship in the absence of SloR. Competition assays revealed a compromised ability for GMS284 and GMS584 to survive peroxide challenge compared with their SloC- and SloR-proficient counterparts. These findings are consistent with a role for SloC and SloR in S. mutans aerotolerance. We also predicted differential expression of oxidative stress tolerance genes in GMS584 versus UA159 and GMS585 when grown aerobically. The results of quantitative RT-PCR experiments revealed S. mutans sod, tpx, and sloC expression that was upregulated in GMS584 compared with UA159 and GMS585, indicating that the impact of oxidative stress on S. mutans is more severe in the absence of SloR than in its presence. The results of electrophoretic mobility shift assays indicate that SloR does not bind to the sod or tpx promoter regions directly, implicating intermediaries that may arbitrate the SloR response to oxidative stress.

Iron-dependent gene expression in Actinomyces oris

Background

Actinomyces oris is a Gram-positive bacterium that has been associated with healthy and diseased sites in the human oral cavity. Most pathogenic bacteria require iron to survive, and in order to acquire iron in the relatively iron-scarce oral cavity A. oris has been shown to produce iron-binding molecules known as siderophores. The genes encoding these siderophores and transporters are thought to be regulated by the amount of iron in the growth medium and by the metal-dependent repressor, AmdR, which we showed previously binds to the promoter of proposed iron-regulated genes.

Objective

The purpose of this study was to characterize siderophore and associated iron transport systems in A. oris.

Design

We examined gene expression of the putative iron transport genes fetA and sidD in response to low- and high-iron environments. One of these genes, sidD, encoding a putative Fe ABC transporter protein, was insertionally inactivated and was examined for causing growth defects. To gain a further understanding of the role of iron metabolism in oral diseases, clinical isolates of Actinomyces spp. were examined for the presence of the gene encoding AmdR, a proposed global regulator of iron-dependent gene expression in A. oris.

Results

When A. oris was grown under iron-limiting conditions, the genes encoding iron/siderophore transporters fetA and sidD showed increased expression. One of these genes (sidD) was mutated, and the sidD::Km strain exhibited a 50% reduction in growth in late log and stationary phase cells in media that contained iron. This growth defect was restored when the sidD gene was provided in a complemented strain. We were able to isolate the AmdR-encoding gene in seven clinical isolates of Actinomyces. When these protein sequences were aligned to the laboratory strain, there was a high degree of sequence similarity.

Conclusions

The growth of the sidD::Km mutant in iron-replete medium mirrored the growth of the wild-type strain grown in iron-limiting medium, suggesting that the sidD::Km mutant was compromised in iron uptake. The known iron regulator AmdR is well conserved in clinical isolates of A. oris. This work provides additional insight into iron metabolism in this important oral microbe.

Ni2+-Dependent and PsaR-Mediated Regulation of the Virulence Genes pcpA, psaBCA, and prtA in Streptococcus pneumoniae

Previous studies have shown that the transcriptional regulator PsaR regulates the expression of the PsaR regulon consisting of genes encoding choline binding protein (PcpA), the extracellular serine protease (PrtA), and the Mn2+-uptake system (PsaBCA), in the presence of manganese (Mn2+), zinc (Zn2+), and cobalt (Co2+). In this study, we explore the Ni2+-dependent regulation of the PsaR regulon. We have demonstrated by qRT-PCR analysis, metal accumulation assays, β-galactosidase assays, and electrophoretic mobility shift assays that an elevated concentration of Ni2+ leads to strong induction of the PsaR regulon. Our ICP-MS data show that the Ni2+-dependent expression of the PsaR regulon is directly linked to high, cell-associated, concentration of Ni2+, which reduces the cell-associated concentration of Mn2+. In vitro studies with the purified PsaR protein showed that Ni2+ diminishes the Mn2+-dependent interaction of PsaR to the promoter regions of its target genes, confirming an opposite effect of Mn2+ and Ni2+ in the regulation of the PsaR regulon. Additionally, the Ni2+-dependent role of PsaR in the regulation of the PsaR regulon was studied by transcriptome analysis.

Interactions of the Metalloregulatory Protein SloR from Streptococcus mutans with Its Metal Ion Effectors and DNA Binding Site

Streptococcus mutans is the causative agent of dental caries, a significant concern for human health, and therefore an attractive target for therapeutics development. Previous work in our laboratory has identified a homodimeric, manganese-dependent repressor protein, SloR, as an important regulator of cariogenesis and has used site-directed mutagenesis to map functions to specific regions of the protein. Here we extend those studies to better understand the structural interaction between SloR and its operator and its effector metal ions. The results of DNase I assays indicate that SloR protects a 42-bp region of DNA that overlaps the sloABC promoter on the S. mutans UA159 chromosome, while electrophoretic mobility shift and solution binding assays indicate that each of two SloR dimers binds to this region. Real-time semiquantitative reverse transcriptase PCR (real-time semi-qRT-PCR) experiments were used to determine the individual base pairs that contribute to SloR-DNA binding specificity. Solution studies indicate that Mn(2+) is better than Zn(2+) at specifically activating SloR to bind DNA, and yet the 2.8-Å resolved crystal structure of SloR bound to Zn(2+) provides insight into the means by which selective activation by Mn(2+) may be achieved and into how SloR may form specific interactions with its operator. Taken together, these experimental observations are significant because they can inform rational drug design aimed at alleviating and/or preventing S. mutans-induced caries formation.

IMPORTANCE

This report focuses on investigating the SloR protein as a regulator of essential metal ion transport and virulence gene expression in the oral pathogen Streptococcus mutans and on revealing the details of SloR binding to its metal ion effectors and binding to DNA that together facilitate this expression. We used molecular and biochemical approaches to characterize the interaction of SloR with Mn(2+) and with its SloR recognition element to gain a clearer picture of the regulatory networks that optimize SloR-mediated metal ion homeostasis and virulence gene expression in S. mutans. These experiments can have a significant impact on caries treatment and/or prevention by revealing the S. mutans SloR-DNA binding interface as an appropriate target for the development of novel therapeutic interventions.

Manganese uptake and streptococcal virulence

Streptococcal solute-binding proteins (SBPs) associated with ATP-binding cassette transporters gained widespread attention first as ostensible adhesins, next as virulence determinants, and finally as metal ion transporters. In this mini-review, we will examine our current understanding of the cellular roles of these proteins, their contribution to metal ion homeostasis, and their crucial involvement in mediating streptococcal virulence. There are now more than 35 studies that have collected structural, biochemical and/or physiological data on the functions of SBPs across a broad range of bacteria. This offers a wealth of data to clarify the formerly puzzling and contentious findings regarding the metal specificity amongst this group of essential bacterial transporters. In particular we will focus on recent findings related to biological roles for manganese in streptococci. These advances will inform efforts aimed at exploiting the importance of manganese and manganese acquisition for the design of new approaches to combat serious streptococcal diseases.

In vitro manganese-dependent cross-talk between Streptococcus mutans VicK and GcrR: implications for overlapping stress response pathways

Streptococcus mutans, a major acidogenic component of the dental plaque biofilm, has a key role in caries etiology. Previously, we demonstrated that the VicRK two-component signal transduction system modulates biofilm formation, oxidative stress and acid tolerance responses in S. mutans. Using in vitro phosphorylation assays, here we demonstrate for the first time, that in addition to activating its cognate response regulator protein, the sensor kinase, VicK can transphosphorylate a non-cognate stress regulatory response regulator, GcrR, in the presence of manganese. Manganese is an important micronutrient that has been previously correlated with caries incidence, and which serves as an effector of SloR-mediated metalloregulation in S. mutans. Our findings supporting regulatory effects of manganese on the VicRK, GcrR and SloR, and the cross-regulatory networks formed by these components are more complex than previously appreciated. Using DNaseI footprinting we observed overlapping DNA binding specificities for VicR and GcrR in native promoters, consistent with these proteins being part of the same transcriptional regulon. Our results also support a role for SloR as a positive regulator of the vicRK two component signaling system, since its transcription was drastically reduced in a SloR-deficient mutant. These findings demonstrate the regulatory complexities observed with the S. mutans manganese-dependent response, which involves cross-talk between non-cognate signal transduction systems (VicRK and GcrR) to modulate stress response pathways.

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.

The Expression of the fim Operon Is Crucial for the Survival of Streptococcus parasanguinis FW213 within Macrophages but Not Acid Tolerance

The acquisition of transition metal ions is essential for the viability and in some cases the expression of virulence genes in bacteria. The fimCBA operon of Streptococcus parasanguinis FW213 encodes a Mn²⁺/Fe²⁺-specific ATP-binding cassette transporter. FimA, a lipoprotein in the system, is essential for the development of endocarditis, presumably by binding to fibrin monolayers on the damaged heart tissue. Recent sequence analysis revealed that Spaf_0344 was homologous to Streptococcus gordonii scaR, encoding a metalloregulatory protein for the Sca Mn²⁺-specific transporter. Based on the homology, Spaf_0344 was designated fimR. By using various fim promoter (p_fim) derivatives fused with a promoterless chloramphenicol acetyltransferase gene, the functions of the cis-elements of p_fim were analyzed in the wild-type and fimR-deficient hosts. The result indicated that FimR represses the expression of p_fim and the palindromic sequences 5′ to fimC are involved in repression of p_fim. A direct interaction between FimR and the palindromic sequences was further confirmed by in vitro electrophoresis gel mobility shift assay and in vivo chromatin immunoprecipitation assay (ChIP)-quantitative real-time PCR (qPCR). The result of the ChIP-qPCR analysis also indicated that FimR is activated by Mn²⁺ and, to a lesser degree, Fe²⁺. Functional analysis indicated that the expression of FimA in S. parasanguinis was critical for wild-type levels of survival against oxidative stress and within phagocytes, but not for acid tolerance. Taken together, in addition to acting as an adhesin (FimA), the expression of the fim operon is critical for the pathogenic capacity of S. parasanguinis.

The role of ATP-binding cassette transporters in bacterial pathogenicity

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

Natural history of dental plaque accumulation in mechanically ventilated adults: a descriptive correlational study

OBJECTIVE

The purpose of this study was to describe the pattern of dental plaque accumulation in mechanically ventilated adults. Accumulation of dental plaque and bacterial colonisation of the oropharynx is associated with a number of systemic diseases including ventilator associated pneumonia.

RESEARCH METHODOLOGY/DESIGN

Data were collected from mechanically ventilated critically ill adults (n=137), enrolled within 24 hours of intubation. Dental plaque, counts of decayed, missing and filled teeth and systemic antibiotic use was assessed on study days 1, 3, 5 and 7. Dental plaque averages per study day, tooth type and tooth location were analysed.

SETTING

Medical respiratory, surgical trauma and neuroscience ICU's of a large tertiary care centre in the southeast United States.

RESULTS

Plaque: all surfaces >60% plaque coverage from day 1 to day 7; molars and premolars contained greatest plaque average >70%. Systemic antibiotic use on day 1 had no significant effect on plaque accumulation on day 3 (p=0.73).

CONCLUSIONS

Patients arrive in critical care units with preexisting oral hygiene issues. Dental plaque tends to accumulate in the posterior teeth (molars and premolars) that may be hard for nurses to visualise and reach; this problem may be exacerbated by endotracheal tubes and other equipment. Knowing accumulation trends of plaque will guide the development of effective oral care protocols.

Sequencing and comparative genome analysis of two pathogenic Streptococcus gallolyticus subspecies: genome plasticity, adaptation and virulence

Streptococcus gallolyticus infections in humans are often associated with bacteremia, infective endocarditis and colon cancers. The disease manifestations are different depending on the subspecies of S. gallolyticus causing the infection. Here, we present the complete genomes of S. gallolyticus ATCC 43143 (biotype I) and S. pasteurianus ATCC 43144 (biotype II.2). The genomic differences between the two biotypes were characterized with comparative genomic analyses. The chromosome of ATCC 43143 and ATCC 43144 are 2,36 and 2,10 Mb in length and encode 2246 and 1869 CDS respectively. The organization and genomic contents of both genomes were most similar to the recently published S. gallolyticus UCN34, where 2073 (92%) and 1607 (86%) of the ATCC 43143 and ATCC 43144 CDS were conserved in UCN34 respectively. There are around 600 CDS conserved in all Streptococcus genomes, indicating the Streptococcus genus has a small core-genome (constitute around 30% of total CDS) and substantial evolutionary plasticity. We identified eight and five regions of genome plasticity in ATCC 43143 and ATCC 43144 respectively. Within these regions, several proteins were recognized to contribute to the fitness and virulence of each of the two subspecies. We have also predicted putative cell-surface associated proteins that could play a role in adherence to host tissues, leading to persistent infections causing sub-acute and chronic diseases in humans. This study showed evidence that the S. gallolyticus still possesses genes making it suitable in a rumen environment, whereas the ability for S. pasteurianus to live in rumen is reduced. The genome heterogeneity and genetic diversity among the two biotypes, especially membrane and lipoproteins, most likely contribute to the differences in the pathogenesis of the two S. gallolyticus biotypes and the type of disease an infected patient eventually develops.

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

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

Genome-wide characterization of the SloR metalloregulome in Streptococcus mutans

Streptococcus mutans is the primary causative agent of human dental caries, a ubiquitous infectious disease for which effective treatment strategies remain elusive. We investigated a 25-kDa SloR metalloregulatory protein in this oral pathogen, along with its target genes that contribute to cariogenesis. Previous studies have demonstrated manganese- and SloR-dependent repression of the sloABCR metal ion transport operon in S. mutans. In the present study, we demonstrate that S. mutans coordinates this repression with that of certain virulence attributes. Specifically, we noted virulence gene repression in a manganese-containing medium when SloR binds to promoter-proximal sequence palindromes on the S. mutans chromosome. We applied a genome-wide approach to elucidate the sequences to which SloR binds and to reveal additional "class I" genes that are subject to SloR- and manganese-dependent repression. These analyses identified 204 S. mutans genes that are preceded by one or more conserved palindromic SloR recognition elements (SREs). We cross-referenced these genes with those that we had identified previously as SloR and/or manganese modulated in microarray and real-time quantitative reverse transcription-PCR (qRT-PCR) experiments. From this analysis, we identified a number of S. mutans virulence genes that are subject to transcriptional upregulation by SloR and noted that such "class II"-type regulation is dependent on direct SloR binding to promoter-distal SREs. These observations are consistent with a bifunctional role for the SloR metalloregulator and implicate it as a target for the development of therapies aimed at alleviating S. mutans-induced caries formation.

Characterization and structure of the manganese-responsive transcriptional regulator ScaR

The streptococcal coaggregation regulator (ScaR) of Streptococcus gordonii is a manganese-dependent transcriptional regulator. When intracellular manganese concentrations become elevated, ScaR represses transcription of the scaCBA operon, which encodes a manganese uptake transporter. A member of the DtxR/MntR family of metalloregulators, ScaR shares sequence similarity with other family members, and many metal-binding residues are conserved. Here, we show that ScaR is an active dimer, with two dimers binding the 46 base pair scaC operator. Each ScaR subunit binds two manganese ions, and the protein is activated by a variety of other metal ions, including Cd(2+), Co(2+), and Ni(2+) but not Zn(2+). The crystal structure of apo-ScaR reveals a tertiary and quaternary structure similar to its homologue, the iron-responsive regulator DtxR. While each DtxR subunit binds a metal ion in two sites, labeled primary and ancillary, crystal structures of ScaR determined in the presence of Cd(2+) and Zn(2+) show only a single occupied metal-binding site that is novel to ScaR. The site analogous to the primary site in DtxR is unoccupied, and the ancillary site is absent from ScaR. Instead, metal ions bind to ScaR at a site labeled "secondary", which is composed of Glu80, Cys123, His125, and Asp160 and lies roughly 5 A away from where the ancillary site would be predicted to exist. This difference suggests that ScaR and its closely related homologues are activated by a mechanism distinct from that of either DtxR or MntR.

Streptococcus adherence and colonization

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

Development of genetic tools for in vivo virulence analysis of Streptococcus sanguinis

Completion of the genome sequence of Streptococcus sanguinis SK36 necessitates tools for further characterization of this species. It is often desirable to insert antibiotic resistance markers and other exogenous genes into the chromosome; therefore, we sought to identify a chromosomal site for ectopic expression of foreign genes, and to verify that insertion into this site did not affect important cellular phenotypes. We designed three plasmid constructs for insertion of erm, aad9 or tetM resistance determinants into a genomic region encoding only a small (65 aa) hypothetical protein. To determine whether this insertion affected important cellular properties, SK36 and its erythromycin-resistant derivative, JFP36, were compared for: (i) growth in vitro, (ii) genetic competence, (iii) biofilm formation and (iv) virulence for endocarditis in the rabbit model of infective endocarditis (IE). The spectinomycin-resistant strain, JFP56, and tetracycline-resistant strain, JFP76, were also tested for virulence in vivo. Insertion of erm did not affect growth, competence or biofilm development of JFP36. Recovery of bacteria from heart valves of co-inoculated rabbits was similar to wild-type for JFP36, JFP56 and JFP76, indicating that IE virulence was not significantly affected. The capacity for mutant complementation in vivo was explored in an avirulent ssaB mutant background. Expression of ssaB from its predicted promoter in the target region restored IE virulence. Thus, the chromosomal site utilized is a good candidate for further manipulations of S. sanguinis. In addition, the resistant strains developed may be further applied as controls to facilitate screening for virulence factors in vivo.

Contribution of lipoproteins and lipoprotein processing to endocarditis virulence in Streptococcus sanguinis

Streptococcus sanguinis is an important cause of infective endocarditis. Previous studies have identified lipoproteins as virulence determinants in other streptococcal species. Using a bioinformatic approach, we identified 52 putative lipoprotein genes in S. sanguinis strain SK36 as well as genes encoding the lipoprotein-processing enzymes prolipoprotein diacylglyceryl transferase (lgt) and signal peptidase II (lspA). We employed a directed signature-tagged mutagenesis approach to systematically disrupt these genes and screen each mutant for the loss of virulence in an animal model of endocarditis. All mutants were viable. In competitive index assays, mutation of a putative phosphate transporter reduced in vivo competitiveness by 14-fold but also reduced in vitro viability by more than 20-fold. Mutations in lgt, lspA, or an uncharacterized lipoprotein gene reduced competitiveness by two- to threefold in the animal model and in broth culture. Mutation of ssaB, encoding a putative metal transporter, produced a similar effect in culture but reduced in vivo competiveness by >1,000-fold. [(3)H]palmitate labeling and Western blot analysis confirmed that the lgt mutant failed to acylate lipoproteins, that the lspA mutant had a general defect in lipoprotein cleavage, and that SsaB was processed differently in both mutants. These results indicate that the loss of a single lipoprotein, SsaB, dramatically reduces endocarditis virulence, whereas the loss of most other lipoproteins or of normal lipoprotein processing has no more than a minor effect on virulence.

Molecular analyses of bacterial DNA in extirpated heart valves from patients with infective endocarditis

BACKGROUND/AIMS

Infective endocarditis (IE) is caused by a microbial infection of the endothelial surface of the heart. Although blood culture examinations are commonly used to determine the associated bacterial species, molecular techniques, which enable rapid identification of targeted bacterial species, have recently been applied in clinical cases.

METHODS

Nine heart valve specimens from IE patients (six subacute cases and three acute cases) were extirpated and collected, then bacterial DNA was extracted. Bacterial species in the specimens were determined by two different molecular methods and the results were compared with those from a conventional blood culture technique. In addition, a comparison between the two molecular methods was carried out using known numbers of six streptococcal species.

RESULTS

The conventional blood culture method revealed the bacterial species in eight cases, while one was found to be negative. Multiple species were identified in most of the cases by both molecular methods; however, those specified by one method were not always consistent with those specified by the other. Furthermore, the species determined by the blood culture technique were not always identified by the molecular methods. We also found that the two molecular methods used in the present study were extremely sensitive to detect from 1 to 100 cells of individual oral streptococcal species.

CONCLUSION

Our results suggest that species specified by molecular methods may have disseminated incidentally into the bloodstream, so interpretation of such results should be carefully undertaken in clinical situations.

Opposite effects of Mn2+ and Zn2+ on PsaR-mediated expression of the virulence genes pcpA, prtA, and psaBCA of Streptococcus pneumoniae

Homeostasis of Zn(2+) and Mn(2+) is important for the physiology and virulence of the human pathogen Streptococcus pneumoniae. Here, transcriptome analysis was used to determine the response of S. pneumoniae D39 to a high concentration of Zn(2+). Interestingly, virulence genes encoding the choline binding protein PcpA, the extracellular serine protease PrtA, and the Mn(2+) uptake system PsaBC(A) were strongly upregulated in the presence of Zn(2+). Using random mutagenesis, a previously described Mn(2+)-responsive transcriptional repressor, PsaR, was found to mediate the observed Zn(2+)-dependent derepression. In addition, PsaR is also responsible for the Mn(2+)-dependent repression of these genes. Subsequently, we investigated how these opposite effects are mediated by the same regulator. In vitro binding of purified PsaR to the prtA, pcpA, and psaB promoters was stimulated by Mn(2+), whereas Zn(2+) destroyed the interaction of PsaR with its target promoters. Mutational analysis of the pcpA promoter demonstrated the presence of a PsaR operator that mediates the transcriptional effects. In conclusion, PsaR is responsible for the counteracting effects of Mn(2+) and Zn(2+) on the expression of several virulence genes in S. pneumoniae, suggesting that the ratio of these metal ions exerts an important influence on pneumococcal pathogenesis.

Streptococcus pneumoniae surface protein PcpA elicits protection against lung infection and fatal sepsis

Previous studies have suggested that pneumococcal choline binding protein A (PcpA) is important for the full virulence of Streptococcus pneumoniae, and its amino acid sequence suggests that it may play a role in cellular adherence. PcpA is under the control of a manganese-dependent regulator and is only expressed at low manganese concentrations, similar to those found in the blood and lungs. PcpA expression is repressed under high manganese concentrations, similar to those found in secretions. In this study, we have demonstrated that PcpA elicits statistically significant protection in murine models of pneumonia and sepsis. In the model of pneumonia with each of four challenge strains, statistically fewer S. pneumoniae cells were recovered from the lungs of mice immunized with PcpA and alum versus mice immunized with alum only. The immunizations reduced the median CFU by 4- to 400-fold (average of 28-fold). In the model of sepsis using strain TIGR4, PcpA expression resulted in shorter times to become moribund and subcutaneous immunization with PcpA increased survival times of mice infected with wild-type PcpA-expressing pneumococci.

Manganese affects Streptococcus mutans virulence gene expression

BACKGROUND/AIMS

Studies of trace metals in drinking water and tooth enamel have suggested a caries-promoting potential for manganese (Mn). Additionally, Mn has been shown to be essential for the expression of mutans streptococci virulence factors such as the glucan-binding lectin (GBL) of Streptococcus sobrinus. The Streptococcus mutans glucan-binding protein (Gbp) GbpC is the functional analogue of the S. sobrinus GBL. S. mutans Gbps have been shown to contribute to biofilm architecture and virulence. This study was undertaken to examine the effects of Mn on the transcription of genes encoding S. mutans Gbps, including gbpC, along with other critical S. mutans virulence genes.

METHODS

Microarray analyses suggested the potential for an Mn effect on Gbp genes. Further investigation of the Mn effects on selected genes was undertaken by performing Northern blots, Western blots, and RT-PCR under conditions of planktonic and biofilm growth in Mn-depleted media or in media containing 50 mircoM Mn.

RESULTS

Mn resulted in increased expression of gbpC and gtfB, and decreased expression of wapA, in both planktonic and biofilm cultures. The expression levels of gbpA and gbpD were also decreased in the presence of Mn, but only in biofilms. The expression of gtfC was increased in the presence of Mn only in planktonic cultures. The spaP gene was expressed more highly in Mn-supplemented planktonic cultures but less in Mn-supplemented biofilms.

CONCLUSION

Mn availability affects the expression of multiple S. mutans genes involved in adhesion and biofilm formation. Furthermore, these effects depend on the growth state of the organism.

Effects of manganese on Streptococcus mutans planktonic and biofilm growth

Streptococcus mutans, an agent of dental caries, was tested for growth in the presence or absence of manganese (Mn), since studies have linked Mn levels with cariogenic potential. Seven S. mutans serotype c strains were grown in chemically defined medium under different atmospheric conditions: 5% CO2, O2-enriched 5% CO2 (shaking) and anaerobic. There was significant strain variability with respect to Mn requirements under the various conditions tested. Both sucrose-dependent and sucrose-independent biofilm growth by strain UA159 were affected by the absence of Mn. S. mutans strains show highly variable responses to both high and low Mn concentrations.

Manganese transport and the role of manganese in virulence

Two areas of research have recently converged to highlight important roles for Mn(2+) in pathogenesis: the recognition that both bacterial Nramp homologs and members of LraI family of proteins are Mn(2+) transporters. Their mutation is associated with decreased virulence of various bacterial species. Thus, Mn(2+) appears to be essential for bacterial virulence. This review describes what is currently known about Mn(2+) transport in prokaryotes and how prokaryotic Mn(2+) transport is regulated. Some of the phenotypes that arise when microorganisms lack Mn(2+) are then discussed, with an emphasis on those phenotypes involving pathogenesis. The concluding section describes possible enzymatic roles for Mn(2+) that might help explain why Mn(2+) is necessary for virulence.

The SloR/Dlg metalloregulator modulates Streptococcus mutans virulence gene expression

Metal ion availability in the human oral cavity plays a putative role in Streptococcus mutans virulence gene expression and in appropriate formation of the plaque biofilm. In this report, we present evidence that supports such a role for the DtxR-like SloR metalloregulator (called Dlg in our previous publications) in this oral pathogen. Specifically, the results of gel mobility shift assays revealed the sloABC, sloR, comDE, ropA, sod, and spaP promoters as targets of SloR binding. We confirmed differential expression of these genes in a GMS584 SloR-deficient mutant versus the UA159 wild-type progenitor by real-time semiquantitative reverse transcriptase PCR experiments. The results of additional expression studies support a role for SloR in S. mutans control of glucosyltransferases, glucan binding proteins, and genes relevant to antibiotic resistance. Phenotypic analysis of GMS584 revealed that it forms aberrant biofilms on an abiotic surface, is compromised for genetic competence, and demonstrates heightened incorporation of iron and manganese as well as resistance to oxidative stress compared to the wild type. Taken together, these findings support a role for SloR in S. mutans adherence, biofilm formation, genetic competence, metal ion homeostasis, oxidative stress tolerance, and antibiotic gene regulation, all of which contribute to S. mutans-induced disease.

Transcription of the contiguous sigB, dtxR, and galE genes in Corynebacterium diphtheriae: evidence for multiple transcripts and regulation by environmental factors

The iron-dependent transcriptional regulator DtxR from Corynebacterium diphtheriae is the prototype for a family of metal-dependent regulators found in diverse bacterial species. The structure of DtxR and its action as a repressor have been extensively characterized, but little is known about expression of dtxR. In the current study, we investigated transcription of dtxR as well as the sigB and galE genes located immediately upstream and downstream from dtxR, respectively. We identified two promoters that determine transcription of dtxR. The first, located upstream of sigB, appears to be controlled by an extracytoplasmic function sigma factor. The second, located in the intergenic region between sigB and dtxR, is similar to promoters used by the primary vegetative sigma factors in other actinomycete species. Using quantitative real-time assays, we demonstrated that the number of transcripts initiated upstream from sigB is affected by several environmental factors. In contrast, the presence of sodium dodecyl sulfate was the only factor tested that conclusively affects the number of transcripts initiated in the sigB-dtxR intergenic region. Additionally, we provided evidence for the existence of transcripts that contain sigB, dtxR, and galE. Our studies provide the first quantitative transcriptional analysis of a gene encoding a DtxR family regulator and give new insights into transcriptional regulation in C. diphtheriae.

Mn2+-dependent regulation of multiple genes in Streptococcus pneumoniae through PsaR and the resultant impact on virulence

The concentration of Mn2+ is 1,000-fold higher in secretions than it is at internal sites of the body, making it a potential signal by which bacteria can sense a shift from a mucosal environment to a more invasive site. PsaR, a metal-dependent regulator in Streptococcus pneumoniae, was found to negatively affect the transcription of psaBCA, pcpA, rrgA, rrgB, rrgC, srtBCD, and rlrA in the presence of Mn2+. psaBCA encode an ABC-type transporter for Mn2+. pcpA, rrgA, rrgB, and rrgC encode several outer surface proteins. srtBCD encode a cluster of sortase enzymes, and rlrA encodes a transcriptional regulator. Steady-state RNA levels are high under low Mn2+ concentrations in the wild-type strain and are elevated under both high and low Mn2+ concentrations in a psaR mutant strain. RlrA is an activator of rrgA, rrgB, rrgC, and srtBCD (D. Hava and A. Camilli, Mol. Microbiol. 45:1389-1406, 2002), suggesting that PsaR may indirectly control these genes through rlrA, while PsaR-dependent repression of psaBCA, pcpA, and rlrA transcription is direct. The impact of Mn2+-dependent regulation on virulence was further examined in mouse models of pneumonia and nasopharyngeal carriage. The abilities of DeltapsaR, pcpA, and DeltapsaR DeltapcpA mutant strains to colonize the lung were reduced compared to those of the wild type, confirming that both PcpA-mediated gene regulation and PsaR-mediated gene regulation are required for full virulence in the establishment of pneumonia. Neither PcpA nor PsaR was found to be required for colonization of the nasopharynx in a carriage model. This is the first demonstration of Mn2+ acting as a signal for the expression of virulence factors within different host sites.

Up-regulation of competence- but not stress-responsive proteins accompanies an altered metabolic phenotype in Streptococcus mutans biofilms

Mature biofilm and planktonic cells of Streptococcus mutans cultured in a neutral pH environment were subjected to comparative proteome analysis. Of the 242 protein spots identified, 48 were significantly altered in their level of expression (P<0.050) or were unique to planktonic or biofilm-grown cells. Among these were four hypothetical proteins as well as proteins known to be associated with the maintenance of competence or found to possess a cin-box-like element upstream of their coding gene. Most notable among the non-responsive genes were those encoding the molecular chaperones DnaK, GroEL and GroES, which are considered to be up-regulated by sessile growth. Analysis of the rest of the proteome indicated that a number of cellular functions associated with carbon uptake and cell division were down-regulated. The data obtained were consistent with the hypothesis that a reduction in the general growth rate of mature biofilms of S. mutans in a neutral pH environment is associated with the maintenance of transformation without the concomitant stress response observed during the transient state of competence in bacterial batch cultures.

Identification of virulence determinants for endocarditis in Streptococcus sanguinis by signature-tagged mutagenesis

Streptococcus sanguinis is a gram-positive, facultative anaerobe and a normal inhabitant of the human oral cavity. It is also one of the most common agents of infective endocarditis, a serious endovascular infection. To identify virulence factors for infective endocarditis, signature-tagged mutagenesis (STM) was applied to the SK36 strain of S. sanguinis, whose genome is being sequenced. STM allows the large-scale creation, in vivo screening, and recovery of a series of mutants with altered virulence. Screening of 800 mutants by STM identified 38 putative avirulent and 5 putative hypervirulent mutants. Subsequent molecular analysis of a subset of these mutants identified genes encoding undecaprenol kinase, homoserine kinase, anaerobic ribonucleotide reductase, adenylosuccinate lyase, and a hypothetical protein. Virulence reductions ranging from 2-to 150-fold were confirmed by competitive index assays. One putatively hypervirulent strain with a transposon insertion in an intergenic region was identified, though increased virulence was not confirmed in competitive index assays. All mutants grew comparably to SK36 in aerobic broth culture except for the homoserine kinase mutant. Growth of this mutant was restored by the addition of threonine to the medium. Mutants containing an insertion or in-frame deletion in the anaerobic ribonucleotide reductase gene failed to grow under strictly anaerobic conditions. The results suggest that housekeeping functions such as cell wall synthesis, amino acid and nucleic acid synthesis, and the ability to survive under anaerobic conditions are important virulence factors in S. sanguinis endocarditis.

Both Corynebacterium diphtheriae DtxR(E175K) and Mycobacterium tuberculosis IdeR(D177K) are dominant positive repressors of IdeR-regulated genes in M. tuberculosis

The diphtheria toxin repressor (DtxR) is an important iron-dependent transcriptional regulator of known virulence genes in Corynebacterium diphtheriae. The mycobacterial iron-dependent repressor (IdeR) is phylogenetically closely related to DtxR, with high amino acid similarity in the DNA binding and metal ion binding site domains. We have previously shown that an iron-insensitive, dominant-positive dtxR(E175K) mutant allele from Corynebacterium diphtheriae can be expressed in Mycobacterium tuberculosis and results in an attenuated phenotype in mice. In this paper, we report the M. tuberculosis IdeR(D177K) strain that has the cognate point mutation. We tested four known and predicted IdeR-regulated gene promoters (mbtI, Rv2123, Rv3402c, and Rv1519) using a promoterless green fluorescent protein (GFP) construct. GFP expression from these promoters was abrogated under low-iron conditions in the presence of both IdeR(D177K) and DtxR(E175K), a result confirmed by reverse transcription-PCR. The IdeR regulon can be constitutively repressed in the presence of an integrated copy of ideR containing this point mutation. These data also suggest that mutant IdeR(D177K) has a mechanism similar to that of DtxR(E175K); iron insensitivity occurs as a result of SH3-like domain binding interactions that stabilize the intermediate form of the repressor after ancillary metal ion binding. This construct can be used to elucidate further the IdeR regulon and its virulence genes and to differentiate these from genes regulated by SirR, which does not have this domain.

Endocarditis-associated oral streptococci promote rapid differentiation of monocytes into mature dendritic cells

Endocarditis is frequently attributable to oral streptococci, but mechanisms of pathogenesis are not well understood, although monocytes appear to be important. High titers of interleukin-12 (IL-12) are produced by peripheral blood mononuclear cells (PBMC) after engaging Streptococcus mutans, but monocytes in developing endocardial vegetations tend to disappear rather than become macrophages. These data prompted the hypothesis that streptococcus-infected monocytes differentiate into short-lived IL-12-producing dendritic cells (DCs) rather than macrophages. PBMC from healthy subjects were stimulated with six isolates of oral streptococci, three nonstreptococcal oral bacteria, or IL-4 plus granulocyte-macrophage colony-stimulating factor, and the appearance of cells with markers typical of mature DCs (CD83(+), CD86(+), CD11c(+), and CD14(-)) was monitored. Supernatant fluids from the PBMC cultures were harvested and IL-12 p70 levels were determined. S. mutans-stimulated monocytes were analyzed for their ability to elicit allogeneic mixed-lymphocyte reactions. All streptococci examined, except one strain of Streptococcus oralis (35037), rapidly induced up-regulation of CD83 and CD86 and a loss of CD14 in the CD11c(+) monocyte population within 20 h. Induction of IL-12 was CD14 dependent and correlated with streptococcal isolates that promoted the DC phenotype. Major histocompatibility complex (MHC) class II expression was up-regulated by S. mutans, and these cells were short-lived and elicited potent allogeneic mixed-lymphocyte reactions typical of DCs. In summary, monocytes stimulated with endocarditis-associated oral streptococci rapidly exhibited the DC phenotype and functions. These data suggest that the initiation of bacterial endocarditis by oral streptococci may involve monocyte-to-DC differentiation, and this may help explain the low levels of macrophages in the site.

An improved arbitrary primed PCR method for rapid characterization of transposon insertion sites

Modifications were made to published arbitrary primed polymerase chain reaction (AP-PCR) procedures that resulted in increased specificity and sensitivity. Several arbitrary primer sequences were also evaluated, resulting in recommendations for primer design.

Contribution of glucan-binding protein C of Streptococcus mutans to bacteremia occurrence

Our previous analysis of major cell surface proteins of Streptococcus mutans isolated from the blood of a patient with bacteremia showed variations of glucan-binding protein C (GbpC) expression. In the present study, we analyzed the contribution of GbpC of S. mutans to bacteremia occurrence. A GbpC-defective mutant strain (C1) was significantly less susceptible to phagocytosis by human polymorphonuclear leukocytes than its parent strain (MT8148) (P < 0.001). When 21 rats were injected with C1 or streptomycin-resistant MT8148R into the jugular vein, strain C1 was recovered from blood in larger numbers and for a longer duration than MT8148R. Further, infection with C1 resulted in significant increases in serum sialic acid (SSA) concentrations, and splenomegaly, as well as body weight reduction. We also evaluated GbpC expression in 20 clinical oral isolates by immunoblotting with anti-GbpC serum, and found that expression intensity was positively correlated to phagocytosis rate (P < 0.05). These results suggest that S. mutans GbpC may be associated with systemic virulence, since a weak expression of GbpC causes the organisms to be refractory to phagocytosis, resulting in a longer survival of the bacterium in the bloodstream.

Iron-regulatory proteins DmdR1 and DmdR2 of Streptomyces coelicolor form two different DNA-protein complexes with iron boxes

In high G+C Gram-positive bacteria, the control of expression of genes involved in iron metabolism is exerted by a DmdR [divalent (bivalent) metal-dependent regulatory protein] in the presence of Fe2+ or other bivalent ions. The dmdR1 and dmdR2 genes of Streptomyces coelicolor were overexpressed in Escherichia coli and the DmdR1 and DmdR2 proteins were purified to homogeneity. Electrophoretic mobility-shift assays showed that both DmdR1 and DmdR2 bind to the 19-nt tox and desA iron boxes forming two different complexes in each case. Increasing the concentrations of DmdR1 or DmdR2 protein shifted these complexes from their low-molecular-mass form to the high-molecular-mass complexes. Formation of the DNA-protein complexes was prevented by the bivalent metal chelating agent 2,2'-dipyridyl and by antibodies specific against the DmdR proteins. Cross-linking with glutaraldehyde of pure DmdR1 or DmdR2 proteins showed that DmdR1 forms dimers, whereas DmdR2 is capable of forming dimers and probably tetramers. Ten different iron boxes were found in a search for iron boxes in the genome of S. coelicolor. Most of them correspond to putative genes involved in siderophore biosynthesis. Since the nucleotide sequence of these ten boxes is identical (or slightly different) with the synthetic DNA fragment containing the desA box used in the present study, it is proposed that DmdR1 and DmdR2 bind to the iron boxes upstream of at least ten different genes in S. coelicolor.

A two-component system that controls the expression of pneumococcal surface antigen A (PsaA) and regulates virulence and resistance to oxidative stress in Streptococcus pneumoniae

Recent genomic-based studies have identified 13 two-component signal transduction systems (TCS) in Streptococcus pneumoniae. Bacterial TCSs are important for regulating expression of bacterial genes, including those which are important to the virulence of pathogenic bacteria. We have used virulence assays together with microarray analysis to investigate the importance of pneumococcal TCS04 in the virulence and gene regulation of this pathogen. Deletion mutants of the response regulator of TCS04, rr04, were examined in three independent pneumococcal strains representing three different pneumococcal serotypes. Analysis of the virulence of the three strains enabled us to identify a serotype-specific attenuation of virulence due to deletion of rr04. Microarray comparison of the transcriptional profiles of the wild-type strains with the rr04 mutants allowed us to determine which transcriptional changes were occurring in the rr04 mutants. Virulence-associated changes were demonstrated in the attenuated strain with significant downregulation of a previously determined virulence locus, psaB, psaC and psaA.