Genetic competence and transformation in oral streptococci

Tetracyclic homoisoflavanoid (+)-brazilin: a natural product inhibits c-di-AMP-producing enzyme and Streptococcus mutans biofilms

The tenacious biofilms formed by Streptococcus mutans are resistant to conventional antibiotics and current treatments. There is a growing need for novel therapeutics that selectively inhibit S. mutans biofilms while preserving the normal oral microenvironment. Previous studies have shown that increased levels of cyclic di-AMP, an important secondary messenger synthesized by diadenylate cyclase (DAC), favored biofilm formation in S. mutans. Thus, targeting S. mutans DAC is a novel strategy to inhibit S. mutans biofilms. We screened a small NCI library of natural products using a fluorescence detection assay. (+)-Brazilin, a tetracyclic homoisoflavanoid found in the heartwood of Caesalpinia sappan, was identified as one of the 11 "hits," with the greatest reduction (>99%) in fluorescence at 100 µM. The smDAC inhibitory profiles of the 11 "hits" established by a quantitative high-performance liquid chromatography assay revealed that (+)-brazilin had the most enzymatic inhibitory activity (87% at 100 µM) and was further studied to determine its half maximal inhibitory concentration (IC50 = 25.1 ± 0.98 µM). (+)-Brazilin non-competitively inhibits smDAC's enzymatic activity (Ki = 140.0 ± 27.13 µM), as determined by a steady-state Michaelis-Menten kinetics assay. In addition, (+)-brazilin's binding profile with smDAC (Kd = 11.87 µM) was illustrated by a tyrosine intrinsic fluorescence quenching assay. Furthermore, at low micromolar concentrations, (+)-brazilin selectively inhibited the biofilm of S. mutans (IC50 = 21.0 ± 0.60 µM) and other oral bacteria. S. mutans biofilms were inhibited by a factor of 105 in colony-forming units when treated with 50 µM (+)-brazilin. In addition, a significant dose-dependent reduction in extracellular DNA and glucan levels was evident by fluorescence microscopy imaging of S. mutans biofilms exposed to different concentrations of (+)-brazilin. Furthermore, colonization of S. mutans on a representative model of enamel using suspended hydroxyapatite discs showed a >90% reduction with 50 µM (+)-brazilin. In summary, we have identified a drug-like natural product inhibitor of S. mutans biofilm that not only binds to smDAC but can also inhibit the function of smDAC. (+)-Brazilin could be a good candidate for further development as a potent therapeutic for the prevention and treatment of dental caries.IMPORTANCEThis study represents a significant advancement in our understanding of potential therapeutic options for combating cariogenic biofilms produced by Streptococcus mutans. The research delves into the use of (+)-brazilin, a natural product, as a potent inhibitor of Streptococcus mutans' diadenylate cyclase (smDAC), an enzyme crucial in the formation of biofilms. The study establishes (+)-brazilin as a non-competitive inhibitor of smDAC while providing initial insights into its binding mechanism. What makes this finding even more promising is that (+)-brazilin does not limit its inhibitory effects to S. mutans alone. Instead, it demonstrates efficacy in hindering biofilms in other oral bacteria as well. The broader spectrum of anti-biofilm activity suggests that (+)-brazilin could potentially serve as a versatile tool in a natural product-based treatment for combating a range of conditions caused by resilient biofilms.

Multitasking functions of bacterial extracellular DNA in biofilms

Bacterial biofilms are intricate ecosystems of microbial communities that adhere to various surfaces and are enveloped by an extracellular matrix composed of polymeric substances. Within the context of bacterial biofilms, extracellular DNA (eDNA) originates from cell lysis or is actively secreted, where it exerts a significant influence on the formation, stability, and resistance of biofilms to environmental stressors. The exploration of eDNA within bacterial biofilms holds paramount importance in research, with far-reaching implications for both human health and the environment. An enhanced understanding of the functions of eDNA in biofilm formation and antibiotic resistance could inspire the development of strategies to combat biofilm-related infections and improve the management of antibiotic resistance. This comprehensive review encapsulates the latest discoveries concerning eDNA, encompassing its origins, functions within bacterial biofilms, and significance in bacterial pathogenesis.

Nattokinase, a Subtilisin-like Alkaline-Serine Protease, Reduces Mutacin Activity by Inactivating the Competence-Stimulating Peptide in Streptococcus mutans

Streptococcus mutans is a major cariogenic organism because of its ability to form biofilms on tooth surfaces. Bacteriocins produced by S. mutans (known as mutacins) are indirect pathogenic factors that play a role in the persistence of this microbe in the oral environment. Nattokinase, a subtilisin-like alkaline serine protease, potently inhibits biofilm formation without affecting S. mutans growth. However, effective strategies utilizing nattokinase to control mutacin production by S. mutans are lacking. In this study, we evaluated the effect of nattokinase on mutacin activity in 46 strains of S. mutans with different mutacin genotypes isolated from the dental plaques of pediatric patients with caries. Nattokinase reduced the activity of mutacin against oral streptococci at a concentration of 1 mg/mL in all clinical isolates. Furthermore, nattokinase reduced the expression of non-lantibiotic mutacin structural genes (nlmABCD) and inactivated the extracellular competence-stimulating peptide involved in comDE activation, which regulates non-lantibiotic mutacin gene expression. These results suggest that nattokinase may reduce the virulence of S. mutans and could potentially be used as a new caries-preventive agent as an alternative to conventional drug treatments.

The Distribution of Eight Antimicrobial Resistance Genes in Streptococcus oralis, Streptococcus sanguinis, and Streptococcus gordonii Strains Isolated from Dental Plaque as Oral Commensals

It has been proposed that oral commensal bacteria are potential reservoirs of a wide variety of antimicrobial resistance genes (ARGs) and could be the source of pathogenic bacteria; however, there is scarce information regarding this. In this study, three common streptococci of the mitis group (S. oralis, S. sanguinis, and S. gordonii) isolated from dental plaque (DP) were screened to identify if they were frequent reservoirs of specific ARGs (blaTEM, cfxA, tetM, tetW, tetQ, ermA, ermB, and ermC). DP samples were collected from 80 adults; one part of the sample was cultured, and from the other part DNA was obtained for first screening of the three streptococci species and the ARGs of interest. Selected samples were plated and colonies were selected for molecular identification. Thirty identified species were screened for the presence of the ARGs. From those selected, all of the S. sanguinis and S. oralis carried at least three, while only 30% of S. gordonii strains carried three or more. The most prevalent were tetM in 73%, and blaTEM and tetW both in 66.6%. On the other hand, ermA and cfxA were not present. Oral streptococci from the mitis group could be considered frequent reservoirs of specifically tetM, blaTEM, and tetW. In contrast, these three species appear not to be reservoirs of ermA and cfxA.

Spatial Correlations and Distribution of Competence Gene Expression in Biofilms of Streptococcus mutans

Streptococcus mutans is an important pathogen in the human oral biofilm. It expresses virulent behaviors that are linked to its genetic competence regulon, which is controlled by comX. Expression of comX is modulated by two diffusible signaling peptides, denoted CSP and XIP, and by other environmental cues such as pH and oxidative stress. The sensitivity of S. mutans competence to environmental inputs that may vary on microscopic length scales raises the question of whether the biofilm environment creates microniches where competence and related phenotypes are concentrated, leading to spatial clustering of S. mutans virulence behaviors. We have used two-photon microscopy to characterize the spatial distribution of comX expression among individual S. mutans cells in biofilms. By analyzing correlations in comX activity, we test for spatial clustering that may suggest localized competence microenvironments. Our data indicate that both competence-signaling peptides diffuse efficiently through the biofilm. XIP elicits a population-wide response. CSP triggers a Poisson-like, spatially random comX response from a subpopulation of cells that is homogeneously dispersed. Our data indicate that competence microenvironments if they exist are small enough that the phenotypes of individual cells are not clustered or correlated to any greater extent than occurs in planktonic cultures.

Pharmacology of T2R Mediated Host-Microbe Interactions

Bitter taste receptors (T2Rs) belong to the G protein-coupled receptor superfamily. Humans express 25 T2Rs that are known to detect several bitter compounds including bacterial quorum sensing molecules (QSM). Primarily found to be key receptors for bitter sensation T2Rs are known to play an important role in mediating innate immune responses in oral and extraoral tissues. Several studies have led to identification of Gram-negative and Gram-positive bacterial QSMs as agonists for T2Rs in airway epithelial cells and immune cells. However, the pharmacological characterization for many of the QSM-T2R interactions remains poorly defined. In this chapter, we discuss the extraoral roles including localization of T2Rs in extracellular vesicles, molecular pharmacology of QSM-T2R interactions, role of T2Rs in mediating innate immune responses, and some of the challenges in understanding T2R pharmacology.

Detection and expression analysis of tet(B) in Streptococcus oralis

Tetracycline resistance can be achieved through tet genes, which code for efflux pumps, ribosomal protection proteins and inactivation enzymes. Some of these genes have only been described in either Gram-positive or Gram-negative bacteria. This is the case of tet(B), which codes for an efflux pump and, so far, had only been found in Gram-negative bacteria. In this study, tet(B) was detected in two clinical Streptococcus oralis strains isolated from the gingival sulci of two subjects. In both cases, the gene was completely sequenced, yielding 100% shared identity and coverage with other previously published sequences of tet(B). Moreover, we studied the expression of tet(B) using RT-qPCR in the isolates grown with and without tetracycline, detecting constitutive expression in only one of the isolates, with no signs of expression in the other one.

This is the first time that the presence and expression of the tet(B) gene has been confirmed in Gram-positive bacteria, which highlights the potential of the genus Streptococcus to become a reservoir and a disseminator of antibiotic resistance genes in an environment so prone to horizontal gene transfer as is the oral biofilm.

The Impairment of Methyl Metabolism From luxS Mutation of Streptococcus mutans

The luxS gene is present in a wide range of bacteria and is involved in many cellular processes. LuxS mutation can cause autoinducer(AI)-2 deficiency and methyl metabolism disorder. The objective of this study was to demonstrate that, in addition to AI-2-mediated quorum sensing (QS), methyl metabolism plays an important role in LuxS regulation in Streptococcus mutans. The sahH gene from Pseudomonas aeruginosa was amplified and introduced into the S. mutans luxS-null strain to complement the methyl metabolism disruption in a defective QS phenotype. The intracellular activated methyl cycle (AMC) metabolites [S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), homocysteine (HCY), and methionine] were quantified in wild-type S. mutans and its three derivatives to determine the metabolic effects of disrupting the AMC. Biofilm mass and structure, acid tolerance, acid production, exopolysaccharide synthesis of multispecies biofilms and the transcriptional level of related genes were determined. The results indicated that SAH and SAM were relatively higher in S. mutans luxS-null strain and S. mutans luxS null strain with plasmid pIB169 when cultured overnight, and HCY was significantly higher in S. mutans UA159. Consistent with the transcriptional profile, luxS deletion-mediated impairment of biofilm formation and acid tolerance was restored to wild-type levels using transgenic SahH. These results also suggest that methionine methyl metabolism contributes to LuxS regulation in S. mutans to a significant degree.

Genotypic diversity and phenotypic traits of Streptococcus mutans isolates and their relation to severity of early childhood caries

BACKGROUND

Early childhood caries (ECC) is an aggressive condition that can affect teeth of young children. This study aimed to evaluate genotypic diversity and phenotypic traits of S. mutans isolated from dental biofilms of children with different caries status in comparison with caries free (CF) children.

METHODS

Streptococcus mutans strains were isolated from supragingival biofilm samples of CF, ECC and severe-ECC (S-ECC) children and genotyped by arbitrary-primer polymerase chain reaction - AP-PCR. S. mutans genotypes were tested for their ability to reduce the suspension pH through glycolysis, to tolerate extreme acid challenge and by their ability to form biofilm. Response variables were analyzed by ANOVA/Tukey or Kruskal-Wallis/Mann-Whitney tests at a 5% of significance.

RESULTS

There was an increase in the prevalence of Streptococcus mutans in biofilms with the severity of dental caries. No differences in genotypic diversity and in acidogenicity of genotypes were found among CF, ECC and S-ECC children. S mutans strains with genotypes more characteristic for ECC and S-ECC children formed more biofilms than those identified in CF children. The strains isolated from S-ECC children were highly acid tolerant.

CONCLUSION

Although S. mutans genotypic diversity was similar among the groups of children, phenotypic traits of S. mutans, especially the acid tolerance response, could explain the severity of early childhood caries.

Comparative genomics and evolution of the amylase-binding proteins of oral streptococci

Background

Successful commensal bacteria have evolved to maintain colonization in challenging environments. The oral viridans streptococci are pioneer colonizers of dental plaque biofilm. Some of these bacteria have adapted to life in the oral cavity by binding salivary α-amylase, which hydrolyzes dietary starch, thus providing a source of nutrition. Oral streptococcal species bind α-amylase by expressing a variety of amylase-binding proteins (ABPs). Here we determine the genotypic basis of amylase binding where proteins of diverse size and function share a common phenotype.

Results

ABPs were detected in culture supernatants of 27 of 59 strains representing 13 oral Streptococcus species screened using the amylase-ligand binding assay. N-terminal sequences from ABPs of diverse size were obtained from 18 strains representing six oral streptococcal species. Genome sequencing and BLAST searches using N-terminal sequences, protein size, and key words identified the gene associated with each ABP. Among the sequenced ABPs, 14 matched amylase-binding protein A (AbpA), 6 matched amylase-binding protein B (AbpB), and 11 unique ABPs were identified as peptidoglycan-binding, glutamine ABC-type transporter, hypothetical, or choline-binding proteins. Alignment and phylogenetic analyses performed to ascertain evolutionary relationships revealed that ABPs cluster into at least six distinct, unrelated families (AbpA, AbpB, and four novel ABPs) with no phylogenetic evidence that one group evolved from another, and no single ancestral gene found within each group. AbpA-like sequences can be divided into five subgroups based on the N-terminal sequences. Comparative genomics focusing on the abpA gene locus provides evidence of horizontal gene transfer.

Conclusion

The acquisition of an ABP by oral streptococci provides an interesting example of adaptive evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-1005-7) contains supplementary material, which is available to authorized users.

Effectiveness of a polyhexanide irrigation solution on methicillin-resistant Staphylococcus aureus biofilms in a porcine wound model

Irrigation and removal of necrotic debris can be beneficial for proper healing. It is becoming increasingly evident that wounds colonized with biofilm forming bacteria, such as Staphylococcus aureus (SA), can be more difficult to eradicate. Here we report our findings of the effects of an irrigation solution containing propyl-betaine and polyhexanide (PHMB) on methicillin-resistant Staphylococcus aureus (MRSA) biofilms in a porcine wound model. Thirty-nine deep partial thickness wounds were created with six wounds assigned to one of six treatment groups: (i) PHMB, (ii) Ringer's solution, (iii) hypochlorous acid/sodium hypochlorite, (iv) sterile water, (v) octenidine dihydrochloride, and (vi) octenilin. Wounds were inoculated with MRSA and covered with a polyurethane dressing for 24 hours to allow biofilm formation. The dressings were then removed and the wounds were irrigated twice daily for 3 days with the appropriate solution. MRSA from four wounds were recovered from each treatment group at 3 days and 6 days hours after initial treatment. Irrigation of wounds with the PHMB solution resulted in 97·85% and 99·64% reductions of MRSA at the respective 3 days and 6 days assessment times when compared to the untreated group. Both of these reductions were statistically significant compared to all other treatment groups (P values <0·05).

An Essential Role for (p)ppGpp in the Integration of Stress Tolerance, Peptide Signaling, and Competence Development in Streptococcus mutans

The microbes that inhabit the human oral cavity are subjected to constant fluctuations in their environment. To overcome these challenges and gain a competitive advantage, oral streptococci employ numerous adaptive strategies, many of which appear to be intertwined with the development of genetic competence. Here, we demonstrate that the regulatory circuits that control development of competence in Streptococcus mutans, a primary etiological agent of human dental caries, are integrated with key stress tolerance pathways by the molecular alarmone (p)ppGpp. We first observed that the growth of a strain that does not produce (p)ppGpp (ΔrelAPQ, (p)ppGpp⁰) is not sensitive to growth inhibition by comXinducing peptide (XIP), unlike the wild-type strain UA159, even though XIP-dependent activation of the alternative sigma factor comX by the ComRS pathway is not impaired in the (p)ppGpp⁰ strain. Overexpression of a (p)ppGpp synthase gene (relP) in the (p)ppGpp⁰ mutant restored growth inhibition by XIP. We also demonstrate that exposure to micromolar concentrations of XIP elicited changes in (p)ppGpp accumulation in UA159. Loss of the RelA/SpoT homolog (RSH) enzyme, RelA, lead to higher basal levels of (p)ppGpp accumulation, but to decreased sensitivity to XIP and to decreases in comR promoter activity and ComX protein levels. By introducing single amino acid substitutions into the RelA enzyme, the hydrolase activity of the enzyme was shown to be crucial for full com gene induction and transformation by XIP. Finally, loss of relA resulted in phenotypic changes to ΔrcrR mutants, highlighted by restoration of transformation and ComX protein production in the otherwise non-transformable ΔrcrR-NP mutant. Thus, RelA activity and its influence on (p)ppGpp pools appears to modulate competence signaling and development through RcrRPQ and the peptide effectors encoded within rcrQ. Collectively, this study provides new insights into the molecular mechanisms that integrate intercellular communication with the physiological status of the cells and the regulation of key virulence-related phenotypes in S. mutans.

Effects of Carbohydrate Source on Genetic Competence in Streptococcus mutans

The capacity to internalize and catabolize carbohydrates is essential for dental caries pathogens to persist and cause disease. The expression of many virulence-related attributes by Streptococcus mutans, an organism strongly associated with human dental caries, is influenced by the peptide signaling pathways that control genetic competence. Here, we demonstrate a relationship between the efficiency of competence signaling and carbohydrate source. A significant increase in the activity of the promoters for comX, comS, and comYA after exposure to competence-stimulating peptide (CSP) was observed in cells growing on fructose, maltose, sucrose, or trehalose as the primary carbohydrate source, compared to cells growing on glucose. However, only cells grown in the presence of trehalose or sucrose displayed a significant increase in transformation frequency. Notably, even low concentrations of these carbohydrates in the presence of excess glucose could enhance the expression of comX, encoding a sigma factor needed for competence, and the effects on competence were dependent on the cognate sugar:phosphotransferase permease for each carbohydrate. Using green fluorescent protein (GFP) reporter fusions, we observed that growth in fructose or trehalose resulted in a greater proportion of the population activating expression of comX and comS, encoding the precursor of comX-inducing peptide (XIP), after addition of CSP, than growth in glucose. Thus, the source of carbohydrate significantly impacts the stochastic behaviors that regulate subpopulation responses to CSP, which can induce competence in S. mutans

IMPORTANCE

The signaling pathways that regulate development of genetic competence in Streptococcus mutans are intimately intertwined with the pathogenic potential of the organism, impacting biofilm formation, stress tolerance, and expression of known virulence determinants. Induction of the gene for the master regulator of competence, ComX, by competence-stimulating peptide (CSP) occurs in a subpopulation of cells. Here, we show that certain carbohydrates that are common in the human diet enhance the ability of CSP to activate transcription of comX and that a subset of these carbohydrates stimulates progression to the competent state. The cognate sugar:phosphotransferase permeases for each sugar are needed for these effects. Interestingly, single-cell analysis shows that the carbohydrates that increase com gene expression do so by enhancing the proportion of cells that respond to CSP. A mathematical model is developed to explain how carbohydrates modulate bistable behavior in the system via the ComRS pathway and ComX stability.

Cyclic di-AMP mediates biofilm formation

Cyclic di-AMP (c-di-AMP) is an emerging second messenger in bacteria. It has been shown to play important roles in bacterial fitness and virulence. However, transduction of c-di-AMP signaling in bacteria and the role of c-di-AMP in biofilm formation are not well understood. The level of c-di-AMP is modulated by activity of di-adenylyl cyclase that produces c-di-AMP and phosphodiesterase (PDE) that degrades c-di-AMP. In this study, we determined that increased c-di-AMP levels by deletion of the pdeA gene coding for a PDE promoted biofilm formation in Streptococcus mutans. Deletion of pdeA upregulated expression of gtfB, the gene coding for a major glucan producing enzyme. Inactivation of gtfB blocked the increased biofilm by the pdeA mutant. Two c-di-AMP binding proteins including CabPA (SMU_1562) and CabPB (SMU_1708) were identified. Interestingly, only CabPA deficiency inhibited both the increased biofilm formation and the upregulated expression of GtfB observed in the pdeA mutant. In addition, CabPA but not CabPB interacted with VicR, a known transcriptional factor that regulates expression of gtfB, suggesting that a signaling link between CabPA and GtfB through VicR. Increased biofilm by the pdeA deficiency also enhanced bacterial colonization of Drosophila in vivo. Taken together, our studies reveal a new role of c-di-AMP in mediating biofilm formation through a CabPA/VicR/GtfB signaling network in S. mutans.

Sharply Tuned pH Response of Genetic Competence Regulation in Streptococcus mutans: a Microfluidic Study of the Environmental Sensitivity of comX

Genetic competence in Streptococcus mutans is a transient state that is regulated in response to multiple environmental inputs. These include extracellular pH and the concentrations of two secreted peptides, designated CSP (competence-stimulating peptide) and XIP (comX-inducing peptide). The role of environmental cues in regulating competence can be difficult to disentangle from the effects of the organism's physiological state and its chemical modification of its environment. We used microfluidics to control the extracellular environment and study the activation of the key competence gene comX. We find that the comX promoter (PcomX) responds to XIP or CSP only when the extracellular pH lies within a narrow window, about 1 pH unit wide, near pH 7. Within this pH range, CSP elicits a strong PcomX response from a subpopulation of cells, whereas outside this range the proportion of cells expressing comX declines sharply. Likewise, PcomX is most sensitive to XIP only within a narrow pH window. While previous work suggested that comX may become refractory to CSP or XIP stimulus as cells exit early exponential phase, our microfluidic data show that extracellular pH dominates in determining sensitivity to XIP and CSP. The data are most consistent with an effect of pH on the ComR/ComS system, which has direct control over transcription of comX 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.

A unique open reading frame within the comX gene of Streptococcus mutans regulates genetic competence and oxidative stress tolerance

Streptococcus mutans displays complex regulation of genetic competence, with ComX controlling late competence gene transcription. The rcrRPQ operon has been shown to link oxidative stress tolerance, (p)ppGpp metabolism and competence in S. mutans. Importantly, an rcrR polar (ΔrcrR-P) mutant is hyper-transformable, but an rcrR non-polar (ΔrcrR-NP) mutant cannot be transformed. Transcriptome comparisons of the rcrR mutants using RNA-Seq and quantitative real-time polymerase chain reaction revealed little expression in the 5' region of comX in ΔrcrR-NP, but high level expression in the 3' region. Northern blotting with comX probes revealed two distinct transcripts in the ΔrcrR-P and ΔrcrR-NP strains, and 5' Rapid Amplification of cDNA Ends mapped the 5' terminus of the shorter transcript to nt +140 of the comX structural gene, where a unique 69-aa open reading frame, termed XrpA, was encoded in a different reading frame than ComX. Two single-nucleotide substitution mutants (comX::T162C; comX::T210A) were introduced to disrupt XrpA without affecting the sequence of ComX. When the mutations were in the ΔrcrR-NP genetic background, ComX production and transformation were restored. Overexpression of xrpA led to impaired growth in aerobic conditions and decreased transformability. These results reveal an unprecedented mechanism for competence regulation and stress tolerance by a gene product encoded within the comX gene that appears unique to S. mutans.

Genetic determinants of reutericyclin biosynthesis in Lactobacillus reuteri

Reutericyclin is a unique antimicrobial tetramic acid produced by some strains of Lactobacillus reuteri. This study aimed to identify the genetic determinants of reutericyclin biosynthesis. Comparisons of the genomes of reutericyclin-producing L. reuteri strains with those of non-reutericyclin-producing strains identified a genomic island of 14 open reading frames (ORFs) including genes coding for a nonribosomal peptide synthetase (NRPS), a polyketide synthase (PKS), homologues of PhlA, PhlB, and PhlC, and putative transport and regulatory proteins. The protein encoded by rtcN is composed of a condensation domain, an adenylation domain likely specific for d-leucine, and a thiolation domain. rtcK codes for a PKS that is composed of a ketosynthase domain, an acyl-carrier protein domain, and a thioesterase domain. The products of rtcA, rtcB, and rtcC are homologous to the diacetylphloroglucinol-biosynthetic proteins PhlABC and may acetylate the tetramic acid moiety produced by RtcN and RtcK, forming reutericyclin. Deletion of rtcN or rtcABC in L. reuteri TMW1.656 abrogated reutericyclin production but did not affect resistance to reutericyclin. Genes coding for transport and regulatory proteins could be deleted only in the reutericyclin-negative L. reuteri strain TMW1.656ΔrtcN, and these deletions eliminated reutericyclin resistance. The genomic analyses suggest that the reutericyclin genomic island was horizontally acquired from an unknown source during a unique event. The combination of PhlABC homologues with both an NRPS and a PKS has also been identified in the lactic acid bacteria Streptococcus mutans and Lactobacillus plantarum, suggesting that the genes in these organisms and those in L. reuteri share an evolutionary origin.

Variation on a theme; an overview of the Tn916/Tn1545 family of mobile genetic elements in the oral and nasopharyngeal streptococci

The oral and nasopharyngeal streptococci are a major part of the normal microbiota in humans. Most human associated streptococci are considered commensals, however, a small number of them are pathogenic, causing a wide range of diseases including oral infections such as dental caries and periodontitis and diseases at other body sites including sinusitis and endocarditis, and in the case of Streptococcus pneumoniae, meningitis. Both phenotypic and sequence based studies have shown that the human associated streptococci from the mouth and nasopharynx harbor a large number of antibiotic resistance genes and these are often located on mobile genetic elements (MGEs) known as conjugative transposons or integrative and conjugative elements of the Tn916/Tn1545 family. These MGEs are responsible for the spread of the resistance genes between streptococci and also between streptococci and other bacteria. In this review we describe the resistances conferred by, and the genetic variations between the many different Tn916-like elements found in recent studies of oral and nasopharyngeal streptococci and show that Tn916-like elements are important mediators of antibiotic resistance genes within this genus. We will also discuss the role of the oral environment and how this is conducive to the transfer of these elements and discuss the contribution of both transformation and conjugation on the transfer and evolution of these elements in different streptococci.

Regulation of competence and gene expression in Streptococcus mutans by the RcrR transcriptional regulator

An intimate linkage between the regulation of biofilm formation, stress tolerance and genetic competence exists in the dental caries pathogen Streptococcus mutans. The rcrRPQ genes encode ABC exporters (RcrPQ) and a MarR-family transcriptional repressor of the rcr operon (RcrR) that play a dominant role in the regulation of the development of genetic competence and connect competence with stress tolerance and (p)ppGpp production in S. mutans. Here we identify the target for efficient RcrR binding in the rcr promoter region using purified recombinant RcrR (rRcrR) protein in electrophoretic mobility shift assays and show that DNA fragments carrying mutations in the binding region were not bound as efficiently by rRcrR in vitro. Mutations in the RcrR binding site impacted expression from the rcrR promoter in vivo and elicited changes in transformation efficiency, competence gene expression, and growth inhibition by competence-stimulating peptide; even when the changes in rcrRPQ transcription were minor. An additional mechanistic linkage of RcrR with competence and (p)ppGpp metabolism was identified by showing that the rRcrR protein could bind to the promoter regions of comX, comYA and relP, although the binding was not as efficient as to the rcrRPQ promoter under the conditions tested. Hence, tightly controlled autogenous regulation of the rcrRPQ operon by RcrR binding to specific target sites is essential for cellular homeostasis, and RcrR contributes to the integration of genetic competence, (p)ppGpp metabolism, and acid and oxidative stress tolerance in S. mutans through both direct and indirect mechanisms.

Intercellular communications in multispecies oral microbial communities

The oral cavity contains more than 700 microbial species that are engaged in extensive cell–cell interactions. These interactions contribute to the formation of highly structured multispecies communities, allow them to perform physiological functions, and induce synergistic pathogenesis. Co-adhesion between oral microbial species influences their colonization of oral cavity and effectuates, to a large extent, the temporal and spatial formation of highly organized polymicrobial community architecture. Individual species also compete and collaborate with other neighboring species through metabolic interactions, which not only modify the local microenvironment such as pH and the amount of oxygen, making it more suitable for the growth of other species, but also provide a metabolic framework for the participating microorganisms by maximizing their potential to extract energy from limited substrates. Direct physical contact of bacterial species with its neighboring co-habitants within microbial community could initiate signaling cascade and achieve modulation of gene expression in accordance with different species it is in contact with. In addition to communication through cell–cell contact, quorum sensing (QS) mediated by small signaling molecules such as competence-stimulating peptides (CSPs) and autoinducer-2 (AI-2), plays essential roles in bacterial physiology and ecology. This review will summarize the evidence that oral microbes participate in intercellular communications with co-inhabitants through cell contact-dependent physical interactions, metabolic interdependencies, as well as coordinative signaling systems to establish and maintain balanced microbial communities.

Bacterial sex in dental plaque

Genes are transferred between bacteria in dental plaque by transduction, conjugation, and transformation. Membrane vesicles can also provide a mechanism for horizontal gene transfer. DNA transfer is considered bacterial sex, but the transfer is not parallel to processes that we associate with sex in higher organisms. Several examples of bacterial gene transfer in the oral cavity are given in this review. How frequently this occurs in dental plaque is not clear, but evidence suggests that it affects a number of the major genera present. It has been estimated that new sequences in genomes established through horizontal gene transfer can constitute up to 30% of bacterial genomes. Gene transfer can be both inter- and intrageneric, and it can also affect transient organisms. The transferred DNA can be integrated or recombined in the recipient's chromosome or remain as an extrachromosomal inheritable element. This can make dental plaque a reservoir for antimicrobial resistance genes. The ability to transfer DNA is important for bacteria, making them better adapted to the harsh environment of the human mouth, and promoting their survival, virulence, and pathogenicity.

Microfluidic study of competence regulation in Streptococcus mutans: environmental inputs modulate bimodal and unimodal expression of comX

Streptococcus mutans regulates genetic competence through a complex network that receives inputs from a number of environmental stimuli, including two signalling peptides designated as CSP and XIP. The response of the downstream competence genes to these inputs shows evidence of stochasticity and bistability and has been difficult to interpret. We have used microfluidic, single-cell methods to study how combinations of extracellular signals shape the response of comX, an alternative sigma factor governing expression of the late competence genes. We find that the composition of the medium determines which extracellular signal (XIP or CSP) can elicit a response from comX and whether that response is unimodal or bimodal across a population of cells. In a chemically defined medium, exogenous CSP does not induce comX, whereas exogenous XIP elicits a comX response from all cells. In complex medium, exogenous XIP does not induce comX, whereas CSP elicits a bimodal comX response from the population. Interestingly, bimodal behaviour required an intact copy of comS, which encodes the precursor of XIP. The comS-dependent capability for both unimodal and bimodal response suggests that a constituent - most likely peptides - of complex medium interacts with a positive feedback loop in the competence regulatory network.

The role of hydrogen peroxide in environmental adaptation of oral microbial communities

Oral streptococci are able to produce growth-inhibiting amounts of hydrogen peroxide (H(2)O(2)) as byproduct of aerobic metabolism. Several recent studies showed that the produced H(2)O(2) is not a simple byproduct of metabolism but functions in several aspects of oral bacterial biofilm ecology. First, the release of DNA from cells is closely associated to the production of H(2)O(2) in Streptococcus sanguinis and Streptococcus gordonii. Extracellular DNA is crucial for biofilm development and stabilization and can also serve as source for horizontal gene transfer between oral streptococci. Second, due to the growth inhibiting nature of H(2)O(2), H(2)O(2) compatible species associate with the producers. H(2)O(2) production therefore might help in structuring the initial biofilm development. On the other hand, the oral environment harbors salivary peroxidases that are potent in H(2)O(2) scavenging. Therefore, the effects of biofilm intrinsic H(2)O(2) production might be locally confined. However, taking into account that 80% of initial oral biofilm constituents are streptococci, the influence of H(2)O(2) on biofilm development and environmental adaptation might be under appreciated in current research.

Five questions about Streptococcus mutans: theoretical study of its transmission and colonisation

PURPOSE

This theoretical study aimed to identify the decisive (and controllable) factors involved in Streptococcus mutans (Sm) infection through addressing questions about (i) the time and prevalence pattern (including the raison d'etre of the discrete period for the infection or WI) of initial Sm colonisation and (ii) the infant's selection of bacterial types and their diversity, which are not yet definitely answered by empirical works.

METHOD

A model of Sm infection (within-host type) was developed. For questions (i): using the basic model, stochastic simulation was performed to reproduce longitudinal observations of the initial colonisation time. A symmetrical or right-skewed gamma distribution was assumed for the maximum colonisable area (K(max)) and transmission rate (mx). Additionally, 3 or 4 developmental modes of colonisable area [K(t)] were assigned based on the K(max) value. For (ii): by extending the basic model to the two-bacterial type model, intraspecific competition analysis focusing on the differences in mx (received by the infancy) and colonisation ability (θD) was performed.

RESULTS

The basic model simulation showed that mx and K(t) played a pivotal role in determining the individual time of initial colonisation and their variations among infants in forming its prevalence patterns (with or without WI). The competition model simulation showed that higher mx could be more advantageous in competitive colonisation than higher θD under repeated invasions. Accordingly, it played a decisive role in infant's selection of initially, persistently and transiently colonising bacterial types, and thus in their diversity.

CONCLUSIONS

(i) The mx is the primary and controllable (risk) factor that extensively affects various aspects of the Sm infection process. (ii) Also, the growing carrying capacity, i.e., K(t) is another important factor when considering how to effectively delay the onset of the colonisation. (iii) Thus, currently, the most feasible and effective control measure for the infection should be microbiological interventions in the primary host with concurrent oral hygiene and dietary control in the exposed child.

Microbial chemical signaling: a current perspective

Communication among microorganisms is mediated through quorum sensing. The latter is defined as cell-density linked, coordinated gene expression in microbial populations as a response to threshold signal concentrations followed by induction of a synchronized population response. This phenomenon is used by a variety of microbes to optimize their survival in a constantly challenging, dynamic milieu, by correlating individual cellular functions to community-based requirements. The synthesis, secretion, and perception of quorum-sensing molecules and their target response play a pivotal role in quorum sensing and are tightly controlled by complex, multilayered and interconnected signal transduction pathways that regulate diverse cellular functions. Quorum sensing exemplifies interactive social behavior innate to the microbial world that controls features such as, virulence, biofilm maturation, antibiotic resistance, swarming motility, and conjugal plasmid transfer. Over the past two decades, studies have been performed to rationalize bacterial cell-to-cell communication mediated by structurally and functionally diverse small molecules. This review describes the theoretical aspects of cellular and quorum-sensing mechanisms that affect microbial physiology and pathobiology.

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.

Biofilm growth alters regulation of conjugation by a bacterial pheromone

Conjugation is an important mode of horizontal gene transfer in bacteria, enhancing the spread of antibiotic resistance. In clinical settings, biofilms are likely locations for antibiotic resistance transfer events involving nosocomial pathogens such as Enterococcus faecalis. Here we demonstrate that growth in biofilms alters the induction of conjugation by a sex pheromone in E. faecalis. Mathematical modelling suggested that a higher plasmid copy number in biofilm cells would enhance a switch-like behaviour in the pheromone response of donor cells with a delayed, but increased response to the mating signal. Alterations in plasmid copy number, and a bimodal response to induction of conjugation in populations of plasmid-containing donor cells were both observed in biofilms, consistent with the predictions of the model. The pheromone system may have evolved such that donor cells in biofilms are only induced to transfer when they are in extremely close proximity to potential recipients in the biofilm community. These results may have important implications for development of chemotherapeutic agents to block resistance transfer and treat biofilm-related clinical infections.

Competence-dependent endogenous DNA rearrangement and uptake of extracellular DNA give a natural variant of Streptococcus mutans without biofilm formation

The production of water-insoluble glucan (WIG) enables Streptococcus mutans to survive and persist in the oral niche. WIG is produced from sucrose by glucosyltransferase encoded tandemly by the highly homologous gtfB and gtfC genes. Conversely, a single hybrid gene from the endogenous recombination of gtfB and gtfC is easily generated using RecA, resulting in S. mutans UA159 WIG- (rate of ∼1.0×10(-3)). The pneumococcus recA gene is regulated as a late competence gene. comX gene mutations did not lead to the appearance of WIG- cells. The biofilm collected from the flow cell had more WIG- cells than among the planktonic cells. Among the planktonic cells, WIG- cells appeared after 16 h and increased ∼10-fold after 32 h of cultivation, suggesting an increase in planktonic WIG- cells after longer culture. The strain may be derived from the biofilm environment. In coculture with donor WIG+ and recipient WIG- cells, the recipient cells reverted to WIG+ and acquired an intact gtfBC region from the environment, indicating that the uptake of extracellular DNA resulted in the phenotypic change. Here we demonstrate that endogenous DNA rearrangement and uptake of extracellular DNA generate WIG- cells and that both are induced by the same signal transducer, the com system. Our findings may help in understanding how S. mutans can adapt to the oral environment and may explain the evolution of S. mutans.

[Streptococcus mutans and oral streptococci in dental plaque]

The human oral microbial biota represents a highly diverse biofilm. Twenty-five species of oral streptococci inhabit the human oral cavity and represent about 20 % of the total oral bacteria. Taxonomy of these bacteria is complex and remains provisional. Oral streptococci encompass friends and foes bacteria. Each species has developed specific properties for colonizing the different oral sites subjected to constantly changing conditions, for competing against competitors, and for resisting external agressions (host immune system, physico-chemical shocks, and mechanical frictions). Imbalance in the indigenous microbial biota generates oral diseases, and under proper conditions, commensal streptococci can switch to opportunistic pathogens that initiate disease in and damage to the host. The group of "mutans streptococci" was described as the most important bacteria related to the formation of dental caries. Streptococcus mutans, although naturally present among the human oral microbiota, is the microbial species most strongly associated with carious lesions. This minireview describes the oral streptococci ecology and their biofilm life style by focusing on the mutans group, mainly S. mutans. Virulence traits, interactions in the biofilm, and influence of S. mutans in dental caries etiology are discussed.

Characterization of DNA binding sites of the ComE response regulator from Streptococcus mutans

In Streptococcus mutans, both competence and bacteriocin production are controlled by ComC and the ComED two-component signal transduction system. Recent studies of S. mutans suggested that purified ComE binds to two 11-bp direct repeats in the nlmC-comC promoter region, where ComE activates nlmC and represses comC. In this work, quantitative binding studies and DNase I footprinting analysis were performed to calculate the equilibrium dissociation constant and further characterize the binding site of ComE. We found that ComE protects sequences inclusive of both direct repeats, has an equilibrium dissociation constant in the nanomolar range, and binds to these two direct repeats cooperatively. Furthermore, similar direct repeats were found upstream of cslAB, comED, comX, ftf, vicRKX, gtfD, gtfB, gtfC, and gbpB. Quantitative binding studies were performed on each of these sequences and showed that only cslAB has a similar specificity and high affinity for ComE as that seen with the upstream region of comC. A mutational analysis of the binding sequences showed that ComE does not require both repeats to bind DNA with high affinity, suggesting that single site sequences in the genome may be targets for ComE-mediated regulation. Based on the mutational analysis and DNase I footprinting analysis, we propose a consensus ComE binding site, TCBTAAAYSGT.

Deletion of competence-induced genes over-expressed in biofilms caused transformation deficiencies in Streptococcus mutans

Previous studies identified nine genes with increased expression in Streptococcus mutans biofilms of which six possessed putative ComX promoter sequences and were homologous to competence-induced genes in Streptococcus pneumoniae, Streptococcus gordonii and Bacillus subtilis. As competence increases in biofilms, a study was undertaken into the roles that these biofilm-induced genes might play in transformation. Only five of the nine gene deletions had a significant effect on transformation efficiency. Deletion of the genes for recombinase A, recA, DNA processing protein, dprA and single-stranded DNA-binding protein, ssbA, produced results comparable with those from other bacteria, supporting the contention that these proteins have similar functions in S. mutans competence. The uncharacterized genes SMU.769 and SMU.836 produced results in variance to deletion mutants of putative homologues in S. pneumoniae. Deletion of SMU.769 reduced chromosomal transformation 2.3-fold. SMU.769 belongs to a family of conserved genes induced by the competence-stimulating peptide and which have no established function. In contrast, deletion of SMU.836 reduced transformation of both plasmid and chromosomal DNA to <3%. Homology searches suggested that Smu.836 belongs to a family of competence-induced peptidoglycan hydrolases with a conserved enzyme domain and a species-variable cell-binding domain for which the best characterized member is the choline-binding protein D, CbpD, of S. pneumoniae.

Point-of-care salivary microbial tests for detection of cariogenic species--clinical relevance thereof--review

Dental caries is a highly prevalent multifactorial disease that can result in serious health impairment. It was shown that oral bacteria play a significant role in caries development. Point-of-care (POC) salivary microbial tests for detection of cariogenic species have been investigated as a potential tool for caries risk assessment. This review aims to evaluate clinical relevance of these tests in the light of recent scientific evidence. Methodology involved PubMed search using key words salivary microbial tests, cariogenic bacteria and caries risk prediction. Articles obtained by the search were cross-referenced to obtain further sources. Specificity and negative-predictive value of these tests are higher than their sensitivity and positive value. Predictive power of the POC salivary microbial tests as a single predictor is generally weak, although it increases when included in multifactorial models for caries prediction. Literature findings support the use of these tests for screening of at-risk individuals in a population of young preschool children without visible caries and for motivation of subjects on individual level. POC salivary microbial tests are simple and inexpensive and, therefore, may be advantageous from public health perspective.

Pathogenesis of A Clinical Ocular Strain of Streptococcus pneumoniae and the Interaction of Pneumolysin with Corneal Cells

Streptococcus pneumoniae is an important cause of bacterial keratitis, an infectious disease of the cornea. This study aimed to determine the importance of pneumolysin (PLY), a pneumococcal virulence factor, in keratitis using a clinical keratitis isolate (K1263) and its isogenic mutant deficient in PLY (K1263ΔPLY) and determine the effect of these strains on primary rabbit corneal epithelial (RCE) cells. Each strain was injected into the corneal stromas of rabbits, clinical examinations were performed, and the recovered bacterial loads were determined. Bacterial extracts were exposed to RCE cells, and morphology and viability were assessed. The mutant strain deficient in PLY, K1263ΔPLY, caused significantly lower ocular disease scores than the parent strain (K1263), although a higher bacterial load was recovered from corneas infected with the mutant strain. Histological examination showed increased inflammatory cells in the anterior chamber and increased edema in eyes infected with the parent strain. RCE cells exposed to the parent strain had significantly decreased cell viability and showed increased evidence of cellular damage. This study confirms that in a strain that can cause clinical keratitis, PLY is a significant cause of the damage associated with pneumococcal keratitis. It also shows for the first time that the results from an in vitro model using RCE cells correlates with in vivo results thereby establishing a less invasive way to study the mechanisms of pneumococcal keratitis.

Proteases of an early colonizer can hinder Streptococcus mutans colonization in vitro

Streptococcus mutans is the primary cariogen that produces several virulence factors that are modulated by a competence-stimulating peptide (CSP) signaling system. In this study, we sought to determine if proteases produced by early dental plaque colonizers such as Streptococcus gordonii interfere with the subsequent colonization of S. mutans BM71 on the existing streptococcal biofilms. We demonstrated that S. mutans BM71 colonized much less efficiently in vitro on streptococcal biofilms than on Actinomyces naeslundii biofilms. Several oral streptococci, relative to A. naeslundii, produced proteases that inactivated the S. mutans CSP. We further demonstrated that cell protein extracts from S. gordonii, but not from A. naeslundii, interfered with S. mutans BM71 colonization. In addition, S. mutans BM71 colonized more efficiently on the sgc protease knockout mutant of S. gordonii than on the parent biofilms. In conclusion, proteases of early colonizers can interfere with subsequent colonization by S. mutans in vitro.

Streptococcus mutans inhibits Candida albicans hyphal formation by the fatty acid signaling molecule trans-2-decenoic acid (SDSF)

In the human mouth, fungi and several hundred species of bacteria coexist. Here we report a case of interkingdom signaling in the oral cavity: A compound excreted by the caries bacterium Streptococcus mutans inhibits the morphological transition from yeast to hyphae, an important virulence trait, in the opportunistic fungus Candida albicans. The compound excreted by S. mutans was originally studied because it inhibited signaling by the universal bacterial signal autoinducer-2 (AI-2), determined by the luminescence of a Vibrio harveyi sensor strain. The inhibitor was purified from cell-free culture supernatants of S. mutans guided by its activity. Its chemical structure was elucidated by using NMR spectroscopy and GC-MS and proved to be trans-2-decenoic acid. We show that trans-2-decenoic acid does not inhibit AI-2-specific signaling, but rather the luciferase reaction used for its detection. A potential biological role of trans-2-decenoic acid was then discovered. It is able to suppress the transition from yeast to hyphal morphology in the opportunistic human pathogen Candida albicans at concentrations that do not affect growth. The expression of HWP1, a hyphal-specific signature gene of C. albicans, is abolished by trans-2-decenoic acid. trans-2-Decenoic acid is structurally similar to the diffusible signal factor (DSF) family of interkingdom-signaling molecules and is the first member of this family from a Gram-positive organism (Streptococcus DSF, SDSF). SDSF activity was also found in S. mitis, S. oralis, and S. sanguinis, but not in other oral bacteria. SDSF could be relevant in shaping multispecies Candida bacteria biofilms in the human body.

Examination of the hdrRM regulon yields insight into the competence system of Streptococcus mutans

Previous studies have identified the hdrRM operon as a novel regulatory system induced by conditions of high cell density. Little is known about the genes under the control of this system, but a variety of important phenotypes are associated with either hdrR overexpression or mutation of hdrM. To characterize the regulatory function of the HdrRM system in Streptococcus mutans we used a microarray approach to compare the transcriptional profiles of an hdrR overexpression strain with an hdrM mutant. Both strains exhibited almost identical profiles, which included all of the known late competence genes as well as a variety of competence-induced bacteriocins. Through a combination of real-time reverse transcription-polymerase chain reaction (PCR), reporter gene analysis and random amplification of complementary DNA ends PCR, we confirmed the role of comX as a central intermediate regulator of numerous genes in the hdrRM regulon. Through these studies, we also identified novel comX-regulated genes required for natural competence. Taken together, our results suggest that the primary function of the HdrRM system is to regulate the late competence genes together with various bacteriocins. This occurs independently of the ComCDE system, even though both systems regulate nearly identical genes. This suggests that S. mutans has multiple parallel input sensory systems that control the same output response: the induction of natural competence and concurrent production of bacteriocins.

A genetic determinant in Streptococcus gordonii Challis encodes a peptide with activity similar to that of enterococcal sex pheromone cAM373, which facilitates intergeneric DNA transfer

Enterococcus faecalis strains secrete multiple peptides representing different sex pheromones that induce mating responses by bacteria carrying specific conjugative plasmids. The pheromone cAM373, which induces a response by the enterococcal plasmid pAM373, has been of interest because a similar activity is also secreted by Streptococcus gordonii and Staphylococcus aureus. The potential to facilitate intergeneric DNA transfer from E. faecalis is of concern because of extensive multiple antibiotic resistance, including vancomycin resistance, that has emerged among enterococci in recent years. Here, we characterize the related pheromone determinant in S. gordonii and show that the peptide it encodes, gordonii-cAM373, does indeed induce transfer of plasmid DNA from E. faecalis into S. gordonii. The streptococcal determinant camG encodes a lipoprotein with a leader sequence, the last 7 residues of which represent the gordonii-cAM373 heptapeptide SVFILAA. Synthetic forms of the peptide had activity similar to that of the enterococcal cAM373 AIFILAS. The lipoprotein moiety bore no resemblance to the lipoprotein encoded by E. faecalis. We also identified determinants in S. gordonii encoding a signal peptidase and an Eep-like zinc metalloprotease (lspA and eep, respectively) similar to those involved in processing certain pheromone precursors in E. faecalis. Mutations generated in camG, lspA, and eep each resulted in the ablation of gordonii-cAM373 activity in culture supernatants. This is the first genetic analysis of a potential sex pheromone system in a commensal oral streptococcal species, which may have implications for intergeneric gene acquisition in oral biofilms.

Observing the biofilm matrix of Staphylococcus epidermidis ATCC 35984 grown using the CDC biofilm reactor

Bacteria flourish in nearly every environment on earth. Contributing to their ability to grow in many esoteric locations is their development into a biofilm structure. In an effort to more accurately model the growth environment of biofilms in nature, a Center for Disease Control and Prevention (CDC) biofilm reactor has been developed that mimics nature-like shear forces and renewable nutrient sources. To date, there has been no confirmation by scanning electron microscopy (SEM) that mature biofilms develop on a surface when grown using the CDC biofilm reactor. Three different SEM methods were used to collect images of Staphylococcus epidermidis ATCC 35984 that was to be grown using the CDC biofilm reactor. In addition, two different fixative techniques were used in each of the imaging methods. Results indicated that after 48 hours of growth in the reactor, S. epidermidis ATCC 35984 does produce a significant network of matrix components and 3D mushroom- or pillar-like structures with signs of water channel development. In conclusion, S. epidermidis ATCC 35984 grown using the CDC biofilm reactor does appear to display signs of mature biofilm development. These results could be important for studies wherein mature biofilms are needed for in vitro and/or in vivo applications.

The hdrRM operon of Streptococcus mutans encodes a novel regulatory system for coordinated competence development and bacteriocin production

The Streptococcus mutans hdrRM operon encodes a novel two-gene regulatory system induced by high cell density. Previous studies identified hdrM as the only known negative regulator of competence development in S. mutans. In the present study, we demonstrated that the HdrRM system bypasses the prototypical competence gene regulators ComC and ComDE in the transcriptional regulation of the competence-specific sigma factor comX and the late competence genes. Similarly, the HdrRM system can abrogate the requirement for ComE to produce the bacteriocin mutacin IV. To further probe the regulatory mechanism of hdrRM, we created an hdrR overexpression strain and showed that it could reproduce each of the hdrM competence and mutacin phenotypes, indicating that HdrM acts as a negative regulator of HdrR activity. Using a mutacin IV-luciferase reporter, we also demonstrated that the hdrRM system utilizes the same promoter elements recognized by ComE and thus appears to comprise a novel regulatory pathway parallel to ComCDE.

Functional analysis of the quorum-sensing streptococcal invasion locus (sil)

Group A streptococcus (GAS) causes a wide variety of human diseases, and at the same time, GAS can also circulate without producing symptoms, similar to its close commensal relative, group G streptococcus (GGS). We previously identified, by transposon-tagged mutagenesis, the streptococcal invasion locus (sil). sil is a quorum-sensing regulated locus which is activated by the autoinducer peptide SilCR through the two-component system SilA-SilB. Here we characterize the DNA promoter region necessary for SilA-mediated activation. This site is composed of two direct repeats of 10 bp, separated by a spacer of 11 bp. Fusion of this site to gfp allowed us to systematically introduce single-base substitutions in the repeats region and to assess the relative contribution of various positions to promoter strength. We then developed an algorithm giving different weights to these positions, and performed a chromosome-wide bioinformatics search which was validated by transcriptome analysis. We identified 13 genes, mostly bacteriocin related, that are directly under the control of SilA. Having developed the ability to quantify SilCR signaling via GFP accumulation prompted us to search for GAS and GGS strains that sense and produce SilCR. While the majority of GAS strains lost sil, all GGS strains examined still possess the locus and ∼63% are able to respond to exogenously added SilCR. By triggering the autoinduction circle using a minute concentration of synthetic SilCR, we identified GAS and GGS strains that are capable of sensing and naturally producing SilCR, and showed that SilCR can be sensed across these streptococci species. These findings suggest that sil may be involved in colonization and establishment of commensal host-bacterial relationships.

Cell-to-cell communication in bacteria is termed quorum-sensing (QS), which is triggered by signaling molecules called autoinducers. In streptococci, autoinducers are synthesized as immature peptides that are processed, secreted, and then sensed by two-component systems (TCSs). As a result, the autoinducer's own expression is upregulated (autoinduction), subsequently creating an ultrasensitive switch that turns on more genes. Group A streptococcus (GAS) is a human pathogen that causes many infections, including necrotizing fasciitis (NF). Previously, we identified in a NF GAS strain a QS locus termed streptococcal invasion locus (sil). Due to a mutation in the autoinducer peptide-SilCR, it is not produced by this strain. Here we sought to better explore sil and to examine if SilCR can be produced by other GAS strains, or strains of its close relative group G streptococcus (GGS). To this end, we characterized the DNA promoter region responsible for the TCS-mediated activation upon sensing of SilCR, and based on bioinformatics and transcriptome analyses we identified genes that are directly affected by the autoinducer peptide. By converting SilCR response to fluorescence production and turning on the autoinduction circle with minute concentrations of synthetic SilCR, we discovered naturally SilCR-producing GAS and GGS strains, and showed that SilCR can be sensed across these species. Our study describes a novel way of cell-to-cell communications among streptococci.

Bacterial and host interactions of oral streptococci

The oral microbial flora comprises one of the most diverse human-associated biofilms. Its development is heavily influenced by oral streptococci, which are considered the main group of early colonizers. Their initial attachment determines the composition of later colonizers in the oral biofilm and impacts the health or disease status of the host. Thus, the role of streptococci in the development of oral diseases is best described in the context of bacterial ecology, which itself is further influenced by interactions with host epithelial cells, the immune system, and salivary components. The tractability of the oral biofilm makes it an excellent model system for studies of complex, biofilm-associated polymicrobial diseases. Using this system, numerous cooperative and antagonistic bacterial interactions have been demonstrated to occur within the community and with the host. In this review, several recent identified interactions are presented.

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.

Independent gene duplications of the YidC/Oxa/Alb3 family enabled a specialized cotranslational function

YidC/Oxa/Alb3 family proteins catalyze the insertion of integral membrane proteins in bacteria, mitochondria, and chloroplasts, respectively. Unlike gram-negative organisms, gram-positive bacteria express 2 paralogs of this family, YidC1/SpoIIIJ and YidC2/YgjG. In Streptococcus mutans, deletion of yidC2 results in a stress-sensitive phenotype similar to that of mutants lacking the signal recognition particle (SRP) protein translocation pathway, while deletion of yidC1 has a less severe phenotype. In contrast to eukaryotes and gram-negative bacteria, SRP-deficient mutants are viable in S. mutans; however, double SRP-yidC2 mutants are severely compromised. Thus, YidC2 may enable loss of the SRP by playing an independent but overlapping role in cotranslational protein insertion into the membrane. This is reminiscent of the situation in mitochondria that lack an SRP pathway and where Oxa1 facilitates cotranslational membrane protein insertion by binding directly to translation-active ribosomes. Here, we show that OXA1 complements a lack of yidC2 in S. mutans. YidC2 also functions reciprocally in oxa1-deficient Saccharomyces cerevisiae mutants and mediates the cotranslational insertion of mitochondrial translation products into the inner membrane. YidC2, like Oxa1, contains a positively charged C-terminal extension and associates with translating ribosomes. Our results are consistent with a gene-duplication event in gram-positive bacteria that enabled the specialization of a YidC isoform that mediates cotranslational activity independent of an SRP pathway.