Probing oral microbial functionality--expression of spxB in plaque samples

Aspects of oral streptococcal metabolic diversity: Imagining the landscape beneath the fog

It is widely acknowledged that the human-associated microbial community influences host physiology, systemic health, disease progression, and even behavior. There is currently an increased interest in the oral microbiome, which occupies the entryway to much of what the human initially encounters from the environment. In addition to the dental pathology that results from a dysbiotic microbiome, microbial activity within the oral cavity exerts significant systemic effects. The composition and activity of the oral microbiome is influenced by (1) host-microbial interactions, (2) the emergence of niche-specific microbial "ecotypes," and (3) numerous microbe-microbe interactions, shaping the underlying microbial metabolic landscape. The oral streptococci are central players in the microbial activity ongoing in the oral cavity, due to their abundance and prevalence in the oral environment and the many interspecies interactions in which they participate. Streptococci are major determinants of a healthy homeostatic oral environment. The metabolic activities of oral Streptococci, particularly the metabolism involved in energy generation and regeneration of oxidative resources vary among the species and are important factors in niche-specific adaptations and intra-microbiome interactions. Here we summarize key differences among streptococcal central metabolic networks and species-specific differences in how the key glycolytic intermediates are utilized.

Identification and Characterization of Negative Regulators of Rgg1518 Quorum Sensing in Streptococcus pneumoniae

Streptococcus pneumoniae is an agent of otitis media, septicemia, and meningitis and remains the leading cause of community-acquired pneumonia regardless of vaccine use. Of the various strategies that S. pneumoniae takes to enhance its potential to colonize the human host, quorum sensing (QS) is an intercellular communication process that provides coordination of gene expression at a community level. Numerous putative QS systems are identifiable in the S. pneumoniae genome, but their gene-regulatory activities and contributions to fitness have yet to be fully evaluated. To contribute to assessing regulatory activities of rgg paralogs present in the D39 genome, we conducted transcriptomic analysis of mutants of six QS regulators. Our results find evidence that at least four QS regulators impact the expression of a polycistronic operon (encompassing genes spd_1517 to spd_1513) that is directly controlled by the Rgg/SHP1518 QS system. As an approach to unravel the convergent regulation placed on the spd_1513-1517 operon, we deployed transposon mutagenesis screening in search of upstream regulators of the Rgg/SHP1518 QS system. The screen identified two types of insertion mutants that result in increased activity of Rgg1518-dependent transcription, one type being where the transposon inserted into pepO, an annotated endopeptidase, and the other type being insertions in spxB, a pyruvate oxidase. We demonstrate that pneumococcal PepO degrades SHP1518 to prevent activation of Rgg/SHP1518 QS. Moreover, the glutamic acid residue in the conserved "HExxH" domain is indispensable for the catalytic function of PepO. Finally, we confirmed the metalloendopeptidase property of PepO, which requires zinc ions, but not other ions, to facilitate peptidyl hydrolysis. IMPORTANCE Streptococcus pneumoniae uses quorum sensing to communicate and regulate virulence. In our study, we focused on one Rgg quorum sensing system (Rgg/SHP1518) and found that multiple other Rgg regulators also control it. We further identified two enzymes that inhibit Rgg/SHP1518 signaling and revealed and validated one enzyme's mechanisms for breaking down quorum sensing signaling molecules. Our findings shed light on the complex regulatory network of quorum sensing in Streptococcus pneumoniae.

Simultaneous Chemical Mapping of Live Biofilm Microenvironmental pH and Hydrogen Peroxide in Real Time with a Triple Scanning Electrochemical Microscopy Tip

Dental plaque biofilm is a complex ecosystem. The distribution of microbial species in the biofilm is heavily influenced by local chemical interactions that result from diverse metabolic activities and the nature of the released molecules. As a relevant example, H2O2-producing bacteria can antagonize disease-associated bacteria, leading to the maintenance of a healthy oral microbiome. Herein, we report the development of a triple-sensor (redox, pH, and H2O2) scanning electrochemical microscopy (SECM) tip capable of simultaneously mapping the pH and H2O2 concentration produced by a dental plaque-derived multispecies biofilm grown on hydroxyapatite. The pH sensor of the triple SECM tip showed a near Nernstian slope of -71.1 ± 2 mV/pH (N = 3), whereas the H2O2 sensor showed a slope of -0.052 ± 0.002 nA/μM H2O2 at pH 7.2 and a detection limit of 1.0 ± 0.2 μM (N = 7). There is no significant difference in the sensitivities of H2O2 sensors at pH 6.2, 7.2, and 8.2 at 95% CI (N = 7). The pH and H2O2 sensors demonstrated excellent reversibility with response times of 3 and 5 s, respectively, along with reliable stability over 4 h at 37 °C. The sensors did not show any cross talk between pH and H2O2 concentration ([H2O2]) measurements, highlighting the accuracy and versatility of the SECM tip. Simultaneous chemical imaging of pH and [H2O2] across the biofilm revealed a clustered distribution of local H2O2 concentrations, ranging from 0 to 17 μM. Conversely, the local pH remained constant at 7.2. The relation of local chemical profiles and the distribution of bacterial species within the oral microbiome was experimentally investigated in the context of bacterial H2O2 antagonism. The benefit of clustered H2O2 production was that the total area of H2O2 produced by smaller clusters was 67% more than that of a single cluster with the same starting number of bacteria. Thus, this triple SECM tip can potentially be used to study local molecular mechanisms that result in dysbiosis of the oral microbiome.

Antagonistic interactions by a high H2 O2 -producing commensal streptococcus modulate caries development by Streptococcus mutans

Dental caries (tooth-decay) is caused by biofilms harboring polymicrobial communities on teeth that leads to the onset of localized areas of enamel demineralization. Streptococcus mutans has been clinically associated with severe caries in childhood. Although commensal bacteria can combat S. mutans using self-generated antimicrobials such as hydrogen peroxide (H2 O2 ), constant sugar-rich diet consumption disrupts microbial homeostasis shifting toward cariogenic community. Recently, Streptococcus oralis subsp. tigurinus strain J22, an oral isolate, was identified as a uniquely potent H2 O2 producer. Here, we assess whether a high H2 O2 -producing commensal streptococcus can modulate the spatial organization and virulence of S. mutans within biofilms. Using an experimental biofilm model, we find that the presence of S. oralis J22 can effectively inhibit the clustering, accumulation, and spatial organization of S. mutans on ex vivo human tooth surface, resulting in significant reduction of enamel demineralization. Notably, the generation of H2 O2 via pyruvate oxidase (SpxB) from S. oralis J22 is not repressed by sugars (a common repressor in other mitis group streptococci), resulting in enhanced inhibition of S. mutans growth (vs. Streptococcus gordonii). We further investigate its impact on biofilm virulence using an in vivo rodent caries model under sugar-rich diet. Coinfection of S. mutans with S. oralis results in reduced caries development compared to either species infected alone, whereas coinfection with S. gordonii has negligible effects, suggesting that the presence of an efficient, high H2 O2 -producer can disrupt S. mutans virulence. This work demonstrates that oral isolates with unusual high H2 O2 production may be capable of modulating biofilm cariogenicity in vivo. The findings also highlight the importance of bacterial antagonistic interactions within polymicrobial communities in health and in disease-causing state.

Induction of Apoptotic Cell Death by Oral Streptococci in Human Periodontal Ligament Cells

Initiation and progression of oral infectious diseases are associated with streptococcal species. Bacterial infection induces inflammatory responses together with reactive oxygen species (ROS), often causing cell death and tissue damage in the host. In the present study, we investigated the effects of oral streptococci on cytotoxicity and ROS production in human periodontal ligament (PDL) cells. Streptococcus gordonii showed cell cytotoxicity in a dose- and time-dependent manner. The cytotoxicity might be due to apoptosis since S. gordonii increased annexin V-positive cells, and the cytotoxicity was reduced by an apoptosis inhibitor, Z-VAD-FMK. Other oral streptococci such as Streptococcus mitis, Streptococcus sanguinis, and Streptococcus sobrinus also induced apoptosis, whereas Streptococcus mutans did not. All streptococci tested except S. mutans triggered ROS production in human PDL cells. Interestingly, however, streptococci-induced apoptosis appears to be ROS-independent, as the cell death induced by S. gordonii was not recovered by the ROS inhibitor, resveratrol or n-acetylcysteine. Instead, hydrogen peroxide (H₂O₂) appears to be important for the cytotoxic effects of streptococci since most oral streptococci except S. mutans generated H₂O₂, and the cytotoxicity was dramatically reduced by catalase. Furthermore, streptococcal lipoproteins are involved in cytotoxicity, as we observed that cytotoxicity induced by the lipoprotein-deficient S. gordonii mutant was less potent than that by the wild-type and was attenuated by anti-TLR2-neutralizing antibody. Indeed, lipoproteins purified from S. gordonii alone were sufficient to induce cytotoxicity. Notably, S. gordonii lipoproteins did not induce H₂O₂ or ROS but cooperatively induced cell death when co-treated with H₂O₂. Taken together, these results suggest that most oral streptococci except S. mutans efficiently induce damage to human PDL cells by inducing apoptotic cell death with bacterial H₂O₂ and lipoproteins, which might contribute to the progression of oral infectious diseases such as apical periodontitis.

Post-translational modification of Streptococcus sanguinis SpxB influences protein solubility and H2 O2 production

Streptococcal pyruvate oxidase (SpxB) is a hydrogen peroxide-generating enzyme and plays a critical role in Streptococcus sanguinis interspecies interactions, but less is known about its biochemistry. We examined SpxB subcellular localization using protein fractionation and microscopy and found SpxB to be primarily cytoplasmic, but a small portion is also membrane associated. Potential post-translational modifications of SpxB were determined using coimmunoprecipitation and mass spectrometry. Two mutant strains were constructed to further validate the presence of predicted site-specific post-translational modifications. These site mutated SpxB proteins exhibited reduced solubility in vivo, which likely contributes to the observed phenotypic changes in colony morphology, bacterial growth, and H₂ O₂ production. Overall, our data suggest that SpxB post-translational modifications likely play a major role to regulate SpxB function in S. sanguinis.

Quantification and gene expression of Lactobacillus casei group species associated with dentinal lesions in early childhood caries

Background

Considering that the Lactobacillus casei group is strongly associated with caries progression, the use of lactobacilli as probiotics must be balanced due to their possible involvement in dental caries.

Objective

This study aimed to detect and quantify L. paracasei, L. rhamnosus, and L. casei group species in the active and arrested dentinal lesions of preschoolers. It also aimed to determine the expression profiles of lactobacilli genes related to adhesion, extracellular polymeric substance regulation, and pyruvate oxidation.

Methods

Total ribonucleic acid (RNA) was extracted from dentinal lesion samples (25 active, 13 arrested) of children between 2 and 5 years of age. The samples were converted to complementary deoxyribonucleic acid (cDNA), and quantitative polymerase chain reaction (qPCR) analyses were performed to quantify and determine the relative abundance (measured by percentage of total bacteria) of L. paracasei, L. rhamnosus, and L. casei group species. The expression profiles of L. paracasei/casei genes (spaC and spxB) and L. rhamnosus genes (spaE and wzb) were assessed. The Student t-test and the Mann-Whitney U test were used for comparisons.

Results

The L. casei group species were found to be part of the viable microbial community in dentinal caries. L. paracasei (p = 0.001), L. rhamnosus (p = 0.022), and L. casei (p = 0.004) group species were abundant in the active dentinal lesions compared to the arrested dentinal lesions. Only the wzb gene (p = 0.006) exhibited a statistically significant difference between the active and arrested lesions in terms of its expression profile; it was expressed to a higher extent in the active dentinal lesions.

Conclusions

The L. casei group species presented in large numbers in the active dentinal caries lesions, indicating that these microorganisms are related to caries activity, and the wzb gene may play an important role in caries progression.

Where in the world do bacteria experience oxidative stress?

Reactive oxygen species - superoxide, hydrogen peroxide and hydroxyl radicals - have long been suspected of constraining bacterial growth in important microbial habitats and indeed of shaping microbial communities. Over recent decades, studies of paradigmatic organisms such as Escherichia coli, Salmonella typhimurium, Bacillus subtilis and Saccharomyces cerevisiae have pinpointed the biomolecules that oxidants can damage and the strategies by which microbes minimize their injuries. What is lacking is a good sense of the circumstances under which oxidative stress actually occurs. In this MiniReview several potential natural sources of oxidative stress are considered: endogenous ROS formation, chemical oxidation of reduced species at oxic-anoxic interfaces, H₂ O₂ production by lactic acid bacteria, the oxidative burst of phagocytes and the redox-cycling of secreted small molecules. While all of these phenomena can be reproduced and verified in the lab, the actual quantification of stress in natural habitats remains lacking - and, therefore, we have a fundamental hole in our understanding of the role that oxidative stress actually plays in the biosphere.

The activation of the oxidative stress response transcription factor SKN-1 in Caenorhabditis elegans by mitis group streptococci

The mitis group, a member of the genetically diverse viridans group streptococci, predominately colonizes the human oropharynx. This group has been shown to cause a wide range of infectious complications in humans, including bacteremia in patients with neutropenia, orbital cellulitis and infective endocarditis. Hydrogen peroxide (H₂O₂) has been identified as a virulence factor produced by this group of streptococci. More importantly, it has been shown that Streptococcus oralis and S. mitis induce epithelial cell and macrophage death via the production of H₂O₂. Previously, H₂O₂ mediated killing was observed in the nematode Caenorhabditis elegans in response to S. oralis and S. mitis. The genetically tractable model organism C. elegans is an excellent system to study mechanisms of pathogenicity and stress responses. Using this model, we observed rapid H₂O₂ mediated killing of the worms by S. gordonii in addition to S. mitis and S. oralis. Furthermore, we observed colonization of the intestine of the worms when exposed to S. gordonii suggesting the involvement of an infection-like process. In response to the H₂O₂ produced by the mitis group, we demonstrate the oxidative stress response is activated in the worms. The oxidative stress response transcription factor SKN-1 is required for the survival of the worms and provides protection against H₂O₂ produced by S. gordonii. We show during infection, H₂O₂ is required for the activation of SKN-1 and is mediated via the p38-MAPK pathway. The activation of the p38 signaling pathway in the presence of S. gordonii is not mediated by the endoplasmic reticulum (ER) transmembrane protein kinase IRE-1. However, IRE-1 is required for the survival of worms in response to S. gordonii. These finding suggests a parallel pathway senses H₂O₂ produced by the mitis group and activates the phosphorylation of p38. Additionally, the unfolded protein response plays an important role during infection.

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

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

Distinct Regulatory Role of Carbon Catabolite Protein A (CcpA) in Oral Streptococcal spxB Expression

Pyruvate oxidase (SpxB)-dependent H₂O₂ production is under the control of carbon catabolite protein A (CcpA) in the oral species Streptococcus sanguinis and Streptococcus gordonii Interestingly, both species react differently to the presence of the preferred carbohydrate source glucose. S. gordonii CcpA-dependent regulation of spxB follows classical carbon catabolite repression. Conversely, spxB expression in S. sanguinis is not influenced by glucose but is repressed by CcpA. Here, we constructed strains expressing the heterologous versions of CcpA or the spxB promoter region to learn if the distinct regulation of spxB expression is transferable from S. gordonii to S. sanguinis and vice versa. While cross-species binding of CcpA to the spxB promoter is conserved in vitro, we were unable to swap the species-specific regulation. This suggests that a regulatory mechanism upstream of CcpA most likely is responsible for the observed difference in spxB expression. Moreover, the overall ecological significance of differential spxB regulation in the presence of various glucose concentrations was tested with additional oral streptococcus isolates and demonstrated that carbohydrate-dependent and carbohydrate-independent mechanisms exist to control expression of spxB in the oral biofilm. Overall, our data demonstrate the unexpected finding that metabolic pathways between two closely related oral streptococcal species can be regulated differently despite an exceptionally high DNA sequence identity.IMPORTANCE Polymicrobial diseases are the result of interactions among the residential microbes, which can lead to a dysbiotic community. Streptococcus sanguinis and Streptococcus gordonii are considered commensal species that are present in the healthy dental biofilm. Both species are able to produce significant amounts of H₂O₂ via the enzymatic action of the pyruvate oxidase SpxB. H₂O₂ is able to inhibit species associated with oral diseases. SpxB and its gene-regulatory elements present in both species are highly conserved. Nonetheless, a differential response to the presence of glucose was observed. Here, we investigate the mechanisms that lead to this differential response. Detailed knowledge of the regulatory mechanisms will aid in a better understanding of oral disease development and how to prevent dysbiosis.

Plasticity of the Pyruvate Node Modulates Hydrogen Peroxide Production and Acid Tolerance in Multiple Oral Streptococci

Commensal Streptococcus sanguinis and Streptococcus gordonii are pioneer oral biofilm colonizers. Characteristic for both is the SpxB-dependent production of H₂O₂, which is crucial for inhibiting competing biofilm members, especially the cariogenic species Streptococcus mutans H₂O₂ production is strongly affected by environmental conditions, but few mechanisms are known. Dental plaque pH is one of the key parameters dictating dental plaque ecology and ultimately oral health status. Therefore, the objective of the current study was to characterize the effects of environmental pH on H₂O₂ production by S. sanguinis and S. gordoniiS. sanguinis H₂O₂ production was not found to be affected by moderate changes in environmental pH, whereas S. gordonii H₂O₂ production declined markedly in response to lower pH. Further investigation into the pyruvate node, the central metabolic switch modulating H₂O₂ or lactic acid production, revealed increased lactic acid levels for S. gordonii at pH 6. The bias for lactic acid production at pH 6 resulted in concomitant improvement in the survival of S. gordonii at low pH and seems to constitute part of the acid tolerance response of S. gordonii Differential responses to pH similarly affect other oral streptococcal species, suggesting that the observed results are part of a larger phenomenon linking environmental pH, central metabolism, and the capacity to produce antagonistic amounts of H₂O₂IMPORTANCE Oral biofilms are subject to frequent and dramatic changes in pH. S. sanguinis and S. gordonii can compete with caries- and periodontitis-associated pathogens by generating H₂O₂ Therefore, it is crucial to understand how S. sanguinis and S. gordonii adapt to low pH and maintain their competitiveness under acid stress. The present study provides evidence that certain oral bacteria respond to environmental pH changes by tuning their metabolic output in favor of lactic acid production, to increase their acid survival, while others maintain their H₂O₂ production at a constant level. The differential control of H₂O₂ production provides important insights into the role of environmental conditions for growth competition of the oral flora.

Real-Time Metabolic Interactions between Two Bacterial Species Using a Carbon-Based pH Microsensor as a Scanning Electrochemical Microscopy Probe

We have developed a carbon-based, fast-response potentiometric pH microsensor for use as a scanning electrochemical microscopy (SECM) chemical probe to quantitatively map the microbial metabolic exchange between two bacterial species, commensal Streptococcus gordonii and pathogenic Streptococcus mutans. The 25 μm diameter H+ ion-selective microelectrode or pH microprobe showed a Nernstian slope of 59 mV/pH and high selectivity against major ions such Na+, K+, Ca2+, and Mg2+. In addition, the unique conductive membrane composition aided us in performing an amperometric approach curve to position the probe and obtain a high-resolution pH map of the microenvironment produced by the lactate-producing S. mutans biofilm. The x-directional pH scan over S. mutans also showed the influence of the pH profile on the metabolic activity of another species, H2O2-producing S. gordonii. When these bacterial species were placed in close spatial proximity, we observed an initial increase in the local H2O2 concentration of approximately 12 ± 5 μM above S. gordonii, followed by a gradual decrease in H2O2 concentration (>30 min) to almost zero as lactate was produced, and a subsequent decrease in pH with a more pronounced metabolic output of S. mutans. These results were supported by gene expression and confocal fluorescence microscopic studies. Our findings illustrate that H2O2-producing S. gordonii is dominant while the buffering capacity of saliva is valid (∼pH 6.0) but is gradually taken over by S. mutans as the latter species slowly starts decreasing the local pH to 5.0 or less by producing lactic acid. Our observations demonstrate the unique capability of our SECM chemical probes for studying real-time metabolic interactions between two bacterial species, which would not otherwise be achievable in traditional assays.

[The origin of hydrogen peroxide in oral cavity and its role in oral microecology balance]

Hydrogen peroxide, an important antimicrobial agent in oral cavity, plays a significant role in the balance of oral microecology. At the early stage of biofilm formation, about 80% of the detected initial colonizers belong to the genus Streptococcus. These oral streptococci use different oxidase to produce hydrogen peroxide. Recent studies showed that the produced hydrogen peroxide plays a critical role in modulating oral microecology. Hydrogen peroxide modulates biofilm development attributed to its growth inhibitory nature. Hydrogen peroxide production is closely associated with extracellular DNA(eDNA) release from microbe and the development of its competent cell which are critical for biofilm development and also serves as source for horizontal gene transfer. Microbe also can reduce the damage to themselves through several detoxification mechanisms. Moreover, hydrogen peroxide is also involved in the regulation of interactions between oral microorganisms and host. Taken together, hydrogen peroxide is an imperative ecological factor that contributes to the microbial equilibrium in the oral cavity. Here we will give a brief review of both the origin and the function in the oral microecology balance of hydrogen peroxide.

The road less traveled - defining molecular commensalism with Streptococcus sanguinis

The commensal oral microbial flora has evolved with the human host to support colonization of the various intraoral sites without triggering a significant immune response. In exchange, the commensal microbes provide critical protection against invading pathogens. The intrinsic ability of the oral flora to create a symbiotic microbial community with the host can be disturbed, selecting for the overgrowth of a dysbiotic community that can result in dental diseases, such as caries and periodontitis. Although the mechanisms of molecular pathogenesis in oral diseases are well characterized, much less is known about the molecular mechanisms used by the commensal flora to maintain oral health. Here we focus on the commensal species Streptococcus sanguinis, which is found in abundance in the early oral biofilm and is strongly correlated with oral health. Streptococcus sanguinis exhibits a variety of features that make it ideally suited as a model organism to explore the molecular basis for commensalism. As such, this review will describe our current mechanistic understanding of S. sanguinis commensalism and speculate upon its molecular traits that may be exploitable to maintain or restore oral health under conditions that would otherwise lead to disease.

Streptococcus sanguinis induces neutrophil cell death by production of hydrogen peroxide

Streptococcus is the dominant bacterial genus in the human oral cavity and a leading cause of infective endocarditis. Streptococcus sanguinis belongs to the mitis group of streptococci and produces hydrogen peroxide (H2O2) by the action of SpxB, a pyruvate oxidase. In this study, we investigated the involvement of SpxB in survival of S. sanguinis in human blood and whether bacterial H2O2 exhibits cytotoxicity against human neutrophils. Results of a bactericidal test with human whole blood revealed that the spxB mutation in S. sanguinis is detrimental to its survival in blood. When S. sanguinis strains were exposed to isolated neutrophils, the bacterial survival rate was significantly decreased by spxB deletion. Furthermore, human neutrophils exposed to the S. sanguinis wild-type strain, in contrast to those exposed to an spxB mutant strain, underwent cell death with chromatin de-condensation and release of web-like extracellular DNA, reflecting induction of neutrophil extracellular traps (NETs). Since reactive oxygen species-mediated NET induction requires citrullination of arginine residues in histone proteins and subsequent chromatin de-condensation, we examined citrullination levels of histone in infected neutrophils. It is important to note that the citrullinated histone H3 was readily detected in neutrophils infected with the wild-type strain, as compared to infection with the spxB mutant strain. Moreover, decomposition of streptococcal H2O2 with catalase reduced NET induction. These results suggest that H2O2 produced by S. sanguinis provokes cell death of neutrophils and NET formation, thus potentially affecting bacterial survival in the bloodstream.

The spxB gene as a target to identify Lactobacillus casei group species in cheese

This study focused on the spxB gene, which encodes for pyruvate oxidase. The presence of spxB in the genome and its transcription could be a way to produce energy and allow bacterial growth during carbohydrate starvation. In addition, the activity of pyruvate oxidase, which produces hydrogen peroxide, could be a mechanism for interspecies competition. Because this gene seems to provide advantages for the encoding species for adaptation in complex ecosystems, we studied spxB in a large set of cheese isolates belonging to the Lactobacillus casei group. Through this study, we demonstrated that this gene is widely found in the genomes of members of the L. casei group and shows variability useful for taxonomic studies. In particular, the HRM analysis method allowed for a specific discrimination between Lactobacillus rhamnosus, Lactobacillus paracasei and L. casei. Regarding the coding region, the spxB functionality in cheese was shown for the first time by real-time PCR, and by exploiting the heterogeneity between the L. casei group species, we identified the bacterial communities encoding the spxB gene in this ecosystem. This study allowed for monitoring of the active bacterial community involved in different stages of ripening by following the POX pathway.

Comparison of genes required for H2O2 resistance in Streptococcus gordonii and Streptococcus sanguinis

Hydrogen peroxide (H2O2) is produced by several members of the genus Streptococcus mainly through the pyruvate oxidase SpxB under aerobic growth conditions. The acute toxic nature of H2O2 raises the interesting question of how streptococci cope with intrinsically produced H2O2, which subsequently accumulates in the microenvironment and threatens the closely surrounding population. Here, we investigate the H2O2 susceptibility of oral Streptococcus gordonii and Streptococcus sanguinis and elucidate potential mechanisms of how they protect themselves from the deleterious effect of H2O2. Both organisms are considered primary colonizers and occupy the same intraoral niche making them potential targets for H2O2 produced by other species. We demonstrate that S. gordonii produces relatively more H2O2 and has a greater ability for resistance to H2O2 stress. Functional studies show that, unlike in Streptococcus pneumoniae, H2O2 resistance is not dependent on a functional SpxB and confirms the important role of the ferritin-like DNA-binding protein Dps. However, the observed increased H2O2 resistance of S. gordonii over S. sanguinis is likely to be caused by an oxidative stress protection machinery present even under anaerobic conditions, while S. sanguinis requires a longer period of time for adaptation. The ability to produce more H2O2 and be more resistant to H2O2 might aid S. gordonii in the competitive oral biofilm environment, since it is lower in abundance yet manages to survive quite efficiently in the oral biofilm.

The impact of horizontal gene transfer on the adaptive ability of the human oral microbiome

The oral microbiome is composed of a multitude of different species of bacteria, each capable of occupying one or more of the many different niches found within the human oral cavity. This community exhibits many types of complex interactions which enable it to colonize and rapidly respond to changes in the environment in which they live. One of these interactions is the transfer, or acquisition, of DNA within this environment, either from co-resident bacterial species or from exogenous sources. Horizontal gene transfer in the oral cavity gives some of the resident bacteria the opportunity to sample a truly enormous metagenome affording them considerable adaptive potential which may be key to survival in such a varying environment. In this review the underlying mechanisms of HGT are discussed in relation to the oral microbiome with numerous examples described where the direct acquisition of exogenous DNA has contributed to the fitness of the bacterial host within the human oral cavity.

Two-component system VicRK regulates functions associated with establishment of Streptococcus sanguinis in biofilms

Streptococcus sanguinis is a commensal pioneer colonizer of teeth and an opportunistic pathogen of infectious endocarditis. The establishment of S. sanguinis in host sites likely requires dynamic fitting of the cell wall in response to local stimuli. In this study, we investigated the two-component system (TCS) VicRK in S. sanguinis (VicRKSs), which regulates genes of cell wall biogenesis, biofilm formation, and virulence in opportunistic pathogens. A vicK knockout mutant obtained from strain SK36 (SKvic) showed slight reductions in aerobic growth and resistance to oxidative stress but an impaired ability to form biofilms, a phenotype restored in the complemented mutant. The biofilm-defective phenotype was associated with reduced amounts of extracellular DNA during aerobic growth, with reduced production of H2O2, a metabolic product associated with DNA release, and with inhibitory capacity of S. sanguinis competitor species. No changes in autolysis or cell surface hydrophobicity were detected in SKvic. Reverse transcription-quantitative PCR (RT-qPCR), electrophoretic mobility shift assays (EMSA), and promoter sequence analyses revealed that VicR directly regulates genes encoding murein hydrolases (SSA_0094, cwdP, and gbpB) and spxB, which encodes pyruvate oxidase for H2O2 production. Genes previously associated with spxB expression (spxR, ccpA, ackA, and tpK) were not transcriptionally affected in SKvic. RT-qPCR analyses of S. sanguinis biofilm cells further showed upregulation of VicRK targets (spxB, gbpB, and SSA_0094) and other genes for biofilm formation (gtfP and comE) compared to expression in planktonic cells. This study provides evidence that VicRKSs regulates functions crucial for S. sanguinis establishment in biofilms and identifies novel VicRK targets potentially involved in hydrolytic activities of the cell wall required for these functions.