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Open Access

Peer-reviewed

Research Article

Streptococcus sanguinis induces neutrophil cell death by production of hydrogen peroxide

  • Ryuichi Sumioka,

    Affiliations Department of Oral and Molecular Microbiology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan, Department of Restorative Dentistry and Endodontology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯
  • Masanobu Nakata ,

    nakata@dent.osaka-u.ac.jp

    Affiliation Department of Oral and Molecular Microbiology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯
  • Nobuo Okahashi,

    Affiliation Center for Frontier Oral Science, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯
  • Yixuan Li,

    Affiliation Department of Oral and Molecular Microbiology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯
  • Satoshi Wada,

    Affiliation Department of Oral and Molecular Microbiology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯
  • Masaya Yamaguchi,

    Affiliation Department of Oral and Molecular Microbiology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯
  • Tomoko Sumitomo,

    Affiliation Department of Oral and Molecular Microbiology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯
  • Mikako Hayashi,

    Affiliation Department of Restorative Dentistry and Endodontology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯
  • Shigetada Kawabata

    Affiliation Department of Oral and Molecular Microbiology, Osaka University Graduate School of Dentistry, Suita-Osaka, Japan

    ⨯

Abstract

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.

Citation: Sumioka R, Nakata M, Okahashi N, Li Y, Wada S, Yamaguchi M, et al. (2017) Streptococcus sanguinis induces neutrophil cell death by production of hydrogen peroxide. PLoS ONE 12(2): e0172223. https://doi.org/10.1371/journal.pone.0172223

Editor: Nades Palaniyar, Hospital for Sick Children, CANADA

Received: August 10, 2016; Accepted: February 1, 2017; Published: February 21, 2017

Copyright: © 2017 Sumioka et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper.

Funding: This work was supported in part by KAKENHI Grants-in-Aid for Scientific Research (B) (#JP15H05012), (C) (#JP26463112), and (C) (#JP26462779) awarded to SK, NO, and MN, respectively, from the Japan Society for the Promotion of Science (https://www.jsps.go.jp/english/).

Competing interests: The authors have declared that no competing interests exist.

Introduction

Streptococcus sanguinis, a member of the mitis group of streptococci, is an initial colonizer on tooth surfaces and promotes dental biofilm formation in an oral environment [1,2]. The mitis group of streptococci is comprised of that and other oral streptococci, such as Streptococcus gordonii, Streptococcus oralis, and the human pathogen Streptococcus pneumoniae [3], and is the leading cause of community-acquired infective endocarditis [4,5]. S. sanguinis, originally isolated from a patient with subacute bacterial endocarditis [6], has been shown to be related to bacteremia caused by dental treatment, daily toothbrushing, and ongoing dental lesions [7]. Hence, epidemiological data may have clinical relevance to show the possibility that S. sanguinis is endowed with the ability to extend its survival in blood.

Neutrophils, the most abundant type of phagocyte present in blood, constitute an essential part of the host innate immune system and possess various strategies to combat invading bacteria [8]. In addition to phagocytosis, neutrophils also kill bacteria in an extracellular manner by the use of neutrophil extracellular traps (NETs), which are DNA-based web-like structures possessing antimicrobial components, including histones, defensins, myeloperoxidase, and neutrophil elastase, and have functions to ensnare and kill pathogens [9]. Several lines of evidence have indicated that either in vitro infection with a pathogenic bacterial strain or stimulation with agents such as interleukin 8, lipopolysaccharide, and phorbol 12-myristate 13-acetate (PMA) trigger the active process, by which NET-associated cell death, i.e., NETosis, and subsequent release of NETs are induced [10,11]. Despite being dependent on a stimulus, reactive oxygen species are indispensable for NET formation, as neutrophils defective in NADPH oxidase activity are not able to undergo NETosis in response to PMA [10]. In PMA-induced NETosis, following activation of the Raf-MEK-ERK pathway, citrullination of histone occurs in a manner dependent on peptidyl arginine deiminase type 4 (PAD4) [12]. Resistance by the mitis group of streptococci to neutrophil bactericidal activity has been indicated as a potential virulent determinant in an endocarditis infection model, thus the ability of S. sanguinis to combat neutrophil killing strategies including NETs is most likely associated with development of vascular diseases, such as infective endocarditis [13]. However, the detailed mechanism by which S. sanguinis evades the immune system remains unclear.

Oral streptococci, including S. sanguinis, metabolize carbohydrates and produce lactate and pyruvate via glycolytic pathways. Under aerobic growth conditions, pyruvate is preferentially converted to acetyl-CoA and acetyl phosphate [14]. The pyruvate oxidase SpxB, encoded by a gene that is considerably conserved among the majority of oral streptococci [15], catalyses the conversion of pyruvate, inorganic phosphate, and oxygen to hydrogen peroxide, carbon dioxide, and acetyl phosphate [14], with acetyl phosphate serving as a precursor for ATP biosynthesis [16]. The mitis group of streptococci produces H2O2 at concentrations ranging from 0.7–1.6 mM in mixed-species biofilm [17], which is antagonistic against Streptococcus mutans present in oral biofilm [18]. This production of H2O2 is mostly attributable to SpxB enzymatic activity. S. pneumoniae SpxB has been implicated in carbohydrate metabolism, modulation of fatty acid composition in the cell membrane, resistance to oxidative stress, biofilm formation, induction of competence, and bacterial virulence [19–26]. The involvement of SpxB in cytotoxicity and virulence has been investigated using animal infection models and in vitro cytotoxicity assays, which showed that H2O2 produced by S. pneumoniae reacts with host nitric oxide to generate toxic peroxynitrite, which causes cellular damage, such as that seen in pneumococcal meningitis [27]. Furthermore, pneumococcal H2O2 also impedes the ciliary beat of human epithelium, which may contribute to the pathogenesis of respiratory tract infections [28]. Additionally, Rai et al. [29] reported that H2O2 produced by S. pneumoniae induces DNA double strand breaks in human alveolar epithelial cells, followed by induction of apoptosis. It has also been suggested that other mitis group streptococci including S. sanguinis exert virulence via H2O2 cytotoxic activity [30].

We previously reported that H2O2 produced by S. oralis and S. sanguinis was sufficient to induce cell death of epithelial cells and THP-1 macrophages [30–32]. More recently, we also demonstrated that H2O2 produced by S. oralis induces lysosomal impairment of infected RAW 264 macrophages, leading to cell death [33]. Of note, the cytotoxicity of H2O2 and its induction of cell death were found to be dependent on its concentration, and varied among cell types. In the present study, we examined whether H2O2 produced by S. sanguinis exhibits cytotoxicity against neutrophils, as well as its contribution to bacterial survival in blood and evasion from killing by neutrophils.

Materials and methods

Bacterial strains and culture conditions

The wild-type S. sanguinis strain SK36 (kindly provided by Dr. Kilian) [34], hereafter referred to as WT, and its derivatives were routinely cultured in Todd-Hewitt broth (TH, Becton Dickinson, NJ, USA) at 37°C. Overnight cultures were diluted 100-fold in 3 ml of TH broth and cultured at 37°C in a normal atmosphere until the optical density at 600 nm (OD600) reached 0.9 (late-exponential phase), unless otherwise stated. For infection of neutrophils, bacteria were washed with PBS, then the OD600 value of the suspension was adjusted to 0.2 using RPMI1640 medium (Wako, Japan).

Construction of spxB-deletion mutant strain of SK36

Construction of spxB-deletion (KO) and revertant (Wr) strains of WT was performed using the temperature sensitive shuttle vector pSET6s, as previously described [35]. Briefly, the upstream and downstream flanking region DNAs of spxB were connected by overlap PCR, and cloned into pSET6s, then the resulting constructs were transformed into SK36 using a synthetic competence stimulating peptide. In the presence of chloramphenicol, a merodiploid mutant strain was created after the initial allelic replacement, and then resolved to possess either mutant or wild-type alleles after the second allelic replacement by decreasing the culture temperature to 28°C and removing chloramphenicol from the culture. To examine the effects of secondary mutations that may have arisen during mutagenesis, a clone possessing the wild-type allele was defined as a revertant strain and utilized in this study. Both in-frame deletion mutant and revertant strains arose from the same merodiploid ancestor. Deletion of the spxB gene or reversion to the wild-type allele was confirmed by site-specific PCR.

Evaluation of H2O2 production

H2O2 production by S. sanguinis strains was evaluated following growth on agar plates, as previously reported [36]. Overnight cultures of S. sanguinis strains were diluted 100-fold in 3 ml of TH broth and then cultured until the exponential phase (OD600 = 0.5) at 37°C in a normal atmosphere. Next, 5-μl aliquots were spotted on brain heart infusion (Becton Dickinson) agar medium containing 1 mg/ml ferric chloride (FeCl3·6H2O, Wako) and 1 mg/ml potassium hexacyanoferrate (III) (K3[FeIII(CN)6], Wako) (henceforth, Prussian blue agar). The combination of hexacyanoferrate (III) and iron (III) ions in an aqueous solution yields blue precipitates of Prussian blue in the presence of H2O2, allowing for detection of H2O2 production around colonies by the appearance of blue halos. The plates were incubated at 37°C for 24 h, then the areas of blue halos were determined using Image J software (http://imagej.net/Welcome). The concentration of H2O2 in liquid cultures was quantified using an Amplex Red Hydrogen Peroxide/Peroxidase Assay Kit (Life Technologies, CA, USA), according to the manufacturer's protocol. Briefly, supernatants of overnight cultures were reacted with 10-acetyl-3, 7-dihydroxyphenoxazine and generation of the red oxidation product resorufin was examined by measuring absorbance at 550 nm (A550).

Blood bactericidal test

To evaluate bactericidal activity in human whole blood, a Lancefield bactericidal assay was performed with minor modifications [37]. Human whole blood was collected from a healthy donor. Next, 180 μl of heparinized blood was mixed with 20 μl of an S. sanguinis suspension containing 1x105 colony forming units (CFU). Following 0.5–3 h of incubation at 37°C on a rotary mixer, the mixture was seeded onto TH agar plates, then after 24 h of incubation, grown colonies were counted. Survival rate was calculated as follows; survival rate = recovered CFU/inoculated CFU x100.

Neutrophil bactericidal assay

We also investigated the bactericidal activity of neutrophils, which were isolated from heparinized human blood using a previously described method [35]. Cells were suspended in RPMI medium containing 10% fetal calf serum (FCS, SAFC Biosciences, KS, USA) with or without catalase (100 units/ml, Sigma-Aldrich). Next, 5x105 cells were mixed with a bacterial suspension at a multiplicity of infection (MOI) of 10, then incubated at 37°C for 1 or 3 h in an atmosphere containing 5% CO2. Survival rate was calculated using the same formula described above.

Neutrophil cell death induced by S. sanguinis

Neutrophils were infected with S. sanguinis strains at an MOI of 10. As a positive control, cells were incubated with 200 nM of PMA (Wako). After incubation for 1–3 h, lactate dehydrogenase (LDH) released into culture supernatant was evaluated using a CytoTox 96 Non-Radioactive Cytotoxicity Assay kit (Promega, WI, USA), as previously described [38]. The absorbance of cell lysates treated with PBS containing 0.25% Triton X-100 (Wako) was set at 100%. Cytotoxicity was calculated using the following equation: [(absorbance of lysate of cells infected with S. sanguinis)-(absorbance of lysate of non-infected cells)]/(absorbance of cells treated with PBS containing 0.25% Triton X-100) x100 (%).

NET formation induced by S. sanguinis and H2O2

Neutrophils (5x105 cells) were seeded into each well of poly-L-lysine (Sigma Aldrich)-coated 8-well chamber slides (Nalge Nunc, NY, USA) and incubated for 30 min to allow the cells to attach to the well bottom. Following a medium change, cells were infected with the S. sanguinis strains at an MOI of 10. Exogenous addition of H2O2 or 200 nM of PMA served as a positive control. After 3 h of incubation, cells were fixed with PBS containing 4% paraformaldehyde at room temperature for 20 min. Next, cells were blocked overnight with PBS containing 1% bovine serum albumin (Sigma Aldrich) at 4°C and reacted with a goat anti-human elastase polyclonal antibody (1:2000, Santa Cruz Biotechnology, CA, USA) at room temperature for 1 h. After washing with PBS, cells were incubated with Alexa Fluor 594-conjugated anti-goat IgG (1:1000, Molecular Probes, OR, USA) at room temperature for 1 h. Finally, each slide was enclosed with ProLong Gold Antifade Reagent with DAPI (Life Technologies) and observed using a Carl Zeiss Axioplan 2 fluorescent microscope system.

Detection of histone citrullination in infected neutrophils

Neutrophils (1.5x106 cells) were incubated for 30 min in a 24-well polystyrene plate. Following a medium change, cells were infected with S. sanguinis at an MOI of 10. Cells treated with 200 nM PMA served as a positive control. After 1 h of incubation at 37°C, cells were suspended with Laemmli gel loading buffer [6.25 mM Tris-HCl (pH 6.8), 4% sodium dodecylsulfate, 10% glycerol, 0.005% bromophenol blue, 50 mM dithiothreitol]. Histone citrullination was detected by immunoblot analysis with a monoclonal antibody against citrullinated histone H3 (rabbit, 1:2000, Abcam, Cambridge, UK). As a loading control, β-actin was detected using an anti-β-actin antibody (rabbit, 1:2000, Cell Signaling, MA, USA). Horseradish peroxidase (HRP)-conjugated antibody against rabbit IgG (1:2000, Cell Signaling) was used as a secondary antibody. Immunoreactive bands were detected using Prime Western Blotting Detection Reagent (GE Healthcare Life Sciences, IL, USA).

Quantitation of NETs

Neutrophils (5x105 cells) were incubated for 30 min in a 96-well plate. The culture medium was changed to RPMI1640 medium with or without catalase (100 units/ml, Sigma-Aldrich). Next, cells were infected with S. sanguinis at an MOI of 10 and incubated for 1–3 h at 37°C, then washed with PBS and reacted with 50 nM of SYTOX Green (Life Technologies), a cell-impermeable DNA-staining dye, for 10 min. The fluorescence of each sample was measured at an excitation/emission wavelength of 485/535 nm, with background fluorescence for non-infected cells subtracted from each obtained value.

Nitroblue tetrazolium test

Neutrophils were preincubated for 1 h with 20 μM Z-VAD-FMK or dimethyl sulfoxide, as a vehicle control, followed by addition of nitroblue tetrazolium (0.4 mg/ml, Sigma Aldrich) and infection with the WT strain at an MOI of 10. After a 1-h incubation at 37°C, the same volume of 0.5 M hydrochloric acid was added to terminate the reaction. Next, the plate was centrifuged at 2000 x g for 10min, after which the supernatants were gently discarded. Formazan was dissolved in dimethyl sulfoxide and absorbance at 550 nm was measured. Non-infected cells served as a control.

Statistical analysis

The significance of differences between groups was evaluated using the GraphPad Prism software package (ver. 7.0). Statistical tests utilized for each experiment are shown in legends accompanying the figures. A confidence interval with a p value of <0.05 was deemed to indicate significance.

Ethics statement

Human venous blood was obtained from healthy volunteers (age range, 27–31 years) with no systemic disease, and no recent history of infectious disease or medications that could alter neutrophil function. Written informed consent was obtained according to a protocol approved by the institutional review board of Osaka University Graduate School of Dentistry. The ethics committee specifically approved this study (approval no. H26-E43).

Results

SpxB contributes to H2O2 production of S. sanguinis SK36

To investigate the role of H2O2 produced by S. sanguinis in bacterial survival in blood, we constructed spxB deletion mutant (KO) and revertant (Wr) strains of S. sanguinis SK36 (WT). The growth rate of the KO strain was similar to that of the WT and Wr strains (data not shown). Overnight cultures of S. sanguinis strains were spotted onto Prussian blue agar to visualize H2O2 generation. After 24 h of incubation in an atmospheric condition, the KO strain exhibited remarkably less prominent blue halos around the colonies, as compared to the WT and Wr strains (Fig 1A), and densitometric analysis indicated that the remarkable reduction in halo area was caused by spxB deletion (Fig 1B). H2O2 in S. sanguinis cultures was also quantified using a peroxidase reaction and Amplex Red, a colorimetric substrate, which showed that its concentration in culture supernatants of the WT and Wr strains was 2.9±0.5 and 3.6±0.8 mM, respectively (Fig 1C), thus deletion of spxB led to a significant decrease in H2O2 concentration (0.9±0.3 mM). Differences in H2O2 concentration between the WT/Wr and KO strains were also noted under an atmospheric condition with 5% CO2 (Fig 1C). Under the anaerobic condition, no halos were observed around the colonies, indicating the requirement of oxygen for SpxB activity (data not shown). Thus, in agreement with previous reports [16, 25, 39, 40], SpxB is the main enzyme responsible for oxygen-dependent H2O2 production by S. sanguinis SK36.

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Fig 1. Effect of spxB deletion on H2O2 production by S. sanguinis.

(A) H2O2 production by S. sanguinis strains was examined using Prussian blue agar plates. Overnight cultures were spotted on the plates and incubated for 24 h. BHI medium was used as a negative control. Bar, 5 mm. (B) After 24 h of incubation, the area of blue halos around colonies was determined using Image J software. Data are shown as the mean ± SD of triplicate samples from 3 independent experiments. Statistically significant differences were evaluated using one-way ANOVA and Tukey’s multiple comparison test. *p<0.01. (C) Concentrations of H2O2 in culture supernatants of S. sanguinis strains grown overnight were quantified using peroxidase reactions. Data are shown as the mean ± SD from 3 (B) or 4 independent experiments. Statistically significant differences were evaluated using two-way ANOVA and Tukey’s multiple comparison test. *p<0.01.

https://doi.org/10.1371/journal.pone.0172223.g001

H2O2 produced by S. sanguinis abrogates bactericidal activity of human whole blood

We then examined whether SpxB is involved in survival of S. sanguinis in blood. S. sanguinis strains were exposed to heparinized human blood and the temporal survival rate was measured at 0.5, 1.5, and 3 h after infection (Fig 2A). At both 0.5 and 1.5 h after infection, the KO strain showed a significantly lower survival rate than the WT and Wr strains. At 0.5 h after infection, the survival rate of the WT and Wr strains was 20.3% and 20%, respectively, which was decreased by 3.3% and 4.0%, respectively, at 3 h after infection. On the other hand, the survival rate of the KO strain at 0.5 and 3 h after infection was 7.5% and 1.5%, respectively. These findings suggested that deletion of spxB renders S. sanguinis more susceptible to blood bactericidal activity.

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Fig 2. Bactericidal tests with S. sanguinis strains.

(A) Human whole blood (180 μl) was mixed with the WT, KO, or Wr strain (1x105 CFU). Following incubation for 0.5–3 h at 37°C, each mixture was seeded onto TH agar plates. Grown colonies were counted and the survival rate of each S. sanguinis strain is shown as % inoculum. Data are presented as the mean ± SD of triplicate samples from 3 independent experiments. Statistically significant differences were evaluated using one-way ANOVA and Tukey’s multiple comparison test. *p<0.01, **p<0.05. (B) Neutrophils were incubated with the S. sanguinis strains at an MOI of 10 with or without catalase (100 units/ml). Following incubation for 1 or 3 h at 37°C, each mixture was plated on TH agar plates. The survival rate of S. sanguinis is shown as % inoculum. Data are presented as the mean ± SD of triplicate samples from 3 independent experiments. Statistically significant differences were evaluated using two-way ANOVA and Tukey’s multiple comparison test. *p<0.01, **p<0.05.

https://doi.org/10.1371/journal.pone.0172223.g002

H2O2 produced by S. sanguinis abrogates bactericidal activity of neutrophils

Since neutrophils are the most abundant phagocytes in blood, we next examined whether their bactericidal activity is influenced by H2O2 produced by S. sanguinis. Human neutrophils with or without the H2O2-detoxifying enzyme catalase were infected with S. sanguinis strains, then the temporal survival rate of each strain was monitored for several hours. In the absence of catalase, the KO strain displayed a marked reduction in survival rate over time as compared with the WT and Wr strains (Fig 2B), findings that were fairly consistent with those obtained in the whole blood bactericidal assays. On the other hand, addition of catalase reduced the rate of survival of the WT and Wr strains at 1 h after infection, whereas there was no significant change in survival rate of the KO strain at that time point. Furthermore, the difference in survival between the WT/Wr strains and KO strain disappeared at 3 h after infection by addition of catalase. Therefore, we consider that SpxB-mediated H2O2 production and/or other phenotypic change caused by the spxB mutation contributes to the sensitivity of S. sanguinis to bactericidal activity of neutrophils.

To examine whether the differences in susceptibility to bacteriocidal activity of neutrophils is caused by an effect of the spxB mutation on phagocytosis efficiency, the bacterial association with neutrophils was examined using infected neutrophils subjected to Giemsa staining (Fig 3). At 0.5 h after infection, the KO strain showed a greater level of phagocytosis by neutrophils as compared to the WT and Wr strains, while a difference was also observed between the KO and Wr strains at 1 h after infection. These results suggested that the survival differences between the WT/Wr and KO strains, as shown in Fig 2B, were attributable, at least in part, to different levels of phagocytosis of each strain.

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Fig 3. Bacterial association with human neutrophils.

Human neutrophils were pre-incubated for 30 min, then infected with the WT, KO, or Wr strain at an MOI of 10. Following incubation for 0.5 or 1 h, cells were fixed with methanol and visualized by Giemsa staining. Using phase-contrast microscopy, 10 random fields (x 1000) were observed and the amount of bacteria demonstrating phagocytosis by neutrophils was determined by counting bacterial chains. (A) Representative images. (B) The numbers of bacterial chains associated with neutrophils at 0.5 and 1 h after infection are shown as the mean ± SD of 5 independent experiments. Statistically significant differences were evaluated using one-way ANOVA and Tukey’s multiple comparison test. *p<0.01, **p<0.05.

https://doi.org/10.1371/journal.pone.0172223.g003

H2O2 produced by S. sanguinis is cytotoxic towards human neutrophils

As previously reported in studies that used monocyte and epithelial cell lines, S. sanguinis induces cell death via H2O2-mediated cytotoxicity [30–33]. Thus, we examined whether the decreased bactericidal activity of neutrophils is caused by cytotoxic effects of streptococcal H2O2. Using culture supernatants of neutrophils exposed to either the S. sanguinis stains at an MOI of 10 or positive control PMA for 1 or 3 h, cytotoxicity against neutrophils was evaluated by measuring the release of LDH. That released from cells infected with WT or Wr strain was nearly the same as that from PMA-treated cells (Fig 4). On the other hand, it is important to note that the spxB mutation impaired the release of LDH from neutrophils, though the KO strain showed more efficient phagocytosis by neutrophils at 0.5 h after infection as compared to the WT and Wr strains, as well as at 1.0 h as compared to the Wr strain (Fig 3). These findings suggest that H2O2 produced by S. sanguinis provokes neutrophil cell death, which most likely affects the net bactericidal activity of neutrophils.

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Fig 4. H2O2 produced by S. sanguinis showed cytotoxicity against neutrophils.

Human neutrophils were mixed with the S. sanguinis WT, KO, or Wr strain at an MOI of 10. As a positive control, PMA was added at concentrations of 200 nM. Following incubation for 1 or 3 h, LDH released into culture supernatant was examined. Cytotoxicity was determined by relative absorbance, with the absorbance of cells lysed with PBS containing 0.25% Triton X-100 set at 100%. Data are presented as the mean ± SD from 3 independent experiments. Statistically significant differences were evaluated using one-way ANOVA and Tukey’s multiple comparison test. *p<0.01.

https://doi.org/10.1371/journal.pone.0172223.g004

H2O2 produced by S. sanguinis induces NET formation

Since reactive oxygen species are involved in NET induction, we also examined whether extracellular H2O2 produced by S. sanguinis induces NETs. DNA release from neutrophils, a characteristic of NETs, was evaluated by staining extracellular DNA with SYTOX green, which is unable to penetrate an intact membrane (Fig 5). Those results revealed the presence of abundant web-like extracellular DNA from cells infected with either the WT or Wr strain, indicating NET formation, which was similar to the results obtained with PMA-treated cells. In contrast, the staining pattern was scarcely detectable in non-treated and KO strain-infected cells. Exogenously added 1 mM H2O2 also induced release of extracellular DNA. To examine the nucleus morphology of infected cells, neutrophil elastase and DNA were fluorescently labeled. Neutrophils infected with the WT or Wr strain exhibited de-condensed and non-lobulated nuclei, which were not observed in non-infected or KO strain-infected cells (S1 Fig). Cells treated with PMA or 1 mM H2O2 also showed de-condensed and non-lobulated nuclei. Therefore, it is conceivable that S. sanguinis induces NETs by cytotoxicity of self-generated H2O2.

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Fig 5. H2O2 produced by S. sanguinis induced release of NETs from infected neutrophils.

Neutrophils were infected with the S. sanguinis strains at an MOI of 10. Exogenously added H2O2 (1 mM) or PMA (200 nM) served as a control. Following incubation for 3 h, cells were reacted with SYTOX Green (50 nM), then observed with a fluorescent microscope. Bar, 20 μm.

https://doi.org/10.1371/journal.pone.0172223.g005

H2O2 produced by S. sanguinis induces citrullination of histones in infected human neutrophils

It has been reported that citrullination of arginine residues in histones H3 and H4 is a crucial step leading to loss of positive charge and subsequent chromatin unfolding during PMA-driven NET induction [32]. Immunoblot analysis revealed acceleration of the citrullination level in neutrophils infected with the WT or Wr strain, as compared with non-infected cells (Fig 6), and that level was equivalent to that induced by PMA. In contrast, neutrophils infected with the KO strain showed a background level of citrullination. Based on these results, we consider it likely that the form of neutrophil cell death induced by S. sanguinis H2O2 includes NETosis.

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Fig 6. Detection of histone citrullination in S. sanguinis-infected neutrophils.

Neutrophils were infected with the WT, KO, or Wr strain at an MOI of 10. Following incubation for 1 h at 37°C, cells were suspended in Laemmli gel loading buffer. Samples were immunoblotted with a rabbit anti-histone H3 antibody and HRP-conjugated anti-rabbit IgG antibody (upper panel). As a loading control, β-actin was detected using an anti-β-actin antibody and HRP-conjugated anti-rabbit IgG antibody (lower panel).

https://doi.org/10.1371/journal.pone.0172223.g006

H2O2 produced by S. sanguinis stimulates NET formation

Finally, neutrophil extracellular DNA was evaluated by measuring fluorescent intensity following SYTOX green staining. The quantity of extracellular DNA from neutrophils infected with the KO strain was significantly lower as compared to that from cells infected with the WT or Wr strain (Fig 7). Furthermore, exogenous addition of catalase decreased the amount of extracellular DNA from neutrophils infected with the WT or Wr strain, whereas no effects were observed with KO strain-infected or PMA-treated cells. We also examined the amount of extracellular DNA derived from S. sanguinis cells [41]. When the S. sanguinis strains were cultured without neutrophils under the same condition used for the infection study, the level of fluorescent intensity was nearly the same as that of the background, indicating that S. sanguinis releases far lower amounts or no DNA under the present test conditions, as compared to neutrophils. These findings show that H2O2 produced by S. sanguinis mediates survival of bacteria encountering neutrophils via cytotoxicity and NET induction.

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Fig 7. Extracellular DNA from neutrophils increased by streptococcal H2O2.

Neutrophils were infected with the S. sanguinis strains at an MOI of 10 with or without catalase (100 units/ml), and cultured for 1–3 h. Neutrophils treated with 200 nM of PMA served as a control. Cells were reacted with SYTOX Green and fluorescent intensity was determined. Data are presented as the mean ± SD of triplicate samples from 3 independent experiments. Statistically significant differences were evaluated using two-way ANOVA and Tukey’s multiple comparison test. *p<0.01.

https://doi.org/10.1371/journal.pone.0172223.g007

Discussion

Routine dental treatment leads to substantial invasion of oral bacteria into the bloodstream resulting in transient bacteremia [42–44], followed by systemic dissemination and occasional onset of extra-oral diseases, including infective endocarditis and polymicrobial abscess [42, 45]. Although infective endocarditis is a rare disease with an annual incidence rate ranging from 3 to 10 episodes per 100,000 individuals in the general population, the mortality rate is considerably high at 10–26% [43, 46, 47]. It is noteworthy that S. sanguinis is one of the most common species detected in related lesions [42]. However, the molecular mechanisms involved in dissemination of S. sanguinis from the oral cavity to the bloodstream, as well as the adaptation mechanism required for persistence in both the bloodstream and vegetative lesions are largely unknown. In the light of recent findings showing that H2O2 produced by mitis group streptococci is cytotoxic towards various types of cells [30–33] and that neutrophils are related to the pathology of infective endocarditis [48, 49], we examined the cytotoxic effects of H2O2 produced by S. sanguinis against neutrophils and the role of SpxB for survival in blood in the present study.

Our findings provide important clues regarding the role of SpxB in survival of S. sanguinis organisms when encountering neutrophils. The present results showed that SpxB of S. sanguinis contributes to cytotoxicity against neutrophils (Figs 2 and 3). Based on our findings showing an inhibitory effect of catalase on cytotoxicity, we consider that H2O2 may play a role in that effect. In addition, NETosis seems to be, at least in part, the cell death form of neutrophils, as extracellular DNA and citrullinated histone were clearly detected in infected neutrophils, while they were less apparent following catalase treatment (Figs 4–7).

It was previously reported that activation of peptidyl arginine deiminase-4 is necessary for PMA-induced NET formation, which leads to replacement of charged arginine residues in histone proteins with neutrally charged citrulline residues, allowing for de-condensation of nuclear chromatin [10]. We found that induction of apoptosis was not relevant to cytotoxicity, since a broad-range apoptosis inhibitor had no effects on the ability of infected neutrophils to produce ROS, as determined with a nitroblue tetrazolium test (data not shown). Diverse cellular responses to exogenous H2O2 have been noted in mammalian cells, while distinct responses are dependent on its concentration [30–33, 50–53]. Since SpxB is involved in carbohydrate metabolism and requires oxygen for enzymatic activity, the environment for streptococcal growth undoubtedly has an impact on SpxB enzymatic activity and H2O2 production. Therefore, in contrast to the situation in blood, the level of cytotoxicity against neutrophils and possible cell death form may differ in the oral cavity, the inherent niche.

At onset, infective endocarditis becomes established on vulnerable sites of heart valves where endothelium has become exfoliated, then blood coagulum consisting of extracellular matrix proteins, plasma proteins, and platelets is subsequently formed upon contact between sub-endothelium and blood. Thereafter, disease progression is considered to be a multistep process [5], with the first step involving attachment and colonization of circulating bacteria on damaged heart tissues. Several surface proteins of S. sanguinis have been implicated in adherence to host tissues and cells [54–58]. The second step is comprised of recruitment and activation of blood monocytes, which induce production of cytokines and procoagulant factors contributing to inflammation and vegetation growth. During this process, colonized bacteria must elude the host defense system and persist in vegetation. Finally, successive accumulation of clots, leukocytes, and bacteria accelerates formation of infected vegetation. Thus, interactions among host tissues, immune cells, and bacteria underlie the pathological condition of infective endocarditis. In addition, a growing body of evidence suggests that neutrophils are also recruited to septic vegetation and NET formation is subsequently induced, thus NET is likely involved in disease pathogenesis as a scaffold for vegetation growth [48, 49, 59, 60]. It has also been reported that activated platelets promote NET induction and subsequent formation of septic thrombi in blood vessels [61]. Moreover, neutrophils play a pivotal role in defense against circulating mitis group streptococci in blood, as demonstrated by the fact that a major risk factor for bacteremia due to mitis group streptococci is severe neutropenia [62–64]. Therefore, the ability of S. sanguinis to induce NETs may be related to development of infective endocarditis.

NETs were originally reported to possess an activity that inhibits growth of ensnared bacteria. We previously showed that S. sanguinis expresses a unique cell wall-associated nuclease, designated SWAN, which allows S. sanguinis to escape from NET bactericidal activity [35]. Thus, S. sanguinis can evade bactericidal activity of neutrophils in two different ways, i.e.; direct killing of neutrophils via H2O2 cytotoxicity and degradation of NETs by SWAN nuclease activity.

Previously, exogenously added H2O2 was shown to promote neutrophil phagocytosis [65]. However, our observations of infected neutrophils indicated that the spxB mutant, which produced less H2O2, was more efficiently phagocytosed after 0.5 and 1 h as compared with the WT/Wr and Wr strains. Therefore, this mutant would be more susceptible to killing by phagocytosis during an early stage of infection, which is probably caused by phenotypic alteration, as reported for the spxB mutant of S. pneumoniae [19–26]. In agreement with those findings, in the present neutrophil bacteriocidal assay, the spxB mutant exhibited a lower survival rate than the WT and revertant strains. However, the difference in regard to bacterial survival rate disappeared with addition of catalase, indicating that H2O2 produced by S. sanguinis mainly influences survival rather than other potential phenotypic changes. In contrast, the cytotoxicity of S. sanguinis against neutrophils was decreased by spxB deletion, suggesting that the cytotoxicity against neutrophils is not mainly attributable to induction of phagocytosis-induced cell death, but rather to NET-induction mediated by H2O2.

The interaction of S. sanguinis with neutrophils was considered to be affected by phenotypic alteration modulated by SpxB. Based on studies of S. pneumoniae SpxB, in addition to reduced H2O2 production, a mutation in the spxB gene imparts multiple phenotypes of S. pneumoniae, which have effects on the bacterial association with neutrophils and susceptibility to their bactericidal activity, including changes in sugar utilization pattern, up-regulation of capsule production, alterations in adhesive properties such as the adhesin expression/membrane fatty acid composition, and susceptibility to H2O2 [19–26]. Although it is unclear whether these phenotypes of the spxB mutant are inherent in S. pneumoniae or common in S. sanguinis, we are unable to exclude the possibility that they influence, either synergistically or separately, the survival of S. sanguinis. Moreover, acetyl phosphate produced by SpxB may serve as a phosphate donor, likely with effects on the global transcriptional profile. These potential phenotypic changes induced by the spxB mutation in S. sanguinis remain to be defined in a future study, which is important for clarifying the impact of SpxB on S. sanguinis virulence.

The interaction of S. gordonii, a member of the mitis group, with neutrophils has been investigated. Using an experimental endocarditis model, Young Lee et al. reported that resistance to neutrophil killing is a potential virulence determinant of S. gordonii, since virulent strains of S. gordonii have an ability to resist the bactericidal activity of neutrophils [13]. They also found that survival of virulent strains was attributable to resistance against the intracellular killing mechanism following attachment to neutrophils and phagocytosis. In a future study, it would be intriguing to examine whether intracellular killing of S. sanguinis is affected by the spxB mutation.

The present results showed that the spxB mutation in S. sanguinis is detrimental to bacterial survival in human blood ex vivo. Thus far, several lines of evidence have provided insight into the role of SpxB in regard to adaptation of mitis group streptococci to various environments, including the bloodstream [20, 24, 26]. However, based on studies of pneumococcal SpxB, it remains controversial whether the spxB mutation is beneficial for survival in blood, as results have varied depending on the infection model and pneumococcal strain used. Syk et al. reported that infection with serotype 1 strains of 2 clonal complexes in both humans and mice gave rise to diverse mutations of the spxB gene, resulting in considerably increased virulence in mouse intravenous infection models and decreased early clearance of bacteria from the bloodstream, though nasopharyngeal colonization after intranasal infection was impaired, indicating that SpxB is required for colonization by serotype 1 strains, but negatively affects survival in blood [26]. Moreover, spxB expression was shown to be decreased in the lungs and bloodstream of mice infected with S. pneumoniae [66, 67]. In contrast, both invasiveness and colonization of the serotype 2 strain D39 were found to be attenuated by spxB deletion [20]. Therefore, adaptation of S. pneumoniae in the bloodstream likely varies among serotypes and strains. Although, typing of S. sanguinis strains is not currently feasible, antigenic variations in surface carbohydrates or proteins are obvious, as clinical isolates occasionally exhibit the Lancefield serotype H antigen [68]. Therefore, it is possible that the requirement of SpxB for survival in blood varies among S. sanguinis strains and future in vivo studies are needed to examine this issue. Furthermore, contradictory reports regarding the involvement of SpxB in pneumococcal survival in blood have been presented, thus the in vivo relevance of the current results is uncertain, since oxygen availability is low and catalase is continuously produced in blood. Nevertheless, our findings indicate that H2O2 produced by S. sanguinis has effects on the course of blood infection and modulates bacterial persistence within the host.

In summary, the present findings demonstrated that S. sanguinis evades neutrophil killing in vitro and counteracts innate immunity in collected blood by the action of SpxB. In a future study, together with phenotypic characterization of the spxB mutant, analysis using an in vivo infection model will be necessary to clarify the role of SpxB/H2O2 during the course of vascular disease progression. Elucidation of the interaction between S. sanguinis and host tissues, especially blood cells and endothelium, may lead to development of prophylaxis and therapeutic intervention for pathologies caused by oral streptococci.

Supporting information

S1 Fig. Neutrophil cell death induced by S. sanguinis.

Neutrophils were infected with the tested S. sanguinis strains at an MOI of 10. Exogenous addition of 1 mM of H2O2 or 200 nM of PMA to neutrophils served as a control. Following incubation for 3 h and subsequent fixation, neutrophil elastase was labeled with a goat anti-human elastase polyclonal antibody and Alexa Fluor 594-conjugated anti-goat IgG. Nuclei were stained with DAPI. Bar, 20 μm.

https://doi.org/10.1371/journal.pone.0172223.s001

(TIF)

Acknowledgments

We appreciate Dr. M. Kilian for kindly providing S. sanguinis strain SK36, and Drs. T. Sekizaki and D. Takamatsu for the pSET6s plasmid.

Author Contributions

  1. Formal analysis: RS MN NO SW MY TS MH SK.
  2. Funding acquisition: MN NO SK.
  3. Investigation: RS MN YL.
  4. Methodology: RS MN NO MY TS.
  5. Project administration: MN NO SK.
  6. Resources: MN NO SW MY TS MH SK.
  7. Supervision: MN NO SK.
  8. Validation: RS MN NO SK.
  9. Visualization: RS.
  10. Writing – original draft: RS MN NO SW MY TS MH SK.
  11. Writing – review & editing: MN NO TS.

References

  1. 1. Rosan B, Lamont RJ. Dental plaque formation. Microbes Infect. 2000;2: 1599–1607. pmid:11113379
  2. 2. Kuramitsu HK, He X, Lux R, Anderson MH, Shi W. Interspecies interactions within oral microbial communities. Microbiol Mol Biol Rev. 2007;71: 653–670. pmid:18063722
  3. 3. Kawamura Y, Hou XG, Sultana F, Miura H, Ezaki T. Determination of 16S rRNA sequences of Streptococcus mitis and Streptococcus gordonii and phylogenetic relationships among members of the genus Streptococcus. Int J Syst Bacteriol. 1995;45: 406–408. pmid:7537076
  4. 4. Benito N, Miró JM, de Lazzari E, Cabell CH, del Río A, Altclas J, et al. Health care-associated native valve endocarditis: importance of non-nosocomial acquisition. Ann Intern Med. 2009;150: 586–594. pmid:19414837
  5. 5. Que YA, Moreillon P. Infective endocarditis. Nat Rev Cardiol. 2011;8: 322–336. pmid:21487430
  6. 6. White JC, Niven CF Jr. Streptococcus S.B.E.: A Streptococcus Associated with Subacute Bacterial Endocarditis. J Bacteriol. 1946;51: 717–722. pmid:16561124
  7. 7. Thornhill MH, Dayer M, Lockhart PB, McGurk M, Shanson D, Prendergast B, et al. Guidelines on prophylaxis to prevent infective endocarditis. Br Dent J. 2016;220: 51–56. pmid:26794105
  8. 8. Mócsai A. Diverse novel functions of neutrophils in immunity, inflammation, and beyond. J Exp Med. 2013;210: 1283–1299. pmid:23825232
  9. 9. Brinkmann V, Reichard U, Goosmann C, Fauler B, Uhlemann Y, Weiss DS, et al. Neutrophil extracellular traps kill bacteria. Science. 2004;303: 1532–1535. pmid:15001782
  10. 10. Brinkmann V, Zychlinsky A. Neutrophil extracellular traps: is immunity the second function of chromatin? J Cell Biol. 2012;198: 773–783. pmid:22945932
  11. 11. Bardoel BW, Kenny EF, Sollberger G, Zychlinsky A. The balancing act of neutrophils. Cell Host Microbe. 2014;15: 526–536. pmid:24832448
  12. 12. Wang Y1, Li M, Stadler S, Correll S, Li P, Wang D, et al. Histone hypercitrullination mediates chromatin decondensation and neutrophil extracellular trap formation. J Cell Biol. 2009;184: 205–213. pmid:19153223
  13. 13. Young Lee S, Cisar JO, Bryant JL, Eckhaus MA, Sandberg AL. Resistance of Streptococcus gordonii to polymorphonuclear leukocyte killing is a potential virulence determinant of infective endocarditis. Infect Immun. 2006;74: 3148–3155. pmid:16714542
  14. 14. Carlsson J, Edlund MB. Pyruvate oxidase in Streptococcus sanguis under various growth conditions. Oral Microbiol Immunol. 1987;2: 10–14. pmid:3295684
  15. 15. Zhu L, Xu Y, Ferretti JJ, Kreth J. Probing oral microbial functionality—expression of spxB in plaque samples. PLoS One. 2014;9: e86685. pmid:24489768
  16. 16. Pericone C.D., Park S., Imlay J.A., and Weiser J.N. Factors contributing to hydrogen peroxide resistance in Streptococcus pneumoniae include pyruvate oxidase (SpxB) and avoidance of the toxic effects of the fenton reaction. J Bacteriol. 2003;185: 6815–6825. pmid:14617646
  17. 17. Liu X, Ramsey MM, Chen X, Koley D, Whiteley M, Bard AJ. Real-time mapping of a hydrogen peroxide concentration profile across a polymicrobial bacterial biofilm using scanning electrochemical microscopy. Proc Natl Acad Sci U S A. 2011;108: 2668–2673. pmid:21282623
  18. 18. Kreth J1, Zhang Y, Herzberg MC. Streptococcal antagonism in oral biofilms: Streptococcus sanguinis and Streptococcus gordonii interference with Streptococcus mutans. J Bacteriol. 2008;190: 4632–4640. pmid:18441055
  19. 19. Carvalho SM, Farshchi Andisi V, Gradstedt H, Neef J, Kuipers OP, Neves AR, et al. Pyruvate oxidase influences the sugar utilization pattern and capsule production in Streptococcus pneumoniae. PLoS One. 2013;8: e68277. pmid:23844180
  20. 20. Spellerberg B, Cundell DR, Sandros J, Pearce BJ, Idanpaan-Heikkila I, Rosenow C, et al. Pyruvate oxidase, as a determinant of virulence in Streptococcus pneumoniae. Mol Microbiol. 1996;19: 803–813. pmid:8820650
  21. 21. Benisty R, Cohen AY, Feldman A, Cohen Z, Porat N. Endogenous H2O2 produced by Streptococcus pneumoniae controls FabF activity. Biochim Biophys Acta. 2010;1801: 1098–1104. pmid:20601114
  22. 22. Blanchette-Cain K, Hinojosa CA, Akula Suresh Babu R, Lizcano A, Gonzalez-Juarbe N, Munoz-Almagro C, et al. Streptococcus pneumoniae biofilm formation is strain dependent, multifactorial, and associated with reduced invasiveness and immunoreactivity during colonization. MBio. 2013;4: e00745–13. pmid:24129258
  23. 23. Bättig P, Mühlemann K. Influence of the spxB gene on competence in Streptococcus pneumoniae. J Bacteriol. 2008;190: 1184–1189. pmid:18065543
  24. 24. Orihuela CJ, Gao G, Francis KP, Yu J, Tuomanen EI. Tissue-specific contributions of pneumococcal virulence factors to pathogenesis. J Infect Dis. 2004;190: 1661–1669. pmid:15478073
  25. 25. Regev-Yochay G, Trzcinski K, Thompson CM, Lipsitch M, Malley R. SpxB is a suicide gene of Streptococcus pneumoniae and confers a selective advantage in an in vivo competitive colonization model. J Bacteriol. 2007;189: 6532–6539. pmid:17631628
  26. 26. Syk A, Norman M, Fernebro J, Gallotta M, Farmand S, Sandgren A, et al. Emergence of hypervirulent mutants resistant to early clearance during systemic serotype 1 pneumococcal infection in mice and humans. J Infect Dis. 2014;210: 4–13. pmid:24443543
  27. 27. Hoffmann O, Zweigner J, Smith SH, Freyer D, Mahrhofer C, Dagand E, et al. Interplay of pneumococcal hydrogen peroxide and host-derived nitric oxide. Infect Immun. 2006;74: 5058–5066. pmid:16926397
  28. 28. Feldman C, Anderson R, Cockeran R, Mitchell T, Cole P, Wilson R. The effects of pneumolysin and hydrogen peroxide, alone and in combination, on human ciliated epithelium in vitro. Respir Med. 2002;96: 580–585. pmid:12195838
  29. 29. Rai P, Parrish M, Tay IJ, Li N, Ackerman S, He F, et al. Streptococcus pneumoniae secretes hydrogen peroxide leading to DNA damage and apoptosis in lung cells. Proc Natl Acad Sci U S A. 2015;112: E3421–3430. pmid:26080406
  30. 30. Okahashi N, Okinaga T, Sakurai A, Terao Y, Nakata M, Nakashima K, et al. Streptococcus sanguinis induces foam cell formation and cell death of macrophages in association with production of reactive oxygen species. FEMS Microbiol Lett. 2011;323: 164–170. pmid:22092716
  31. 31. Okahashi N, Nakata M, Sumitomo T, Terao Y, Kawabata S. Hydrogen peroxide produced by oral Streptococci induces macrophage cell death. PLoS One. 2013;8: e62563. pmid:23658745
  32. 32. Okahashi N, Sumitomo T, Nakata M, Sakurai A, Kuwata H, Kawabata S. Hydrogen peroxide contributes to the epithelial cell death induced by the oral mitis group of streptococci. PLoS One. 2014;9: e88136. pmid:24498253
  33. 33. Okahashi N, Nakata M, Kuwata H, Kawabata S. Streptococcus oralis Induces Lysosomal Impairment of Macrophages via Bacterial Hydrogen Peroxide. Infect Immun. 2016;84: 2042–2050. pmid:27113357
  34. 34. Kilian M, Mikkelsen L, Henrichsen J. Taxonomic study of viridans streptococci: description of Streptococcus gordonii sp. nov. and emended descriptions of Streptococcus sanguis (White and Niven 1946), Streptococcus oralis (Bridge and Sneath 1982), and Streptococcus mitis (Andrewes and Horder 1906). Int J Sys Bacteriol. 1989;39: 471–484.
  35. 35. Morita C, Sumioka R, Nakata M, Okahashi N, Wada S, Yamashiro T, et al. Cell wall-anchored nuclease of Streptococcus sanguinis contributes to escape from neutrophil extracellular trap-mediated bactericidal activity. PLoS One. 2014;9: e103125. pmid:25084357
  36. 36. Saito M, Seki M, Iida K, Nakayama H, Yoshida S. A novel agar medium to detect hydrogen peroxide-producing bacteria based on the prussian blue-forming reaction. Microbiol Immunol. 2007;51(9): 889–892. pmid:17895606
  37. 37. Lancefield RC. Differentiation of group A streptococci with a common R antigen into three serological types, with special reference to the bactericidal test. J Exp Med. 1957;107: 525–544.
  38. 38. Dacheux D, Attree I, Schneider C, Toussaint B. Cell death of human polymorphonuclear neutrophils induced by a Pseudomonas aeruginosa cystic fibrosis isolate requires a functional type III secretion system. Infect Immun. 1999;67: 6164–6167. pmid:10531282
  39. 39. Belanger AE, Clague MJ, Glass JI, Leblanc DJ. Pyruvate oxidase is a determinant of Avery's rough morphology. J Bacteriol. 2004;186: 8164–8171. pmid:15576764
  40. 40. Chen L, Ge X, Dou Y, Wang X, Patel JR, Xu P. Identification of hydrogen peroxide production-related genes in Streptococcus sanguinis and their functional relationship with pyruvate oxidase. Microbiology. 2011;157: 13–20. pmid:20847003
  41. 41. Kreth J, Vu H, Zhang Y, Herzberg MC. Characterization of hydrogen peroxide-induced DNA release by Streptococcus sanguinis and Streptococcus gordonii. J Bacteriol. 2009;191: 6281–6291. pmid:19684131
  42. 42. Douglas CW, Heath J, Hampton KK, Preston FE. Identity of viridans streptococci isolated from cases of infective endocarditis. J Med Microbiol. 1993;39: 179–182. pmid:8366515
  43. 43. Wray D, Ruiz F, Richey R, Stokes T, Guideline Development Group. Prophylaxis against infective endocarditis for dental procedures—summary of the NICE guideline. Br Dent J. 2008;204: 555–557. pmid:18500298
  44. 44. Roberts GJ, Jaffray EC, Spratt DA, Petrie A, Greville C, Wilson M, et al. Duration, prevalence and intensity of bacteraemia after dental extractions in children. Heart. 2006;92: 1274–1277. pmid:16488926
  45. 45. Xiao J, Klein MI, Falsetta ML, Lu B, Delahunty CM, Yates JR 3rd, et al. The exopolysaccharide matrix modulates the interaction between 3D architecture and virulence of a mixed-species oral biofilm. PLoS Pathog. 2012;8: e1002623. pmid:22496649
  46. 46. Stacy A, Everett J, Jorth P, Trivedi U, Rumbaugh KP, Whiteley M. Bacterial fight-and-flight responses enhance virulence in a polymicrobial infection. Proc Natl Acad Sci U S A. 2014;111: 7819–7824. pmid:24825893
  47. 47. Prendergast BD. The changing face of infective endocarditis. Heart. 2006;92: 879–885. pmid:16216860
  48. 48. Jung CJ, Yeh CY, Hsu RB, Lee CM, Shun CT, Chia JS. Endocarditis pathogen promotes vegetation formation by inducing intravascular neutrophil extracellular traps through activated platelets. Circulation. 2005;131: 571–581.
  49. 49. Al-Salih G1, Al-Attar N, Delbosc S, Louedec L, Corvazier E, Loyau S, Michel JB, Pidard D, Duval X, Meilhac O. Role of vegetation-associated protease activity in valve destruction in human infective endocarditis. PLoS One. 2012;7: e45695. pmid:23029186
  50. 50. Chandra J, Samali A, Orrenius S. Triggering and modulation of apoptosis by oxidative stress. Free Rad Biol Med. 2000;29: 323–333. pmid:11035261
  51. 51. Quinn J, Findlay VJ, Dawson K, Millar JB, Jones N, Morgan BA, et al. Distinct regulatory proteins control the graded transcriptional response to increasing H2O2 levels in fission yeast Schizosaccharomyces pombe. Mol Biol Cell. 2002;13: 805–816. pmid:11907263
  52. 52. Giorgio M, Trinei M, Migliaccio E, Pelicci PG. Hydrogen peroxide: a metabolic by-product or a common mediator of ageing signals? Nat Rev Mol Cell Biol. 2007;8: 722–728. pmid:17700625
  53. 53. Pelletier M, Lepow TS, Billingham LK, Murphy MP, Siegel RM. New tricks from an old dog: mitochondrial redox signaling in cellular inflammation. Semin Immunol. 2012;24: 384–392. pmid:23391428
  54. 54. Herzberg MC, Brintzenhofe KL, Clawson CC. Aggregation of human platelets and adhesion of Streptococcus sanguis. Infect Immun. 1983;39: 1457–1469. pmid:6188697
  55. 55. Ford I, Douglas CW. The role of platelets in infective endocarditis. Platelets. 1997;8: 285–294. pmid:16793661
  56. 56. Plummer C, Wu H, Kerrigan SW, Meade G, Cox D, Ian Douglas CW. A serine-rich glycoprotein of Streptococcus sanguis mediates adhesion to platelets via GPIb. Br J Haematol. 2005;129: 101–109. pmid:15801962
  57. 57. Okahashi N, Nakata M, Sakurai A, Terao Y, Hoshino T, Yamaguchi M, et al. Pili of oral Streptococcus sanguinis bind to fibronectin and contribute to cell adhesion. Biochem Biophys Res Commun. 2010;391: 1192–1196. pmid:20004645
  58. 58. Okahashi N, Nakata M, Terao Y, Isoda R, Sakurai A, Sumitomo T, et al. Pili of oral Streptococcus sanguinis bind to salivary amylase and promote the biofilm formation. Microb Pathog. 2011;50: 148–154. pmid:21238567
  59. 59. Sass LA, Ziemba KJ, Heiser EA, Mauriello CT, Werner AL, Aguiar MA, Nyalwidhe JO, Cunnion KM. A 1-Year-Old with Mycobacterium tuberculosis Endocarditis with Mass Spectrometry Analysis of Cardiac Vegetation Composition. J Pediatric Infect Dis Soc. 2016;5: 85–88. pmid:26908495
  60. 60. Augustin P, Alsalih G, Launey Y, Delbosc S, Louedec L, Ollivier V, et al. Predominant role of host proteases in myocardial damage associated with infectious endocarditis induced by Enterococcus faecalis in a rat model. Infect Immun. 2013;81: 1721–1729. pmid:23478315
  61. 61. Fuchs TA, Brill A, Duerschmied D, Schatzberg D, Monestier M, Myers DD Jr, et al. Extracellular DNA traps promote thrombosis. Proc Natl Acad Sci U S A. 2010;107: 15880–15885. pmid:20798043
  62. 62. Barton CD, Waugh LK, Nielsen MJ, Paulus S. Febrile neutropenia in children treated for malignancy. J Infect. 2015;71 Suppl 1: S27–35.
  63. 63. Bhatt V, Saleem A. Drug-induced neutropenia—pathophysiology, clinical features, and management. Ann Clin Lab Sci. 2004;34: 131–137. pmid:15228223
  64. 64. Bochud PY, Calandra T, Francioli P. Bacteremia due to viridans streptococci in neutropenic patients: a review. Am J Med. 1994;97: 256–64. pmid:8092175
  65. 65. Bejarano I, Terrón MP, Paredes SD, Barriga C, Rodríguez AB, Pariente JA. Hydrogen peroxide increases the phagocytic function of human neutrophils by calcium mobilisation. Mol Cell Biochem. 2007;296: 77–84. pmid:16955226
  66. 66. LeMessurier KS, Ogunniyi AD, Paton JC. Differential expression of key pneumococcal virulence genes in vivo. Microbiology. 2006;152: 305–311. pmid:16436418
  67. 67. Mahdi LK, Ogunniyi AD, LeMessurier KS, Paton JC. Pneumococcal virulence gene expression and host cytokine profiles during pathogenesis of invasive disease. Infect Immun. 2008;76: 646–657. pmid:18039836
  68. 68. Farmer ED. Serological subdivisions among the Lancefield group H streptococci. J Gen Microbiol. 1954;11: 131–138. pmid:13211967