Identification of Streptococcus pneumoniae genes specifically induced in mouse lung tissues

Transcriptional regulation of TacL-mediated lipoteichoic acids biosynthesis by ComE during competence impacts pneumococcal transformation

Competence development is essential for bacterial transformation since it enables bacteria to take up free DNA from the surrounding environment. The regulation of teichoic acid biosynthesis is tightly controlled during pneumococcal competence; however, the mechanism governing this regulation and its impact on transformation remains poorly understood. We demonstrated that a defect in lipoteichoic acid ligase (TacL)-mediated lipoteichoic acids (LTAs) biosynthesis was associated with impaired pneumococcal transformation. Using a fragment of tacL regulatory probe as bait in a DNA pulldown assay, we successfully identified several regulatory proteins, including ComE. Electrophoretic mobility shift assays revealed that phosphomimetic ComE, but not wild-type ComE, exhibited specific binding to the probe. DNase I footprinting assays revealed the specific binding sequences encompassing around 30 base pairs located 31 base pairs upstream from the start codon of tacL. Expression of tacL was found to be upregulated in the ΔcomE strain, and the addition of exogenous competence-stimulating peptide repressed the tacL transcription in the wild-type strain but not the ΔcomE mutant, indicating that ComE exerted a negative regulatory effect on the transcription of tacL. Mutation in the JH2 region of tacL upstream regulatory sequence led to increased LTAs abundance and displayed higher transformation efficiency. Collectively, our work identified the regulatory mechanisms that control LTAs biosynthesis during competence and thereby unveiled a repression mechanism underlying pneumococcal transformation.

Methyl anthranilate deteriorates biofilm structure of Streptococcus suis and antagonizes the capsular polysaccharide defence effect

BACKGROUND

Streptococcus suis is an important zoonotic pathogen that often causes biofilm-associated infection. Bacterial biofilm-dependent infection is associated with enhanced drug resistance, making it difficult to eradicate. Novel therapeutic approaches are required urgently to treat infections associated with S. suis biofilm. This study aimed to investigate the effects and mechanisms of methyl anthranilate (MA) on S. suis biofilm.

METHODS

The effect of MA on S. suis biofilm was determined using the crystal violet method, and the microstructure of the biofilm was observed by electron microscopy. The effects on capsular polysaccharides were determined using the phenol-sulphuric acid method and high-performance liquid chromatography. Adhesion and antiphagocytosis properties of S. suis were detected via cell assays. Molecular docking, molecular dynamics simulation and enzyme activity inhibition assays were used to further explore the effect of MA on AI-2 quorum sensing (QS) of S. suis. Finally, the therapeutic effect of MA was investigated using a mouse infection model.

RESULTS

MA destroyed the structure of S. suis biofilm, hindered biofilm formation, and reduced the synthesis of capsular polysaccharides significantly, which further weakened the adhesion and antiphagocytosis ability of S. suis. MA had a docking effect and binding site (SER76 and ASP197) similar to S-adenosylhomocysteine (SAH). Further analysis showed that MA competitively bound 5'-methyladenosine/S-adenosine homocysteine nucleosidase with SAH to interfere with AI-2 QS. In a mouse model, MA reduced the bacterial burden and inflammatory infiltrates effectively.

CONCLUSION

This study revealed the antibiofilm effects of MA, and highlighted its potential as a QS inhibitor against S. suis infection.

Identification of cis-acting elements upstream of regR gene in streptococcus pneumoniae

The identification and characterization of functional cis-acting elements is of fundamental importance for comprehending the regulatory mechanisms of gene transcription and bacterial pathogenesis. The transcription factor RegR has been demonstrated to control both competence and virulence in Streptococcus pneumoniae. Despite the clear contribution of RegR to these pathways, the mechanisms underlying its transcriptional regulation remain poorly understood. In this study, we conducted mutational analysis, gene dissection and luciferase activity assays to characterize the cis-elements situated upstream of the regR gene. Our findings revealed that a 311 bp 3'-terminal DNA sequence of the spd0300 gene represents a central region of the upstream cis-acting element of regR. Further investigations identified two structurally similar enhancer-like sequences within this region which feature prominently in the regulation of regR transcription. Furthermore, employing DNA pull-down assays allowed us to enrich the trans-acting factors with the potential to interact with these cis-acting elements. Notably, we found that the competence regulator ComE was implicated in the regulation of regR transcription, a finding which was corroborated by electrophoretic mobility shift assays (EMSA) and quantitative real-time PCR analyses (qRT-PCR). Taken together, our data thus provide fresh insight into the transcriptional regulation of regR.

spd1672, a novel in vivo-induced gene, affects inflammatory response in a murine model of Streptococcus pneumoniae infection

spd1672, a novel Streptococcus pneumoniae (hereinafter S. pn) gene induced in vivo, has been identified to contribute to the virulence of S. pn; however, the role of spd1672 during host innate immune reaction against S. pn infection is unknown. In the present study, mice were infected with wild-type D39 and mutant D39Δspd1672 strains. Compared with the D39-infected mice, reduced bacterial load and attenuated inflammatory response were observed in the D39Δspd1672-treated mice. The levels of proinflammatory cytokines, including IFN-γ, TNF-α, and IL-1β, in the blood of D39Δspd1672-infected mice were lower than that in the D39-infected group. Additionally, attenuated activation of STAT3 and AKT was observed in the D39Δspd1672-infected mice. In conclusion, our data indicated that spd1672 expression modulates the release of proinflammatory cytokines, and AKT–STAT3 signaling appears to participate in the process. In conclusion, the present study extends our understanding of the role of an in vivo-induced gene in S. pn–host interaction.

Screening assay for inhibitors of a recombinant Streptococcus pneumoniae UDP-glucose pyrophosphorylase

The UDP-glucose pyrophosphorylase of Streptococcus pneumoniae (GalU_Spn) is absolutely required for the biosynthesis of capsular polysaccharide, the sine qua non virulence factor of pneumococcus. Since the eukaryotic enzymes are completely unrelated to their prokaryotic counterparts, we propose that the GalU enzyme is a critical target to fight the pneumococcal disease. A recombinant GalU_Spn was overexpressed and purified. An enzymatic assay that is rapid, sensitive and easy to perform was developed. This assay was appropriate for screening chemical libraries for searching GalU inhibitors. This work represents a fundamental step in the exploration of novel antipneumococcal drugs.

CpsR, a GntR family regulator, transcriptionally regulates capsular polysaccharide biosynthesis and governs bacterial virulence in Streptococcus pneumoniae

Transcriptional regulation of capsule expression is critical for pneumococcal transition from carriage to infection, yet the underlying mechanism remains incompletely understood. Here, we describe the regulation of capsular polysaccharide, one of the most important pneumococcal virulence factor by a GntR family regulator, CpsR. Electrophoretic mobility-shift assays have shown the direct interaction between CpsR and the cps promoter (cpsp), and their interaction could be competitively interfered by glucose. DNase I footprinting assays localized the binding site to a region −146 to −114 base pairs relative to the transcriptional start site of the cps locus in S. pneumoniae D39. We found that CpsR negatively controlled the transcription of the cps locus and hence CPS production, which was confirmed by fine-tuning expression of CpsR in a ΔcpsR complemented strain. Increased expression of CpsR in complemented strain led to a decreased resistance to the whole-blood-mediated killing, suggesting a protective role for CpsR-cpsp interaction in the establishment of invasive infection. Finally, animal experiments showed that CpsR-cpsp interaction was necessary for both pneumococcal colonization and invasive infection. Taken together, our results provide a thorough insight into the regulation of capsule production mediated by CpsR and its important roles in pneumococcal pathogenesis.

A novel protein, RafX, is important for common cell wall polysaccharide biosynthesis in Streptococcus pneumoniae: implications for bacterial virulence

Teichoic acid (TA), together with peptidoglycan (PG), represents a highly complex glycopolymer that ensures cell wall integrity and has several crucial physiological activities. Through an insertion-deletion mutation strategy, we show that ΔrafX mutants are impaired in cell wall covalently attached TA (WTA)-PG biosynthesis, as evidenced by their abnormal banding patterns and reduced amounts of WTA in comparison with wild-type strains. Site-directed mutagenesis revealed an essential role for external loop 4 and some highly conserved amino acid residues in the function of RafX protein. The rafX gene was highly conserved in closely related streptococcal species, suggesting an important physiological function in the lifestyle of streptococci. Moreover, a strain D39 ΔrafX mutant was impaired in bacterial growth, autolysis, bacterial division, and morphology. We observed that a strain R6 ΔrafX mutant was reduced in adhesion relative to the wild-type R6 strain, which was supported by an inhibition assay and a reduced amount of CbpA protein on the ΔrafX mutant bacterial cell surface, as shown by flow cytometric analysis. Finally, ΔrafX mutants were significantly attenuated in virulence in a murine sepsis model. Together, these findings suggest that RafX contributes to the biosynthesis of WTA, which is essential for full pneumococcal virulence.

Hyaluronic acid derived from other streptococci supports Streptococcus pneumoniae in vitro biofilm formation

We investigate the role of hyaluronic acid (HA) on S. pneumoniae in vitro biofilm formation and evaluate gene expressions of virulence and/or biofilm related genes. Biofilms were grown in medium supplied with HA derived from capsule of Streptococcus equi. The biomasses of biofilms were detected by crystal-violet (CV) microtiter plate assay, and the morphology was viewed under scanning electron microscope (SEM). The gene expressions were assessed by relative quantitative RT-PCR. The results showed that the HA support pneumococcal growth in planktonic form and within biofilms. The CV-microtiter plate assay detected significantly increased biofilm growth in medium containing HA. The SEM analysis revealed thick and organized biofilms in positive control and HA supplemented medium. The nanA, nanB, bgaA, strH, luxS, hysA, ugl, and PST-EIIA encoding genes were significantly upregulated in the planktonic cells grown in presence of HA, while the lytA and comA genes were downregulated. Similarly the luxS, hysA, ugl, and PST-EIIA encoding genes were significantly upregulated by more than 2-folds in HA biofilms. The results of this study indicate that the HA derived from capsule of S. equi supports pneumococcal growth in planktonic state and within biofilms and upregulated virulence and biofilm related genes.

Pneumococcal ClpP modulates the maturation and activation of human dendritic cells: implications for pneumococcal infections

DCs are essential for host immune response to pathogens. Pneumococcal diseases still remain to be a major global-health issue, and HSP100/ClpP is a ubiquitously present virulence determinant for Streptococcus pneumoniae. Here, we show that ClpP expression facilitates the uptake and phagocytosis of pneumococci by human DCs, and it could increase apoptosis of DCs infected with pneumococci. Furthermore, pneumococcal ClpP is required for optimal production of inflammatory cytokines and chemokines and an efficient activation of adaptive immune response in DCs. Complementary, purified rClpP protein recognizes TLR4 and functionally activates human DCs by augmenting the expression of surface molecules and the production of inflammatory cytokines and chemokines dependent on MAPKs and NF-κB signaling pathways. Besides, ClpP-treated DCs induce T cell proliferation and contribute to Th1 immune response. This study describes a novel role of ClpP in the interaction of DCs with pneumococci that could provide new insight for the progression of pneumococcal diseases and has important implications for designing pneumococcal protein vaccines.

Identification of genes that contribute to the pathogenesis of invasive pneumococcal disease by in vivo transcriptomic analysis

Streptococcus pneumoniae (the pneumococcus) continues to be responsible for a high level of global morbidity and mortality resulting from pneumonia, bacteremia, meningitis, and otitis media. Here we have used a novel technique involving niche-specific, genome-wide in vivo transcriptomic analyses to identify genes upregulated in distinct niches during pathogenesis after intranasal infection of mice with serotype 4 or 6A pneumococci. The analyses yielded 28 common, significantly upregulated genes in the lungs relative to those in the nasopharynx and 25 significantly upregulated genes in the blood relative to those in the lungs in both strains, some of which were previously unrecognized. The role of five upregulated genes from either the lungs or the blood in pneumococcal pathogenesis and virulence was then evaluated by targeted mutagenesis. One of the mutants (ΔmalX) was significantly attenuated for virulence in the lungs, two (ΔaliA and ΔilvH) were significantly attenuated for virulence in the blood relative to the wild type, and two others (ΔcbiO and ΔpiuA) were completely avirulent in a mouse intranasal challenge model. We also show that the products of aliA, malX, and piuA are promising candidates for incorporation into multicomponent protein-based pneumococcal vaccines currently under development. Importantly, we suggest that this new approach is a viable complement to existing strategies for the discovery of genes critical to the distinct stages of invasive pneumococcal disease and potentially has broad application for novel protein antigen discovery in other pathogens such as S. pyogenes, Haemophilus influenzae type b, and Neisseria meningitidis.

SP0454, a putative threonine dehydratase, is required for pneumococcal virulence in mice

Increasing pressure in antibiotic resistance and the requirement for the design of new vaccines are the objectives of clarifying the putative virulence factors in pneumococcal infection. In this study, the putative threonine dehydratase sp0454 was inactivated by erythromycin-resistance cassette replacement in Streptococcus pneumoniae CMCC 31203 strain. The sp0454 mutant was tested for cell growth, adherence, colonization, and virulence in a murine model. The Δsp0454 mutant showed decreased ability for colonization and impaired ability to adhere to A549 cells. However, the SP0454 polypeptide or its antiserum did not affect pneumococcal CMCC 31203 adhesion to A549 cells. The sp0454 deletion mutant was less virulent in a murine intranasal infection model. Real-time RT-PCR analysis revealed significant decrease of the pneumococcal surface antigen A expression in the sp0454 mutant. These results suggest that SP0454 contributes to virulence and colonization, which could be explained in part by modulating the expression of other virulence factors, such as psaA in pneumococcal infection.

Identification of a novel pneumococcal vaccine antigen preferentially expressed during meningitis in mice

Streptococcus pneumoniae is the most common cause of severe bacterial meningitis in children, the elderly, and immunocompromised individuals. To identify virulence factors preferentially expressed during meningitis, we conducted niche-specific genome-wide in vivo transcriptomic analysis after intranasal infection of mice with serotype 4 or 6A pneumococci. The expression of 34 bacterial genes was substantially altered in brain tissue of mice infected with either of the 2 strains. Ten upregulated genes were common to both strains, 7 of which were evaluated for their role in the development of meningitis. One previously uncharacterized protein, α-glycerophosphate oxidase (GlpO), was cytotoxic for human brain microvascular endothelial cells (HBMECs) via generation of H(2)O(2). A glpO deletion mutant was defective in adherence to HBMECs in vitro as well as in progression from the blood to the brain in vivo. Mutant bacteria also induced markedly reduced meningeal inflammation and brain pathology compared with wild type, despite similar levels of bacteremia. Immunization of mice with GlpO protected against invasive pneumococcal disease and provided additive protection when formulated with pneumolysin toxoid. Our results provide the basis of a strategy that can be adapted to identify genes that contribute to the development of meningitis caused by other pathogens.

Streptococcus pneumoniae can utilize multiple sources of hyaluronic acid for growth

The mechanisms by which Streptococcus pneumoniae obtains carbohydrates for growth during airway colonization remain to be elucidated. The low concentration of free carbohydrates in the normal human airway suggests that pneumococci must utilize complex glycan structures for growth. The glycosaminoglycan hyaluronic acid is present on the apical surface of airway epithelial cells. As pneumococci express a hyaluronate lyase (Hyl) that cleaves hyaluronic acid into disaccharides, we hypothesized that during colonization pneumococci utilize the released carbohydrates for growth. Hyaluronic acid supported significant pneumococcal growth in an hyl-dependent manner. A phosphoenolpyruvate-dependent phosphotransferase system (PTS) and an unsaturated glucuronyl hydrolase (Ugl) encoded downstream of hyl are also essential for growth on hyaluronic acid. This genomic arrangement is present in several other organisms, suggesting conservation of the utilization mechanism between species. In vivo experiments support the hypothesis that S. pneumoniae utilizes hyaluronic acid as a carbon source during colonization. We also demonstrate that pneumococci can utilize the hyaluronic acid capsule of other bacterial species for growth, suggesting an alternative carbohydrate source for pneumococcal growth. Together, these data support a novel function for pneumococcal degradation of hyaluronic acid in vivo and provide mechanistic details of growth on this glycosaminoglycan.

Immunization with DnaJ (hsp40) could elicit protection against nasopharyngeal colonization and invasive infection caused by different strains of Streptococcus pneumoniae

Increasing mortality, morbidity and economic costs have been paid to pneumococcal diseases every year. Currently, vaccination is the most promising strategy to reduce the occurrence of pneumococcal infection. In this study, we investigated the protective efficacy of immunization with recombinant DnaJ (hsp40) protein against infections of different serotypes of Streptococcus pneumoniae. We demonstrated that mucosal immunization with DnaJ antigen could induce both systemic and mucosal antibodies for DnaJ and stimulate the release of high levels of IL-10, IFN-γ and IL-17A. Moreover, this mucosal vaccination could reduce nasal or lung colonization of pneumococcus and elicit protection against different serotypes of invasive pneumococcal infections. As well, we found that intraperitoneal immunization with DnaJ could also protect against invasive infections caused by different serotypes of pneumococcus, and passive immunization with antibodies specific for DnaJ confirmed that this protection was antibody-mediated. Our results therefore support the potential of DnaJ as a conserved pneumococcal protein vaccine.

Pneumococcal proteins PspA and PspC induce CXCL8 production in human neutrophils: implications in pneumococcal infections

Surface-exposed pneumococcal virulence proteins pneumococcal surface protein A (PspA) and pneumococcal surface protein C (PspC) play important roles in the pathogenesis of invasive pneumococcal diseases. Human neutrophils are principle antimicrobial effector cells of the innate and adaptive immune systems. In this study, we investigated the effects of PspA and PspC on the up-regulation of chemokine CXCL8 in human neutrophils, and characterized the underlying intracellular signaling pathways. Both PspA and PspC were found to induce the release of newly synthesized CXCL8. Synergistic effect was observed in the combined treatment of PspA and PspC on the release of CXCL8. Products from PspA-deficient or PspC-deficient mutant pneumococcus that did not express PspA or PspC induced significantly less release of CXCL8 than wild type pneumococcus. Both PspA and PspC could activate p38 MAPK and NF-κB pathways in neutrophils, while inhibition of NF-κB and p38 MAPK could suppress the release of CXCL8 from neutrophils induced by PspA and PspC. Together, our results demonstrated that the induction of CXCL8 in human neutrophils activated by PspA and PspC was regulated by p38 MAPK and NF-κB pathways.

Immunization with a combination of three pneumococcal proteins confers additive and broad protection against Streptococcus pneumoniae Infections in Mice

Pneumococcal polysaccharide-based vaccines are effective in preventing pneumococcus infection; however, some drawbacks preclude their widespread use in developing and undeveloped countries. Here, we evaluated the protective effects of ATP-dependent caseinolytic protease (ClpP), pneumolysin mutant (DeltaA146 Ply), putative lipoate-protein ligase (Lpl), or combinations thereof against pneumococcal infections in mice. Vaccinated mice were intraperitoneally and/or intranasally challenged with different pneumococcal strains. In intraperitoneal challenge models with pneumococcal strain D39 (serotype 2), the most striking protection was obtained with the combination of the three antigens. Similarly, with the intranasal challenge models, (i) additive clearance of bacteria in lungs was observed for the combination of the three antigens and (ii) a combination vaccine conferred complete protection against intranasal infections of three of the four most common pneumococcal strains (serotypes 14, 19F, and 23F) and 80% protection for pneumococcal strain 6B. Even so, immunity to this combination could confer protection against pneumococcal infection with a mixture of four serotypes. Our results showed that the combination vaccine was as effective as the currently used vaccines (PCV7 and PPV23). These results indicate that system immunization with the combination of pneumococcal antigens could provide an additive and broad protection against Streptococcus pneumoniae in pneumonia and sepsis infection models.