Proteomics-based analysis of a counter-oxidative stress system in Porphyromonas gingivalis

Comparative Proteomics of Bacteria Under Stress Conditions

Bacteria are unicellular organisms with the ability to exist in the harshest of climate and cope with sub-optimal fluctuating environmental conditions. They accomplish this by modification of their internal cellular environment. When external conditions are varied, change in the cell is triggered at the transcriptional level, which usually leads to proteolysis and rewiring of the proteome. Changes in cellular homeostasis, modifications in proteome, and dynamics of such survival mechanisms can be studied using various scientific techniques. Our focus in this chapter would be on comparative proteomics of bacteria under stress conditions using approaches like 2D electrophoresis accompanied by N-terminal sequencing and recently, mass spectrometry. More than 170 such studies on bacteria have been accomplished till to date and involve analysis of whole cells as well as that of cellular fractions, i.e., outer membrane, inner membrane, cell envelope, cytoplasm, thylakoid, lipid bodies, etc. Similar studies conducted on gram-negative and gram-positive model organism, i.e., Escherichia coli and Bacillus subtilis, respectively, have been summarized. Vital information, hypothesis about conservation of stress-specific proteome, and conclusions are also presented in the light of research conducted over the last decades.

The involvement of CdhR in Porphyromonas gingivalis during nitric oxide stress

Porphyromonas gingivalis, the causative agent of adult periodontitis, must gain resistance to frequent oxidative and nitric oxide (NO) stress attacks from immune cells in the periodontal pocket to survive. Previously, we found that, in the wild-type and under NO stress, the expression of PG1237 (CdhR), the gene encoding for a putative LuxR transcriptional regulator previously called community development and hemin regulator (CdhR), was upregulated 7.7-fold, and its adjacent gene PG1236 11.9-fold. Isogenic mutants P. gingivalis FLL457 (ΔCdhR::ermF), FLL458 (ΔPG1236::ermF), and FLL459 (ΔPG1236-CdhR::ermF) were made by allelic exchange mutagenesis to determine the involvement of these genes in P. gingivalis W83 NO stress resistance. The mutants were black pigmented and β hemolytic and their gingipain activities varied with strains. FLL457 and FLL459 mutants were more sensitive to NO compared to the wild type, and complementation restored NO sensitivity to that of the wild type. DNA microarray analysis of FLL457 showed that approximately 2% of the genes were upregulated and over 1% of the genes downregulated under NO stress conditions compared to the wild type. Transcriptome analysis of FLL458 and FLL459 under NO stress showed differences in their modulation patterns. Some similarities were also noticed between all mutants. The PG1236-CdhR gene cluster revealed increased expression under NO stress and may be part of the same transcriptional unit. Recombinant CdhR showed binding activity to the predicted promoter regions of PG1459 and PG0495. Taken together, the data indicate that CdhR may play a role in NO stress resistance and be involved in a regulatory network in P. gingivalis.

Modulatory Mechanisms of Pathogenicity in Porphyromonas gingivalis and Other Periodontal Pathobionts

The pathogenesis of periodontitis depends on a sustained feedback loop where bacterial virulence factors and immune responses both contribute to inflammation and tissue degradation. Periodontitis is a multifactorial disease that is associated with a pathogenic shift in the oral microbiome. Within this shift, low-abundance Gram-negative anaerobic pathobionts transition from harmless colonisers of the subgingival environment to a virulent state that drives evasion and subversion of innate and adaptive immune responses. This, in turn, drives the progression of inflammatory disease and the destruction of tooth-supporting structures. From an evolutionary perspective, bacteria have developed this phenotypic plasticity in order to respond and adapt to environmental stimuli or external stressors. This review summarises the available knowledge of genetic, transcriptional, and post-translational mechanisms which mediate the commensal-pathogen transition of periodontal bacteria. The review will focus primarily on Porphyromonas gingivalis.

A putative TetR regulator is involved in nitric oxide stress resistance in Porphyromonas gingivalis

To survive in the periodontal pocket, Porphyromonas gingivalis, the main causative agent of periodontal disease, must overcome oxidative and nitric oxide (NO) stress. Previously, we reported that, in the presence of NO comparable to stress conditions, the transcriptome of P. gingivalis was differentially expressed, and genes belonging to the PG1178-81 cluster were significantly upregulated. To further evaluate their role(s) in NO stress resistance, these genes were inactivated by allelic exchange mutagenesis. Isogenic mutants P. gingivalis FLL460 (ΔPG1181::ermF) and FLL461 (ΔPG1178-81::ermF) were black-pigmented, with gingipain and hemolytic activities comparable to that of the wild-type strain. Whereas the recovery of these isogenic mutants from NO stress was comparable to the wild-type, there was increased sensitivity to hydrogen peroxide-induced stress. RNA-Seq analysis under conditions of NO stress showed that approximately 5 and 8% of the genome was modulated in P. gingivalis FLL460 and FLL461, respectively. The PG1178-81 gene cluster was shown to be part of the same transcriptional unit and is inducible in response to NO stress. In the presence of NO, PG1181, a putative transcriptional regulator, was shown to bind to its own promoter region and that of several other NO responsive genes including PG0214 an extracytoplasmic function σ factor, PG0893 and PG1236. Taken together, the data suggest that PG1181 is a NO responsive transcriptional regulator that may play an important role in the NO stress resistance regulatory network in P. gingivalis.

Environmental stimuli shape biofilm formation and the virulence of periodontal pathogens

Periodontitis is a common inflammatory disease affecting the tooth-supporting structures. It is initiated by bacteria growing as a biofilm at the gingival margin, and communication of the biofilms differs in health and disease. The bacterial composition of periodontitis-associated biofilms has been well documented and is under continual investigation. However, the roles of several host response and inflammation driven environmental stimuli on biofilm formation is not well understood. This review article addresses the effects of environmental factors such as pH, temperature, cytokines, hormones, and oxidative stress on periodontal biofilm formation and bacterial virulence.

Sulfur-34S stable isotope labeling of amino acids for quantification (SULAQ34) of proteomic changes in Pseudomonas fluorescens during naphthalene degradation

The relative quantification of proteins is one of the major techniques used to elucidate physiological reactions. Because it allows one to avoid artifacts due to chemical labeling, the metabolic introduction of heavy isotopes into proteins and peptides is the preferred method for relative quantification. For eukaryotic cells, stable isotope labeling by amino acids in cell culture (SILAC) has become the gold standard and can be readily applied in a vast number of scenarios. In the microbial realm, with its highly versatile metabolic capabilities, SILAC is often not feasible, and the use of other (13)C or (15)N labeled substrates might not be practical. Here, the incorporation of heavy sulfur isotopes is shown to be a useful alternative. We introduce (34)S stable isotope labeling of amino acids for quantification and the corresponding tools required for spectra extraction and disintegration of the isotopic overlaps caused by the small mass shift. As proof of principle, we investigated the proteomic changes related to naphthalene degradation in P. fluorescens ATCC 17483 and uncovered a specific oxidative-stress-like response.

Identification and characterization of novel glycoproteins involved in growth and biofilm formation by Porphyromonas gingivalis

Porphyromonas gingivalis has been implicated as a major pathogen associated with chronic periodontitis. To extend our knowledge of post-translational protein glycosylation in P. gingivalis, a proteomic analysis involving two-dimensional polyacrylamide gel electrophoresis combined with carbohydrate staining and mass spectrometry was performed. Four novel glycoproteins, PGN0743, PGN0876, PGN1513 and PGN0729, in P. gingivalis ATCC 33277 were identified. These four identified glycoproteins possess a range of biochemical activities and cellular localization. PGN0743 contains a sequence motif identifying it as a FKBP-type cis-trans isomerase, which has activity usually associated with chaperone functions. PGN0876 and PGN1513 contain tetratricopeptide repeat domains that mediate protein-protein interactions. PGN0729 encodes the outer membrane protein 41 precursor, which was previously identified as Pgm6, and is homologous to the OmpA protein in Escherichia coli. Several different types of glycoprotein were identified, suggesting that P. gingivalis possesses a general mechanism for protein glycosylation. PGN0743-deficient and PGN0876-deficient mutants were constructed to examine the role(s) of the two identified glycoproteins. Both mutants showed a decreased growth rate under nutrient-limited conditions and reduced biofilm formation activity. These results suggest that the novel glycoproteins PGN0743 and PGN0876 play an important role in the growth and colonization of P. gingivalis.

Deep sequencing of Porphyromonas gingivalis and comparative transcriptome analysis of a LuxS mutant

Porphyromonas gingivalis is a major etiological agent in chronic and aggressive forms of periodontal disease. The organism is an asaccharolytic anaerobe and is a constituent of mixed species biofilms in a variety of microenvironments in the oral cavity. P. gingivalis expresses a range of virulence factors over which it exerts tight control. High-throughput sequencing technologies provide the opportunity to relate functional genomics to basic biology. In this study we report qualitative and quantitative RNA-Seq analysis of the transcriptome of P. gingivalis. We have also applied RNA-Seq to the transcriptome of a ΔluxS mutant of P. gingivalis deficient in AI-2-mediated bacterial communication. The transcriptome analysis confirmed the expression of all predicted ORFs for strain ATCC 33277, including 854 hypothetical proteins, and allowed the identification of hitherto unknown transcriptional units. Twelve non-coding RNAs were identified, including 11 small RNAs and one cobalamin riboswitch. Fifty-seven genes were differentially regulated in the LuxS mutant. Addition of exogenous synthetic 4,5-dihydroxy-2,3-pentanedione (DPD, AI-2 precursor) to the ΔluxS mutant culture complemented expression of a subset of genes, indicating that LuxS is involved in both AI-2 signaling and non-signaling dependent systems in P. gingivalis. This work provides an important dataset for future study of P. gingivalis pathophysiology and further defines the LuxS regulon in this oral pathogen.

Oxidative stress response in the opportunistic oral pathogen Fusobacterium nucleatum

The anaerobic, Gram-negative bacillus Fusobacterium nucleatum plays a vital role in oral biofilm formation and the development of periodontal disease. The organism plays a central bridging role between early and late colonizers within dental plaque and plays a protective role against reactive oxygen species. Using a two-dimensional gel electrophoresis and mass spectrometry approach, we have annotated 78 proteins within the proteome of F. nucleatum subsp. nucleatum and identified those proteins whose apparent intracellular concentrations change in response to either O(2)- or H(2)O(2)-induced oxidative stress. Three major protein systems were altered in response to oxidative stress: (i) proteins of the alkyl hydroperoxide reductase/thioredoxin reductase system were increased in intracellular concentration; (ii) glycolytic enzymes were modified by oxidation (i.e. D-glyceraldehyde 3-phosphate dehydrogenase, and fructose 6-phosphate aldolase) or increased in intracellular concentration, with an accompanying decrease in ATP production; and (iii) the intracellular concentrations of molecular chaperone proteins and related proteins (i.e. ClpB, DnaK, HtpG, and HrcA) were increased.

Characterization of human-type monoclonal antibodies against reduced form of hemin binding protein 35 from Porphyromonas gingivalis

BACKGROUND AND OBJECTIVE

The gram-negative anaerobe Porphyromonas gingivalis has been implicated as an important pathogen in the development of adult periodontitis, and its colonization of subgingival sites is critical in the pathogenic process. We previously identified a 35 kDa surface protein (hemin binding protein 35; HBP35) from P. gingivalis that exhibited coaggregation activity, while additional analysis suggested that this protein possessed an ability to bind heme molecules. For development of passive immunotherapy for periodontal diseases, human-type monoclonal antibodies have been prepared using HBP35 as an antigen in TransChromo mice. In the present study, we focused on a single antibody, TCmAb-h13, which is known to inhibit heme binding to recombinant HBP35. The aim of our investigation was to clarify the redox-related function of HBP35 and consider the benefits of human-type monoclonal antibodies.

MATERIAL AND METHODS

To examine the antigen recognition capability of TCmAbs with immunoblotting and Biacore techniques, we used the native form as well as several Cys-to-Ser variants of recombinant HBP35.

RESULTS

We found that the redox state of recombinant HBP35 was dependent on two Cys residues, (48) C and (51) C, in the thioredoxin active center (WCGxCx). Furthermore, TCmAb-h13 recognized the reduced forms of recombinant HBP35, indicating its inhibitory effect on P. gingivalis growth.

CONCLUSION

Hemin binding protein 35 appears to be an important molecule involved in recognition of the redox state of environmental conditions. In addition, TCmAb-h13 had an inhibitory effect on heme binding to recombinant HBP35, thereby interfering with P. gingivalis growth.

Tetratricopeptide repeat protein-associated proteins contribute to the virulence of Porphyromonas gingivalis

Porphyromonas gingivalis is one of the most etiologically important microorganisms in periodontal disease. We found in a previous study that PG1385 (TprA) protein, a tetratricopeptide repeat (TPR) protein, was upregulated in P. gingivalis wild-type cells placed in a mouse subcutaneous chamber and that a tprA mutant was clearly less virulent in the mouse subcutaneous abscess model (M. Yoshimura et al., Oral Microbiol. Immunol. 23:413-418, 2008). In the present study, we investigated the gene expression profile of tprA mutant cells placed in a mouse subcutaneous chamber and found that 9 genes, including PG2102 (tapA), PG2101 (tapB), and PG2100 (tapC) genes, were downregulated in the tprA mutant compared with those in the wild type. Expression of a cluster of tapA, tapB, and tapC genes of the mutant was also downregulated in an in vitro culture with enriched brain heart infusion medium. The TprA protein has three TPR motifs known as a protein-protein interaction module. Yeast two-hybrid system analysis and in vitro protein binding assays with immunoprecipitation and surface plasmon resonance detection revealed that the TprA protein could bind to TapA and TapB proteins. TprA and TapB proteins were located in the periplasmic space, whereas TapA, which appeared to be one of the C-terminal domain family proteins, was located at the outer membrane. We constructed tapA, tapB, and tapC single mutants and a tapA-tapB-tapC deletion mutant. In the mouse subcutaneous infection experiment, all of the mutants were less virulent than the wild type. These results suggest that TprA, TapA, TapB, and TapC are cooperatively involved in P. gingivalis virulence.

3'-Phosphoadenosine-5'-phosphate phosphatase activity is required for superoxide stress tolerance in Streptococcus mutans

Aerobic microorganisms have evolved different strategies to withstand environmental oxidative stresses generated by various reactive oxygen species (ROS). For the facultative anaerobic human oral pathogen Streptococcus mutans, the mechanisms used to protect against ROS are not fully understood, since it does not possess catalase, an enzyme that degrades hydrogen peroxide. In order to elucidate the genes that are essential for superoxide stress response, methyl viologen (MV)-sensitive mutants of S. mutans were generated via ISS1 mutagenesis. Screening of approximately 2,500 mutants revealed six MV-sensitive mutants, each containing an insertion in one of five genes, including a highly conserved hypothetical gene, SMU.1297. Sequence analysis suggests that SMU.1297 encodes a hypothetical protein with a high degree of homology to the Bacillus subtilis YtqI protein, which possesses an oligoribonuclease activity that cleaves nano-RNAs and a phosphatase activity that degrades 3'-phosphoadenosine-5'-phosphate (pAp) and 3'-phosphoadenosine-5'-phosphosulfate (pApS) to produce AMP; the latter activity is similar to the activity of the Escherichia coli CysQ protein, which is required for sulfur assimilation. SMU.1297 was deleted using a markerless Cre-loxP-based strategy; the SMU.1297 deletion mutant was just as sensitive to MV as the ISS1 insertion mutant. Complementation of the deletion mutant with wild-type SMU.1297, in trans, restored the parental phenotype. Biochemical analyses with purified SMU.1297 protein demonstrated that it has pAp phosphatase activity similar to that of YtqI but apparently lacks an oligoribonuclease activity. The ability of SMU.1297 to dephosphorylate pApS in vivo was confirmed by complementation of an E. coli cysQ mutant with SMU.1297 in trans. Thus, our results suggest that SMU.1297 is involved in superoxide stress tolerance in S. mutans. Furthermore, the distribution of homologs of SMU.1297 in streptococci indicates that this protein is essential for superoxide stress tolerance in these organisms.

Proteome analysis of Porphyromonas gingivalis cells placed in a subcutaneous chamber of mice

INTRODUCTION

Porphyromonas gingivalis, an oral anaerobic bacterium, is considered a major pathogen for chronic periodontitis. Pathogenic bacteria usually upregulate or downregulate gene expression to combat the protective responses of their hosts.

METHODS

To determine what protein is regulated when P. gingivalis cells invade host tissues, we analyzed the proteome of P. gingivalis cells that were placed in a mouse subcutaneous chamber by two-dimensional gel electrophoresis and mass spectrometry.

RESULTS

Fourteen proteins were upregulated, while four proteins were downregulated. We focused on three upregulated proteins, PG1089 (DNA-binding response regulator RprY), PG1385 (TPR domain protein), and PG2102 (immunoreactive 61-kDa antigen), and constructed mutant strains that were defective in these proteins. Mouse abscess model experiments revealed that the mutant strain defective in PG1385 was clearly less virulent than the wild-type parent strain.

CONCLUSION

These results indicate that the PG1385 protein is involved in P. gingivalis virulence and that the method used here is useful when investigating the P. gingivalis proteins responsible for virulence.

Response of Porphyromonas gingivalis to heme limitation in continuous culture

Porphyromonas gingivalis is an anaerobic, asaccharolytic, gram-negative bacterium that has essential requirements for both iron and protoporphyrin IX, which it preferentially obtains as heme. A combination of large-scale quantitative proteomic analysis using stable isotope labeling strategies and mass spectrometry, together with transcriptomic analysis using custom-made DNA microarrays, was used to identify changes in P. gingivalis W50 protein and transcript abundances on changing from heme-excess to heme-limited continuous culture. This approach identified 160 genes and 70 proteins that were differentially regulated by heme availability, with broad agreement between the transcriptomic and proteomic data. A change in abundance of the enzymes of the aspartate and glutamate catabolic pathways was observed with heme limitation, which was reflected in organic acid end product levels of the culture fluid. These results demonstrate a shift from an energy-efficient anaerobic respiration to a less efficient process upon heme limitation. Heme limitation also resulted in an increase in abundance of a protein, PG1374, which we have demonstrated, by insertional inactivation, to have a role in epithelial cell invasion. The greater abundance of a number of transcripts/proteins linked to invasion of host cells, the oxidative stress response, iron/heme transport, and virulence of the bacterium indicates that there is a broad response of P. gingivalis to heme availability.

Functional analysis of the thioredoxin domain in Porphyromonas gingivalis HBP35

Periodontitis is one of the most common oral diseases in humans. This caused by infection by the oral bacterium Porphyromonas gingivalis. Our strategy to prevent this infection is to establish a passive immunization system in which endogenous antibodies can be applied directly to neutralize virulent factors associated with this bacterium. We focused our attention on the P. gingivalis 35 kDa surface protein, or HBP35, since this protein is involved not only in the coaggregation with oral miroflora but also in hemin binding. In addition, nucleotide sequencing of the gene, hbp35, coding for this protein revealed the presence of a catalytic center for thioredoxin, and we further attempted to characterized the protein by amino acid substitution. A total of four Cys residues were substituted for Ser residues by combining the simple method for site-directed mutagenesis and the heterodimer system, an approach designed to construct chimeric plasmids readily. Native and mutagenized hbp35 were introduced into the Eschericha coli dsbA mutant strain, JCB 572, defective in both alkaline phosphatase and motile activities due to inefficient disulfide bond formation. Transformant harboring the native hbp35 could complement the dsbA mutation, suggesting a role of disulfide bond formation of this protein in P. gingivalis cells. Possible roles of the Cys residues in complementation are discussed.

Application of 16O/18O reverse proteolytic labeling to determine the effect of biofilm culture on the cell envelope proteome of Porphyromonas gingivalis W50

Porphyromonas gingivalis is an oral pathogen linked to chronic periodontitis. The bacterium exists as part of a polymicrobial biofilm accreted onto the tooth surface. An understanding of the changes to the proteome especially of the cell envelope of biofilm cells compared with planktonic cells could provide valuable insight into the molecular processes of biofilm formation. To establish which proteins changed in abundance between the planktonic and biofilm growth states, the cell envelope fractions of two biological replicates of P. gingivalis cultivated in a chemostat were analysed. Proteins were separated by 1-D SDS-PAGE, in-gel digested with trypsin in the presence of H216O or H218O and identified and quantified by LC-MALDI TOF/TOF-MS. Using a reverse labeling strategy we identified and quantified the changes in abundance of 81 P. gingivalis cell envelope proteins. No form of bias between the labels was observed. Twenty four proteins increased in abundance and 18 decreased in abundance in the biofilm state. A group of cell-surface located C-Terminal Domain family proteins including RgpA, HagA, CPG70 and PG99 increased in abundance in the biofilm cells. Other proteins that exhibited significant changes in abundance included transport related proteins (HmuY and IhtB), metabolic enzymes (FrdAB) and immunogenic proteins.

Gingipain-dependent interactions with the host are important for survival of Porphyromonas gingivalis

Porphyromonas gingivalis, a major periodontal pathogen, must acquire nutrients from host derived substrates, overcome oxidative stress and subvert the immune system. These activities can be coordinated via the gingipains which represent the most significant virulence factor produced by this organism. In the context of our contribution to this field, we will review the current understanding of gingipain biogenesis, glycosylation, and regulation, as well as discuss their role in oxidative stress resistance and apoptosis. We can postulate a model, in which gingipains may be part of the mechanism for P. gingivalis virulence.

Quantitative proteomics of intracellular Porphyromonas gingivalis

Whole-cell quantitative proteomic analyses were conducted to investigate the change from an extracellular to intracellular lifestyle for Porphyromonas gingivalis, a Gram-negative intracellular pathogen associated with periodontal disease. Global protein abundance data for P. gingivalis strain ATCC 33277 internalized for 18 h within human gingival epithelial cells and controls exposed to gingival cell culture medium were obtained at sufficient coverage to provide strong evidence that these changes are profound. A total of 385 proteins were overexpressed in internalized P. gingivalis relative to controls; 240 proteins were shown to be underexpressed. This represented in total about 28% of the protein encoding ORFs annotated for this organism, and slightly less than half of the proteins that were observed experimentally. Production of several proteases, including the classical virulence factors RgpA, RgpB, and Kgp, was decreased. A separate validation study was carried out in which a 16-fold dilution of the P. gingivalis proteome was compared to the undiluted sample in order to assess the quantitative false negative rate (all ratios truly alternative). Truly null (no change) abundance ratios from technical replicates were used to assess the rate of quantitative false positives over the entire proteome. A global comparison between the direction of abundance change observed and previously published bioinformatic gene pair predictions for P. gingivalis will assist with future studies of P. gingivalis gene regulation and operon prediction.

A semi-quantitative GeLC-MS analysis of temporal proteome expression in the emerging nosocomial pathogen Ochrobactrum anthropi

A semi-quantitative gel-based analysis identifies distinct proteomic profiles associated with specific growth points for the nosocomial pathogen Ochrobactrum anthropi.

Background

The α-Proteobacteria are capable of interaction with eukaryotic cells, with some members, such as Ochrobactrum anthropi, capable of acting as human pathogens. O. anthropi has been the cause of a growing number of hospital-acquired infections; however, little is known about its growth, physiology and metabolism. We used proteomics to investigate how protein expression of this organism changes with time during growth.

Results

This first gel-based liquid chromatography-mass spectrometry (GeLC-MS) temporal proteomic analysis of O. anthropi led to the positive identification of 131 proteins. These were functionally classified and physiochemically characterized. Utilizing the emPAI protocol to estimate protein abundance, we assigned molar concentrations to all proteins, and thus were able to identify 19 with significant changes in their expression. Pathway reconstruction led to the identification of a variety of central metabolic pathways, including nucleotide biosynthesis, fatty acid anabolism, glycolysis, TCA cycle and amino acid metabolism. In late phase growth we identified a number of gene products under the control of the oxyR regulon, which is induced in response to oxidative stress and whose protein products have been linked with pathogen survival in response to host immunity reactions.

Conclusion

This study identified distinct proteomic profiles associated with specific growth points for O. anthropi, while the use of emPAI allowed semi-quantitative analyses of protein expression. It was possible to reconstruct central metabolic pathways and infer unique functional and adaptive processes associated with specific growth phases, thereby resulting in a deeper understanding of the physiology and metabolism of this emerging pathogenic bacterium.

Peroxiredoxins in bacterial antioxidant defense

Peroxiredoxins constitute an important component of the bacterial defense against toxic peroxides. These enzymes use reactive cysteine thiols to reduce peroxides with electrons ultimately derived from reduced pyridine dinucleotides. Studies examining the regulation and physiological roles of AhpC, Tpx, Ohr and OsmC reveal the multilayered nature of bacterial peroxide defense. AhpC is localized in the cytoplasm and has a wide substrate range that includes H2O2, organic peroxides and peroxynitrite. This enzyme functions in both the control of endogenous peroxides, as well as in the inducible defense response to exogenous peroxides or general stresses. Ohr, OsmC and Tpx are organic peroxide specific. Tpx is localized to the periplasm and can be involved in either constitutive peroxide defense or participate in oxidative stress inducible responses depending on the organism. Ohr is an organic peroxide specific defense system that is under the control of the organic peroxide sensing repressor OhrR. In some organisms Ohr homologs are regulated in response to general stress. Clear evidence indicates that AhpC, Tpx and Ohr are involved in virulence. The role of OsmC is less clear. Regulation of OsmC expression is not oxidative stress inducible, but is controlled by multiple general stress responsive regulators.

Role of the Clp system in stress tolerance, biofilm formation, and intracellular invasion in Porphyromonas gingivalis

Clp proteases and chaperones are ubiquitous among prokaryotes and eukaryotes, and in many pathogenic bacteria the Clp stress response system is also involved in regulation of virulence properties. In this study, the roles of ClpB, ClpC, and ClpXP in stress resistance, homotypic and heterotypic biofilm formation, and intracellular invasion in the oral opportunistic pathogen Porphyromonas gingivalis were investigated. Absence of ClpC and ClpXP, but not ClpB, resulted in diminished tolerance to high temperatures. Response to oxidative stress was not affected by the loss of any of the Clp proteins. The clpC and clpXP mutants demonstrated elevated monospecies biofilm formation, and the absence of ClpXP also enhanced heterotypic P. gingivalis-Streptococcus gordonii biofilm formation. All clp mutants adhered to gingival epithelial cells to the same level as the wild type; however, ClpC and ClpXP were found to be necessary for entry into host epithelial cells. ClpB did not play a role in entry but was required for intracellular replication and survival. ClpXP negatively regulated the surface exposure of the minor fimbrial (Mfa) protein subunit of P. gingivalis, which stimulates biofilm formation but interferes with epithelial cell entry. Collectively, these results show that the Clp protease complex and chaperones control several processes that are important for the colonization and survival of P. gingivalis in the oral cavity.

Role of the hemin-binding protein 35 (HBP35) of Porphyromonas gingivalis in coaggregation

Hemin-binding protein 35 (HBP35) in Porphyromonas gingivalis is one of the outer membrane proteins and has been reported to be a non-fimbrial coaggregation factor. In this study, a P. gingivalis HBP35-deficient mutant (MD774) was constructed from wild-type strain FDC381 by insertion mutagenesis in order to provide a better understanding of this protein's role in coaggregation. The intact cells and vesicles in FDC381 were found to have strong aggregation activities with Gram-positive bacteria. But neither the vesicles nor the intact cells showed aggregation activity in MD774. In addition, MD774 reduced autoaggregation activity. Immunoblot analysis of MD774 showed the presence of a non-maturated 45-kDa fimbrillin protein. Electron microscopy showed that the MD774 had no long fimbriae on the cell surface. Arg- and Lys-gingipain activity in MD774 was significantly decreased, compared with FDC381. Real-time RT-PCR demonstrated a significant reduction in the expression of gingipain-associated genes rgpA, rgpB, and kgp. In conclusion, we suggest that the reduction in coaggregation was caused by the combined reduction of a variety of molecules, including HBP35, gingipains, and fimbriae. Our results suggest that the HBP35 protein directly influences not only coaggregation as an adhesion molecule but also indirectly influences the expression of other coaggregation factors.