Haemin-restriction influences haemin-binding, haemagglutination and protease activity of cells and extracellular membrane vesicles of Porphyromonas gingivalis W50

Outer membrane vesicles of Porphyromonas gingivalis: Novel communication tool and strategy

Extracellular vesicles (EVs) have been recognized as a universal method of cellular communications and are reportedly produced in bacteria, archaea, and eukaryotes. Bacterial EVs are often called “Outer Membrane Vesicles” (OMVs) as they were the result of a controlled blebbing of the outer membrane of gram-negative bacteria such as Porphyromonas gingivalis (P. gingivalis). Bacterial EVs are natural messengers, implicated in intra- and inter-species cell-to-cell communication among microorganism populations present in microbiota. Bacteria can incorporate their pathogens into OMVs; the content of OMVs differs, depending on the type of bacteria. The production of distinct types of OMVs can be mediated by different factors and routes. A recent study highlighted OMVs ability to carry crucial molecules implicated in immune modulation, and, nowadays, they are considered as a way to communicate and transfer messages from the bacteria to the host and vice versa. This review article focuses on the current understanding of OMVs produced from major oral bacteria, P. gingivalis: generation, characteristics, and contents as well as the involvement in signal transduction of host cells and systemic diseases. Our recent study regarding the action of P. gingivalis OMVs in the living body is also summarized.

Outer Membrane Vesicle-Host Cell Interactions

Outer membrane vesicles (OMVs) are nanosized proteoliposomes derived from the outer membrane of Gram-negative bacteria. They are ubiquitously produced both in culture and during infection and are now recognized to play crucial roles during host-microbe interactions. OMVs can transport a broad range of chemically diverse cargoes, including lipids and lipopolysaccharides, membrane-embedded and associated proteins and small molecules, peptidoglycan, and nucleic acids. Particularly, virulence factors such as adhesins and toxins are often enriched in OMVs. Here we discuss a variety of ways in which OMVs facilitate host-microbe interactions, including their contributions to biofilm formation, nutrient scavenging, and modulation of host cell function. We particularly examine recent findings regarding OMV-host cell interactions in the oral cavity and the gastrointestinal tract.

Pyocyanina contributory factor in haem acquisition and virulence enhancement of Porphyromonas gingivalis in the lung [corrected]

Several recent studies show that the lungs infected with Pseudomonas aeruginosa are often co-colonised by oral bacteria including black-pigmenting anaerobic (BPA) Porphyromonas species. The BPAs have an absolute haem requirement and their presence in the infected lung indicates that sufficient haem, a virulence up-regulator in BPAs, must be present to support growth. Haemoglobin from micro-bleeds occurring during infection is the most likely source of haem in the lung. Porphyromonas gingivalis displays a novel haem acquisition paradigm whereby haemoglobin must be firstly oxidised to methaemoglobin, facilitating haem release, either by gingipain proteolysis or capture via the haem-binding haemophore HmuY. P. aeruginosa produces the blue phenazine redox compound, pyocyanin. Since phenazines can oxidise haemoglobin, it follows that pyocyanin may also facilitate haem acquisition by promoting methaemoglobin production. Here we show that pyocyanin at concentrations found in the CF lung during P. aeruginosa infections rapidly oxidises oxyhaemoglobin in a dose-dependent manner. We demonstrate that methaemoglobin formed by pyocyanin is also susceptible to proteolysis by P. gingivalis Kgp gingipain and neutrophil elastase, thus releasing haem. Importantly, co-incubation of oxyhaemoglobin with pyocyanin facilitates haem pickup from the resulting methemoglobin by the P. gingivalis HmuY haemophore. Mice intra-tracheally challenged with viable P. gingivalis cells plus pyocyanin displayed increased mortality compared to those administered P. gingivalis alone. Pyocyanin significantly elevated both methaemoglobin and total haem levels in homogenates of mouse lungs and increased the level of arginine-specific gingipain activity from mice inoculated with viable P. gingivalis cells plus pyocyanin compared with mice inoculated with P. gingivalis only. These findings indicate that pyocyanin, by promoting haem availability through methaemoglobin formation and stimulating of gingipain production, may contribute to virulence of P. gingivalis and disease severity when co-infecting with P. aeruginosa in the lung.

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.

HmuY haemophore and gingipain proteases constitute a unique syntrophic system of haem acquisition by Porphyromonas gingivalis

Haem (iron protoporphyrin IX) is both an essential growth factor and virulence regulator for the periodontal pathogen Porphyromonas gingivalis, which acquires it mainly from haemoglobin via the sequential actions of the R- and K-specific gingipain proteases. The haem-binding lipoprotein haemophore HmuY and its cognate receptor HmuR of P. gingivalis, are responsible for capture and internalisation of haem. This study examined the role of the HmuY in acquisition of haem from haemoglobin and the cooperation between HmuY and gingipain proteases in this process. Using UV-visible spectroscopy and polyacrylamide gel electrophoresis, HmuY was demonstrated to wrest haem from immobilised methaemoglobin and deoxyhaemoglobin. Haem extraction from oxyhaemoglobin was facilitated after oxidation to methaemoglobin by pre-treatment with the P. gingivalis R-gingipain A (HRgpA). HmuY was also capable of scavenging haem from oxyhaemoglobin pre-treated with the K-gingipain (Kgp). This is the first demonstration of a haemophore working in conjunction with proteases to acquire haem from haemoglobin. In addition, HmuY was able to extract haem from methaemalbumin, and could bind haem, either free in solution or from methaemoglobin, even in the presence of serum albumin.

Antibacterial action of polyphosphate on Porphyromonas gingivalis

Polyphosphate [poly(P)] has antibacterial activity against various Gram-positive bacteria. In contrast, Gram-negative bacteria are generally resistant to poly(P). Here, we describe the antibacterial characterization of poly(P) against a Gram-negative periodontopathogen, Porphyromonas gingivalis. The MICs of pyrophosphate (Na(4)P(2)O(7)) and all poly(P) (Na(n + 2)P(n)O(3n + 1); n = 3 to 75) tested for the bacterium by the agar dilution method were 0.24% and 0.06%, respectively. Orthophosphate (Na(2)HPO(4)) failed to inhibit bacterial growth. Poly-P75 was chosen for further study. In liquid medium, 0.03% poly-P75 was bactericidal against P. gingivalis irrespective of the growth phase and inoculum size, ranging from 10(5) to 10(9) cells/ml. UV-visible spectra of the pigments from P. gingivalis grown on blood agar with or without poly-P75 showed that poly-P75 reduced the formation of μ-oxo bisheme by the bacterium. Poly-P75 increased hemin accumulation on the P. gingivalis surface and decreased energy-driven uptake of hemin by the bacterium. The expression of the genes encoding hemagglutinins, gingipains, hemin uptake loci, chromosome replication, and energy production was downregulated, while that of the genes related to iron storage and oxidative stress was upregulated by poly-P75. The transmission electron microscope showed morphologically atypical cells with electron-dense granules and condensed nucleoid in the cytoplasm. Collectively, poly(P) is bactericidal against P. gingivalis, in which hemin/heme utilization is disturbed and oxidative stress is increased by poly(P).

Bacterial outer membrane vesicles in disease and preventive medicine

Gram-negative bacteria have the ability to produce outer membrane-derived vesicles (OMVs) that are released into the extracellular milieu. Even though this intriguing phenomenon is well-known since many years, various aspects of bacterial OMVs are not fully described and are still in the process of being characterized in detail. One major reason for this is that depending on the bacterial species and its respective ecological niche, OMVs exhibit an enormous functional diversity. Research of the past years has clearly shown that OMVs of many pathogenic bacteria contribute to the virulence potential by enriching virulence factors and delivering them over long distances, superseding direct bacterial contact with their host. The subsequent interaction of OMVs with the host can occur at different levels regarding the type of immune response or the target cell type and may lead to different outcomes ranging from non-immunogenic activation or a pro-inflammatory response to cytotoxicity. In contrast to being virulence factors, OMVs are used for vaccination purposes in the combat against bacterial pathogens, and recent research thus is focused on to indirectly aim these versatile bacterial weapons against themselves.

LuxS involvement in the regulation of genes coding for hemin and iron acquisition systems in Porphyromonas gingivalis

The periodontal pathogen Porphyromonas gingivalis employs a variety of mechanisms for the uptake of hemin and inorganic iron. Previous work demonstrated that hemin uptake in P. gingivalis may be controlled by LuxS-mediated signaling. In the present study, the expression of genes involved in hemin and iron uptake was determined in parent and luxS mutant strains by quantitative real-time reverse transcription-PCR. Compared to the parental strain, the luxS mutant showed reduced levels of transcription of genes coding for the TonB-linked hemin binding protein Tlr and the lysine-specific protease Kgp, which can degrade host heme-containing proteins. In contrast, there was up-regulation of the genes for another TonB-linked hemin binding protein, HmuR; a hemin binding lipoprotein, FetB; a Fe(2+) ion transport protein, FeoB1; and the iron storage protein ferritin. Differential expression of these genes in the luxS mutant was maximal in early-exponential phase, which corresponded with peak expression of luxS and AI-2 signal activity. Complementation of the luxS mutation with wild-type luxS in trans rescued expression of hmuR. Mutation of the GppX two-component signal transduction pathway caused an increase in expression of luxS along with tlr and lower levels of message for hmuR. Moreover, expression of hmuR was repressed, and expression of tlr stimulated, when the luxS mutant was incubated with AI-2 partially purified from the culture supernatant of wild-type cells. A phenotypic outcome of the altered expression of genes involved in hemin uptake was impairment of growth of the luxS mutant in hemin-depleted medium. The results demonstrate a role of LuxS/AI-2 in the regulation of hemin and iron acquisition pathways in P. gingivalis and reveal a novel control pathway for luxS expression.

Gene expression profiling and characterization under hemin limitation in Porphyromonas gingivalis

Hemin is an important nutrient for Porphyromonas gingivalis growth and pathogenicity. We examined the gene expression profile of P. gingivalis, including genes involved in its pathogenicity, at various growth stages under hemin-standard and limited conditions by using a custom-made microarray. The transcription of many genes decreased after late-log and mid-log phases under hemin-standard and limited conditions, respectively. We focused on two groups of genes while comparing gene expression profiles under hemin-standard and limited conditions by gene tree analysis. Genes belonging to group A maintained high transcriptional levels, whereas genes in group B were expressed at low levels under standard hemin conditions. However, group B genes increased remarkably under hemin-limited conditions. Groups A and B contained genes involved in regulatory functions and protein fate, respectively. Genes related to energy metabolism, transport, and protein binding were present in both groups. Our results suggest that P. gingivalis experienced severe stress under hemin-limited conditions, and growth phase-dependent changes in transcription levels were observed for many genes. Moreover, increased expression of genes involved in energy metabolism suggests that hemin is related not only to pathogenicity, but also energy metabolism.

Signaling system in Porphyromonas gingivalis based on a LuxS protein

The luxS gene of quorum-sensing Vibrio harveyi is required for type 2 autoinducer production. We identified a Porphyromonas gingivalis open reading frame encoding a predicted peptide of 161 aa that shares 29% identity with the amino acid sequence of the LuxS protein of V. harveyi. Conditioned medium from a late-log-phase P. gingivalis culture induced the luciferase operon of V. harveyi, but that from a luxS insertional mutant did not. In P. gingivalis, the expression of luxS mRNA was environmentally controlled and varied according to the cell density and the osmolarity of the culture medium. In addition, differential display PCR showed that the inactivation of P. gingivalis luxS resulted in up-regulation of a hemin acquisition protein and an arginine-specific protease and reduced expression of a hemin-regulated protein, a TonB homologue, and an excinuclease. The data suggest that the luxS gene in P. gingivalis may function to control the expression of genes involved in the acquisition of hemin.

Characterization of a novel outer membrane hemin-binding protein of Porphyromonas gingivalis

Porphyromonas gingivalis is a gram-negative, anaerobic coccobacillus that has been implicated as a major etiological agent in the development of chronic periodontitis. In this paper, we report the characterization of a protein, IhtB (iron heme transport; formerly designated Pga30), that is an outer membrane hemin-binding protein potentially involved in iron assimilation by P. gingivalis. IhtB was localized to the cell surface of P. gingivalis by Western blot analysis of a Sarkosyl-insoluble outer membrane preparation and by immunocytochemical staining of whole cells using IhtB peptide-specific antisera. The protein, released from the cell surface, was shown to bind to hemin using hemin-agarose. The growth of heme-limited, but not heme-replete, P. gingivalis cells was inhibited by preincubation with IhtB peptide-specific antisera. The ihtB gene was located between an open reading frame encoding a putative TonB-linked outer membrane receptor and three open reading frames that have sequence similarity to ATP binding cassette transport system operons in other bacteria. Analysis of the deduced amino acid sequence of IhtB showed significant similarity to the Salmonella typhimurium protein CbiK, a cobalt chelatase that is structurally related to the ATP-independent family of ferrochelatases. Molecular modeling indicated that the IhtB amino acid sequence could be threaded onto the CbiK fold with the IhtB structural model containing the active-site residues critical for chelatase activity. These results suggest that IhtB is a peripheral outer membrane chelatase that may remove iron from heme prior to uptake by P. gingivalis.

Life below the gum line: pathogenic mechanisms of Porphyromonas gingivalis

Porphyromonas gingivalis, a gram-negative anaerobe, is a major etiological agent in the initiation and progression of severe forms of periodontal disease. An opportunistic pathogen, P. gingivalis can also exist in commensal harmony with the host, with disease episodes ensuing from a shift in the ecological balance within the complex periodontal microenvironment. Colonization of the subgingival region is facilitated by the ability to adhere to available substrates such as adsorbed salivary molecules, matrix proteins, epithelial cells, and bacteria that are already established as a biofilm on tooth and epithelial surfaces. Binding to all of these substrates may be mediated by various regions of P. gingivalis fimbrillin, the structural subunit of the major fimbriae. P. gingivalis is an asaccharolytic organism, with a requirement for hemin (as a source of iron) and peptides for growth. At least three hemagglutinins and five proteinases are produced to satisfy these requirements. The hemagglutinin and proteinase genes contain extensive regions of highly conserved sequences, with posttranslational processing of proteinase gene products contributing to the formation of multimeric surface protein-adhesin complexes. Many of the virulence properties of P. gingivalis appear to be consequent to its adaptations to obtain hemin and peptides. Thus, hemagglutinins participate in adherence interactions with host cells, while proteinases contribute to inactivation of the effector molecules of the immune response and to tissue destruction. In addition to direct assault on the periodontal tissues, P. gingivalis can modulate eucaryotic cell signal transduction pathways, directing its uptake by gingival epithelial cells. Within this privileged site, P. gingivalis can replicate and impinge upon components of the innate host defense. Although a variety of surface molecules stimulate production of cytokines and other participants in the immune response, P. gingivalis may also undertake a stealth role whereby pivotal immune mediators are selectively inactivated. In keeping with its strict metabolic requirements, regulation of gene expression in P. gingivalis can be controlled at the transcriptional level. Finally, although periodontal disease is localized to the tissues surrounding the tooth, evidence is accumulating that infection with P. gingivalis may predispose to more serious systemic conditions such as cardiovascular disease and to delivery of preterm infants.

The periodontopathogen Porphyromonas gingivalis binds iron protoporphyrin IX in the mu-oxo dimeric form: an oxidative buffer and possible pathogenic mechanism

Mössbauer spectroscopy was used to re-evaluate iron protoporphyrin IX, FePPIX, binding and the chemical nature of the black iron porphyrin pigment of Porphyromonas gingivalis. We demonstrate that FePPIX is bound to the cell in the mu-oxo dimeric form, [Fe(III)PPIX]2O, and that the iron porphyrin pigment is also composed of this material. P. gingivalis also assimilated monomeric Fe(II)- and Fe(III)PPIX into mu-oxo dimers in vitro. Scatchard analysis revealed a greater binding maximum of cells for mu-oxo dimers than for monomeric Fe(III)-or Fe(II)PPIX, although the relative affinity constant for the dimers was lower. Formation of [Fe(III)PPIX]2O via reactions of Fe(II)PPIX with oxygen, and its toxic derivatives, would serve as an oxidative buffer and permit P. gingivalis and other black-pigmenting anaerobes to engender and maintain a local anaerobic environment. Tying up of free oxygen species with iron protoporphyrin IX would also reduce and limit Fe(II)PPIX-mediated oxygen-radical cell damage. More importantly, formation of a cell-surface mu-oxo dimer layer may function as a protective barrier against assault by reactive oxidants generated by neutrophils. Selective interference with these mechanisms would offer the possibility of attenuating the pathogenicity of P. gingivalis and other iron protoporphyrin IX-binding pathogens whose virulence is regulated by this reactive molecule.

Albumin and hemalbumin degradation by Porphyromonas gingivalis

Degradation of bovine albumin and hemalbumin by Porphyromonas gingivalis W50 cells under non-reducing conditions at 37 degrees C was examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometry. Albumin and hemalbumins with heme:protein molar ratios of 1:1, 4:1 and 8:1 were degraded, yielding protease-resistant 55.6-kDa peptides. Cells of strains WPH 35, 11834 and Bg 381 also produced a similar digestion pattern. N-terminal sequencing of the 55.6-kDa albumin digestion fragment revealed two peptides with the sequences 82glu-thr-tyr-gly-asp-met-ala and 95gln-pro-glu-arg-asn-glu-cys, indicating cleavage in the N-terminal hinge region. Tosyllysylchloromethylketone and N-ethylmaleimide were the most effective in inhibiting breakdown of albumin and hemalbumin with a 1:1 heme:protein ratio. Initial degradation rates of albumin and all hemalbumins were similar, but the total amount of hemalbumins degraded over 7.5 h decreased with increased ratio of bound hemin. The specific proteolysis of hemalbumin may enable P. gingivalis to release hemin from a region of the molecule where heme binding is least avid.

Binding of hemoglobin to the envelope of Porphyromonas gingivalis and isolation of the hemoglobin-binding protein

The binding activity of the Porphyromonas gingivalis envelope and hemoglobin was examined over a wide range of pH values from 4.5 to 9.0. The binding activity in low-pH buffers was much higher than that at high pH; the optimum pHs for the binding were found to be 4.5 and 5.0. Since the hemoglobin bound to the envelope was found to dissociate in the pH 8.5 and 9.0 buffers, the binding is reversible. We hypothesized that hemoglobin-binding protein (HbBP), responsible for the binding to hemoglobin, exists in the envelope and confirmed its presence by dot blot determination with peroxidase-conjugated hemoglobin. Then we attempted to isolate HbBP from the solubilized (by a detergent) materials of the envelope by affinity chromatography. The molecular mass of HbBP was 19 kDa, and the isoelectric point was 4.3.

Characterization of a Tn4351-generated hemin uptake mutant of Porphyromonas gingivalis: evidence for the coordinate regulation of virulence factors by hemin

The ability of Porphyromonas gingivalis to acquire iron in the iron-limited environment of the host is crucial to the colonization of this organism. We report here on the isolation and characterization of a transpositional insertion mutant of P. gingivalis A7436 (designated MSM-3) which is defective in the utilization and transport of hemin. P. gingivalis MSM-3 was selected on the basis of its nonpigmented phenotype on anaerobic blood agar following mutagenesis with the Bacteroides fragilis transposon Tn4351. P. gingivalis MSM-3 grew poorly when supplied with hemin as a sole source of iron; however, growth was observed with hemoglobin or inorganic iron. P. gingivalis MSM-3 grown in either hemin-replete or hemin-depleted conditions bound and transported less [14C]hemin or [59Fe]hemin than did the parent strain. At 4 h, P. gingivalis MSM-3 grown in hemin-replete conditions transported only 10,000 pmol of hemin per mg of protein, or 14% of the amount transported by P. gingivalis A7436. Unlike P. gingivalis A7436, hemin binding and transport by P. gingivalis MSM-3 were not tightly regulated by hemin or iron. Examination of P. gingivalis MSM-3 cultures by electron microscopy revealed an overproduction of membrane vesicles, and determination of the dry weight of purified vesicles indicated that P. gingivalis MSM-3 produced twice as much membrane vesicles as did strain A7436. Extracellular vesicles isolated from P. gingivalis MSM-3 also were found to express increased hemolytic and trypsin-like protease activities compared with the parent strain. When inoculated into subcutaneous chambers implanted in mice, P. gingivalis MSM-3 was highly infectious and more invasive than the parent strain, as indicated by secondary lesion formation and death. Taken together, these results indicate that the decreased transport of hemin by P. gingivalis MSM-3 results in the increased expression of several virulence factors which may be coordinately regulated by hemin.

Binding and accumulation of hemin in Porphyromonas gingivalis are induced by hemin

Although hemin is an essential nutrient for the black-pigmented oral bacterium Porphyromonas gingivalis, the mechanisms involved in hemin binding and uptake are poorly defined. In this study, we have examined the binding of hemin and Congo red (CR) to P. gingivalis whole cells and have defined the conditions for maximal binding. Additionally, the accumulation of hemin by P. gingivalis under growing conditions has been characterized. P. gingivalis A7436 was grown under hemin- or iron-deplete conditions (basal medium [BM] or Schaedler broth with dipyridyl [SBD]) or under hemin- or iron-replete conditions (BM with hemin [BMH] or Schaedler broth [SB]), and hemin and CR binding were assessed spectrophotometrically. Binding of hemin by P. gingivalis whole cells was rapid and was observed in samples obtained from cells grown under hemin- and iron-replete and hemin-deplete conditions but was not observed in cells grown under iron limitation. We also found that P. gingivalis whole cells bound more hemin when grown in BMH or SB than cells grown in BM or SBD. Binding of CR by P. gingivalis A7436 was also enhanced when cells were grown in the presence of hemin or when cells were incubated with hemin prior to CR binding. Hemin binding and accumulation were also assessed using [14C]hemin and [59Fe]hemin under growing conditions. Both [14C]hemin and [59Fe]hemin were accumulated by P. gingivalis, indicating that iron and the porphyrin ring were taken into the cell. Binding and accumulation of hemin under growing conditions were also induced by growth of P. gingivalis in hemin-replete media. Hemin accumulation was inhibited by the addition of KCN to P. gingivalis cultures, indicating that active transport was required for hemin uptake. [14C]hemin binding and accumulation were also inhibited by the addition of either cold hemin or protoporphyrin IX. Taken together, these results indicate that P. gingivalis transports the entire hemin moiety into the cell and that the binding and accumulation of hemin are induced by growth of cultures in the presence of hemin.

Hemin uptake in Porphyromonas gingivalis: Omp26 is a hemin-binding surface protein

A 26-kDa outer membrane protein (Omp26) has been proposed to play a role in hemin acquisition by Porphyromonas gingivalis (T. E. Bramanti and S. C. Holt, J. Bacteriol. 174:5827-5839, 1992). We studied [55Fe]hemin uptake in P. gingivalis grown under conditions of hemin starvation (Omp26 expressed on the outer membrane surface) and hemin excess (Omp26 not expressed on surface). [55Fe]hemin uptake occurred rapidly in hemin-starved cells which incorporated up to 70% of total [55Fe]hemin within 3 min. P. gingivalis grown under hemin-starved conditions or treated with the iron chelator 2,2'-bipyridyl to induce an iron stress took up six times more [55Fe]hemin than hemin-excess-grown cells. Polyclonal monospecific anti-Omp26 antibody added to hemin-starved cells inhibited [55Fe]hemin uptake by more than 50%, whereas preimmune serum had no effect. [55Fe]hemin uptake in hemin-starved P. gingivalis was inhibited (36 to 67%) in the presence of equimolar amounts of unlabeled hemin, protoporphyrin IX, zinz protoporphyrin, and Congo red dye but was not inhibited in the presence of non-hemin-containing iron sources. Heat shock treatment (45 degrees C) of hemin-excess-grown P. gingivalis (which cases translocation of Omp26 to the surface) increased [55Fe]hemin uptake by threefold after 3 min in comparison with cells grown at 37 degrees C. However, no [55Fe] hemin uptake beyond 3 min was observed in either hemin-excess-grown or hemin-starved cells exposed to heat shock. In experiments using heterobifunctional cross-linker analysis, hemin and selected porphyrins were cross-linked to Omp26 in hemin-starved P. gingivalis, but no cross-linking was seen with hemin-excess-grown cells. However, cross-linking of hemin to Omp26 was observed after heat shock treatment of hemin-excess-grown cells. Finally, anti-Omp26 antibody inhibited cross-linked of hemin to Omp26. These findings indicate that hemin binding and transport into P.gingivalis cell mediated by Omp26.