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Landmark & Important Study Article Notes: Virulence Factors of Periodontal Pathogens

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SECTION I: FOUNDATIONAL / LANDMARK STUDIES

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STUDY 1 - The Microbial Complex Framework

• Identified 5 major bacterial complexes consistently observed regardless of analytical method used. Named by color: red, orange, yellow, green, and purple complexes.
• Red complex (the most significant for disease): Porphyromonas gingivalis, Treponema denticola, and Bacteroides forsythus (now Tannerella forsythia) formed a tightly interrelated cluster.
• The red complex correlated strikingly with clinical measures of periodontal disease - particularly pocket depth and bleeding on probing.
• Orange complex (Fusobacterium nucleatum/periodonticum subspecies, Prevotella intermedia, P. nigrescens, Peptostreptococcus micros, Eubacterium nodatum, Campylobacter spp.) acted as a "bridge" complex - red complex organisms were rarely found without orange complex colonization.
• Demonstrated that periodontal infection is driven by bacterial complexes rather than individual species, fundamentally shifting the paradigm from the "specific plaque hypothesis" single-pathogen model.
• A. actinomycetemcomitans serotype b was an outlier with little relation to the 5 major complexes, implying unique pathogenic niche.

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STUDY 2 - The Keystone Pathogen Hypothesis

• Coined the term "keystone pathogen" - a low-abundance microorganism that, despite its small numbers, can orchestrate inflammatory disease by remodelling a normally benign microbiota into a dysbiotic one.
• Applied this concept primarily to P. gingivalis, which at low colonization levels can still cause significant microbial community shifts and bone loss in animal models.
• Mechanistically: P. gingivalis subverts complement (particularly C5a receptor - CR3 crosstalk) and impairs host defense, causing overgrowth of oral commensal bacteria. This dysbiotic community - not P. gingivalis alone - drives complement-dependent inflammation and bone resorption.
• Proposed that the resulting inflammatory environment is nutritionally favorable to P. gingivalis (degraded host proteins as amino acid sources; hemin as iron source), completing a self-reinforcing pathogenic cycle.
• Extended the hypothesis to other diseases (IBD, colorectal cancer) where low-abundance keystone pathogens may similarly orchestrate dysbiosis.
• Distinguished between a "pathogen" (causes disease alone) and a "keystone pathogen" (acts through community manipulation).
• Over 2,294 citations as of 2025.

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STUDY 3 - Community Activist / Complement Subversion (Companion Paper)

• Provided direct experimental evidence that P. gingivalis modulates complement function to change both the quantity and composition of normal oral microbiota.
• In a mouse model of periodontitis, this complement-mediated dysbiosis was responsible for pathologic bone loss - the altered commensal community drove disease, not P. gingivalis directly.
• First formal use of "keystone pathogen" in a dental research journal (companion to the Nature Reviews paper).
• Showed that P. gingivalis creates a "dysbiosis between the host and dental plaque" as the initiating mechanism of periodontitis.
• Demonstrated that even minimal inoculations with P. gingivalis could dramatically alter microbial community composition.

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STUDY 4 - Red Complex Virulence: Comprehensive Overview

• Provided the definitive comparative virulence analysis of all three red complex members in a single authoritative paper.
• For P. gingivalis: detailed gingipains (Arg-gingipain/RgpA, RgpB; Lys-gingipain/Kgp), fimbriae (FimA types I-V, Mfa1), LPS, capsule (6 serotypes K1-K6), hemagglutinins, and outer membrane vesicles (OMVs) as virulence determinants.
• For T. denticola: outlined dentilisin/CTLP (chymotrypsin-like protease) complex (PrcB-PrcA-PrtP), major outer sheath protein (MOSP/Msp), periplasmic flagella and motility, OppA lipoprotein, and MOSP-mediated cell membrane perturbation.
• For T. forsythia: described BspA surface antigen (leucine-rich repeat protein), N-acetylneuraminidase (sialidase), karilysin (metalloprotease), and its requirement for exogenous N-acetylmuramic acid for peptidoglycan synthesis (nutritional dependency linking it to P. gingivalis).
• Emphasized synergistic interactions among red complex members - co-infection is more pathogenic than mono-infection due to metabolic cross-feeding and shared invasion mechanisms.
• Used both animal models (gnotobiotic rats, murine abscess models) and clinical correlation data.

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SECTION II: PORPHYROMONAS GINGIVALIS - SPECIFIC VIRULENCE FACTOR STUDIES

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STUDY 5 - Molecular Strategies of P. gingivalis Virulence

• Gingipains (Arg-gingipain A/B and Lys-gingipain): cysteine proteases that are the signature virulence factors of P. gingivalis. Cleave host proteins including fibrinogen, complement components (C3, C5), cytokines, and IgG. RgpA and Kgp form high-molecular-weight complexes with hemagglutinin domains.
• Type IX Secretion System (T9SS): Detailed the T9SS as the specialized outer membrane transport machine for gingipains and other effectors. Effectors contain a conserved C-terminal domain (CTD) recognized by the T9SS. After transport, they are attached to anionic lipopolysaccharide (A-LPS) at the cell surface - forming a "virulence coat."
• Anionic LPS (A-LPS) and LPS heterogeneity: P. gingivalis produces two distinct LPS forms. A-LPS anchors gingipains/effectors to the surface. O-LPS activates TLR2 weakly and antagonizes TLR4 - a key immune evasion strategy.
• Fimbriae: FimA (major fimbriae, 5 genotypes) mediates adhesion to saliva, cementum, and host cells. Mfa1 (minor fimbriae) mediates binding to Streptococcus gordonii (a key colonization event) and complement receptor CR3.
• Capsular polysaccharide (CPS): Anti-phagocytic; K1-K6 serotypes differ in virulence. K1 is most antiphagocytic.
• Outer membrane vesicles (OMVs): Carry gingipains, LPS, and fimbriae to distant sites. Allow invasion of host cells and systemic dissemination.
• Outlined first T9SS inhibitors - potential therapeutic targets.

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STUDY 6 - Roles of P. gingivalis Virulence Factors in Periodontitis

• Detailed how LPS of P. gingivalis differs structurally from canonical Gram-negative LPS: lipid A has penta-acylated or tetra-acylated forms rather than the hexa-acylated form, activating TLR2 rather than TLR4. This heterodox TLR2 activation leads to immune subversion rather than classical inflammatory killing.
• Gingipain effects on host immunity: RgpA/B and Kgp degrade complement C3, C4, and C5; cleave and inactivate cytokines (IL-6, IL-8, TNF-α); degrade antibodies; and activate protease-activated receptors (PARs) on epithelial/endothelial cells - triggering IL-8 secretion and neutrophil recruitment but then degrading the resulting chemokines.
• Fimbriae-mediated immune subversion: FimA activates Toll-like receptor 2 (TLR2) and induces IL-10 (anti-inflammatory), suppressing the bactericidal response. Mfa1 engages CR3 on macrophages, activating an intracellular PI3K/Akt signaling pathway that blocks intracellular killing.
• Biofilm promotion: Virulence factors promote co-aggregation with S. gordonii, F. nucleatum, T. denticola, and other species - facilitating transition from early- to late-stage dysbiotic biofilm.
• Asaccharolytic metabolism: P. gingivalis derives energy primarily from amino acid fermentation (proteolytic), making the inflamed periodontium (rich in serum proteins) its ideal niche.
• Covered T-cell modulation: gingipains cleave and inactivate CD4/CD8 T cell co-receptors, impairing adaptive immunity.

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STUDY 7 - P. gingivalis: Multiple Tools of Inflammatory Damage (Updated Comprehensive Review)

• TLR2/TLR4 signaling update: Clarified that P. gingivalis LPS signals through both TLR2 and TLR4, but with context-dependent and often opposing effects. TLR2 signaling promotes immune dysregulation; TLR4 antagonism (by atypical lipid A) prevents classical innate killing.
• Peptidyl arginine deiminase (PPAD): P. gingivalis expresses the only known bacterial PAD enzyme. PPAD citrullinates host and bacterial proteins, generating citrullinated antigens that may trigger anti-citrullinated protein antibodies (ACPAs) - mechanistically linking periodontitis to rheumatoid arthritis (RA). A novel PPAD variant was identified correlating with aggressive disease.
• OMVs as neurovirulence factors: OMVs carry gingipains to the CNS via trigeminal nerve pathways and through a compromised blood-brain barrier (BBB). Gingipains were detected ultrastructurally in the substantia nigra of Parkinson's disease patients.
• CRISPR array in virulence: A bacterial CRISPR system was found to regulate virulence gene expression and modulate host inflammatory responses.
• C-terminal domain (CTD) and T9SS: Defined the CTD as the sorting signal for T9SS substrates; identified first T9SS inhibitors with therapeutic potential.
• COVID-19 interactions: P. gingivalis may co-facilitate SARS-CoV-2 entry and worsen systemic outcomes through shared inflammatory pathways.
• Tpr proteases: Trypsin-like Tpr proteases (TprC, TprD, TprI, TprJ, TprK) described with detailed substrate specificity; contribute to ECM degradation and immune evasion.

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STUDY 8 - OMV-Mediated Pathogenesis

• OMV composition: OMVs are nano-sized vesicles (20-200 nm) blebbing from the outer membrane. They contain LPS, gingipains (RgpA, RgpB, Kgp), fimbriae, outer membrane proteins, and periplasmic proteins - representing a concentrated "virulence payload."
• OMV production regulation: Regulated by environmental signals including iron/hemin availability, heat stress, and antibiotic exposure. Gingipains themselves regulate OMV size and content.
• Local effects: OMVs penetrate gingival epithelium, activate TLR2/TLR4 on resident cells, degrade complement, and disrupt epithelial barrier integrity.
• Systemic dissemination: OMVs are detectable in systemic circulation and can cross the blood-brain barrier. They carry virulence factors to distant organs - mechanistically explaining P. gingivalis links to cardiovascular disease, diabetes, and Alzheimer's disease.
• Immune evasion via OMVs: OMVs act as "decoys," absorbing host antibodies and complement, protecting the parent bacterium.
• Invasion via OMVs: OMVs fuse with host cell membranes, delivering gingipains and LPS intracellularly to modulate NF-κB, NLRP3 inflammasome, and autophagy pathways.

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SECTION III: AGGREGATIBACTER ACTINOMYCETEMCOMITANS - SPECIFIC STUDIES

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STUDY 9 - Leukotoxin: Mechanism to Therapeutics

• LtxA (Leukotoxin A): Member of the repeats-in-toxin (RTX) family. Unlike most RTX toxins with broad host range, LtxA shows exquisite human selectivity - it kills only human and Old World primate leukocytes.
• LFA-1 receptor: The functional receptor for LtxA is lymphocyte function-associated antigen-1 (LFA-1, αLβ2 integrin), expressed exclusively on white blood cells. This specificity makes LtxA unique among bacterial toxins.
• Cell death mechanisms vary by cell type:
  • In lymphocytes: LtxA triggers rapid caspase-independent necrotic death (oncosis)
  • In monocytes/macrophages: triggers lysosomal membrane permeabilization and cathepsin release
  • In neutrophils: activates the NLRP3 inflammasome with IL-1β release
  • In dendritic cells: impairs antigen presentation
• Cholesterol dependency: LtxA requires membrane cholesterol for LFA-1 binding; the toxin extracts cholesterol from lipid rafts - a key step in pore formation.
• Highly leukotoxic clones: Certain A. actinomycetemcomitans clones (notably the JP2 clone prevalent in West African-ancestry populations) carry a 530 bp deletion in the ltxCABD operon promoter region, causing 10-20× overexpression of LtxA. These JP2-type clones are strongly associated with localized aggressive periodontitis (LAP) in adolescents.
• LtxA as a therapeutic agent: Because of LFA-1 specificity, LtxA is being investigated as a targeted agent against hematological malignancies (leukemia, lymphoma) and autoimmune disease.

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STUDY 10 - Leukotoxin and Aggressive Periodontitis Association

• Longitudinal evidence for LtxA in disease prediction: Reviewed studies showing A. actinomycetemcomitans colonization status (especially highly leukotoxic clones) can predict future development of aggressive periodontitis - a relatively rare example of a microbial marker with predictive clinical value.
• Clarified the genetic heterogeneity of A. actinomycetemcomitans: multiple serotypes (a-f) and biotypes exist, but LtxA expression level is the primary virulence discriminant between aggressive and minimally virulent strains.
• JP2 clone epidemiology: disproportionately found in individuals of West African and North African descent; accounts for the higher LAP prevalence in these populations.
• LtxA and IL-1β: Documented that LtxA-induced cell death releases large quantities of IL-1β from macrophages, which directly drives alveolar bone resorption by activating osteoclasts via RANK/RANKL.
• Discussed additional A. actinomycetemcomitans virulence factors: cytolethal distending toxin (CDT), fimbriae, OmpA outer membrane protein, iron acquisition, and biofilm formation via PGA exopolysaccharide.

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STUDY 11 - Comprehensive Virulence and Pathogenicity of A. actinomycetemcomitans

• Fimbriae: Flp-1 (Flp fimbriae) and Fim fimbriae mediate initial adhesion to tooth surfaces and host cells. Flp-type fimbriae form a tight biofilm with star-shaped colony morphology characteristic of virulent strains.
• Cytolethal distending toxin (CDT): CDT is a heterotrimeric AB2 toxin. The CdtB subunit has DNase I-like activity, causing double-strand DNA breaks in host cells. This induces cell cycle arrest (G2/M block), apoptosis in lymphocytes/monocytes, and impairs immune responses. CDT is expressed by most A. actinomycetemcomitans strains.
• Lipopolysaccharide (LPS): Contains a Y4 serotype-specific O antigen. A. actinomycetemcomitans LPS activates TLR4 strongly (unlike P. gingivalis), inducing robust IL-6, IL-8, and TNF-α production - driving both tissue destruction and bone loss.
• Iron acquisition: BFR1 (bacterioferritin), lactoferrin-binding proteins, and transferrin-binding proteins allow iron scavenging from host proteins - essential for survival in the nutrient-limited periodontal pocket.
• Outer membrane vesicles: A. actinomycetemcomitans OMVs contain LtxA, CDT, and LPS - amplifying their delivery to host cells.
• Geographic and demographic distribution: Virulent genotypes (especially serotype b + JP2 clone) predominate in specific ethnic and geographic populations, explaining disease incidence disparities.
• Age-dependency: LAP typically manifests at puberty, potentially due to hormonal effects on host susceptibility combined with A. actinomycetemcomitans colonization acquired from family members in early childhood.

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STUDY 12 - Oral Pathogenesis: Full Virulence Factor Review

• Specifically addressed GroEL and DnaK (heat shock proteins) as immunostimulatory virulence factors - they induce cytokine production (IL-1β, TNF-α) through TLR4 signaling, contributing to bone loss independent of direct bacterial invasion.
• HtpG: Another heat shock protein that modulates adaptive immunity and suppresses T-cell proliferation.
• Described the cascade from A. actinomycetemcomitans virulence factor exposure → cytokine induction → osteoclast activation → alveolar bone resorption, providing a mechanistic link between bacterial virulence and the key clinical outcome of periodontitis.
• Summarized evidence for LtxA-independent bone resorption through CDT, LPS, and immunostimulatory proteins.
• Discussed cell-surface materials (PGA exopolysaccharide, autoinducer-2) promoting biofilm persistence and antibiotic resistance.

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SECTION IV: TANNERELLA FORSYTHIA - SPECIFIC STUDIES

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STUDY 13 - Virulence Mechanisms of Tannerella forsythia

• BspA (T. forsythia surface protein A): A leucine-rich repeat (LRR) surface adhesin - the most studied T. forsythia virulence factor. BspA binds fibronectin and fibrinogen, mediates co-aggregation with P. gingivalis and T. denticola, and activates TLR2, inducing NF-κB and pro-inflammatory cytokine production.
• Sialidase (NanH): Neuraminidase that cleaves sialic acid residues from host glycoproteins and mucosal surfaces. This exposes underlying glycan structures (galactose residues) that serve as receptors for bacterial adhesins - facilitating colonization. Also degrades mucin, disrupting the protective mucosal layer. T. forsythia may "scavenge" sialic acid as a metabolic substrate (linking virulence to nutritional strategy).
• Karilysin: A metalloprotease (M43 family) that cleaves complement components (C4, C2, C3, factor B) - enabling complement evasion - and degrades matrix proteins (fibronectin, vitronectin). Karilysin physically interacts with Mfa1 of P. gingivalis, providing a molecular mechanism for their co-pathogenicity.
• CTLP/KTOP-like proteases: Trypsin/chymotrypsin-like protease activities similar to T. denticola, contributing to ECM degradation.
• Peptidoglycan auxotrophy: T. forsythia cannot synthesize N-acetylmuramic acid (MurNAc) and depends on scavenging from P. gingivalis or host tissues. This metabolic dependency physically links T. forsythia to P. gingivalis, explaining their tight ecological and clinical association.
• Reviewed animal model data showing BspA mutants are attenuated in virulence, confirming BspA as a key pathogenicity determinant.

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STUDY 14 - Sialic Acid and Tannerella forsythia

• Described the sialic acid scavenging pathway of T. forsythia in detail: NanH sialidase cleaves sialic acid from host glycoproteins → liberated sialic acid is transported into the bacterium via NanT transporter → catabolized as a carbon/nitrogen source.
• This "feast" on host sialic acid is dual-purpose: nutritional acquisition AND colonization enhancement (by unmasking adhesin receptors).
• Sialidase activity is a shared feature among periodontal pathogens (P. gingivalis, T. denticola also have sialidase activity), suggesting convergent evolution of this virulence strategy.
• Reviewed how sialidase inhibitors (e.g., DANA, zanamivir analogs) could potentially suppress T. forsythia virulence - a therapeutic angle.
• Discussed the role of sialic acid in biofilm formation and inter-species signaling.

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SECTION V: PATHOGENESIS MODELS AND SYSTEMIC LINKS

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STUDY 15 - Periodontal Diseases: Pathogenesis Framework

• Integrated microbial virulence factors with the host inflammatory response - establishing that tissue destruction in periodontitis is largely immunopathological, not just directly bacterial.
• Defined the role of gingival crevicular fluid (GCF) cytokines (IL-1β, IL-6, TNF-α, PGE2) as biomarkers and mediators of periodontitis progression.
• Connected periodontal infection to systemic outcomes - proposed the "focal infection" concept with emerging evidence linking periodontal pathogens to cardiovascular disease, adverse pregnancy outcomes, and diabetes.
• Established the framework of periodontal pathogens using virulence factors to both directly damage tissue (proteases, LPS, toxins) and dysregulate host immune responses - driving the self-sustaining inflammatory cycle.
• Introduced the paradigm of periodontal diagnosis using host biomarkers (IL-1β levels in GCF) not just microbiology.

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STUDY 16 - Comprehensive P. gingivalis Overview

• Detailed P. gingivalis fimA genotypes (I-V): Type II fimA is the most prevalent in periodontitis patients and most associated with clinical disease severity. Type IV is associated with non-progressive disease.
• Described hemagglutinins (HagA, HagB, HagC, HagD, HagE): Outer membrane proteins mediating adhesion and erythrocyte binding; important in iron/hemin acquisition (P. gingivalis is a strict asaccharolyte requiring heme).
• Clarified P. gingivalis metabolism: strictly asaccharolytic; depends on proteolytic breakdown of host proteins and peptides for carbon/energy/iron.
• Reviewed intracellular invasion: P. gingivalis invades epithelial cells, fibroblasts, endothelial cells, and macrophages using fimbriae (FimA, Mfa1) + integrin signaling, surviving intracellularly and evading antibacterial mechanisms.
• Described capsule role in serum resistance and anti-phagocytic protection; non-encapsulated strains are more serum-sensitive and more inflammatory.

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STUDY 17 - Periodontal Pathogens and Neurodegeneration

• Mechanistic virulence links to neurodegeneration: Gingipains of P. gingivalis cleave ApoE and tau proteins → tau hyperphosphorylation and aggregation (Alzheimer's hallmarks). Gingipains also degrade complement, cytokine receptors, and BBB tight junction proteins.
• Routes of CNS access for periodontal pathogens: (1) Hematogenous spread across a compromised BBB; (2) Retrograde travel along trigeminal and olfactory nerves; (3) OMV-mediated transport.
• T. denticola neuroinvasion: Dentilisin activates MMP-2, degrades BBB matrix components; T. denticola has been detected in brain tissue from Alzheimer's patients.
• F. nucleatum as emerging pathogen: Acts as a bridge organism enabling P. gingivalis biofilm invasion; FadA adhesin (F. nucleatum) activates Wnt/β-catenin signaling in epithelial/neuronal cells.
• 20 of 24 longitudinal studies showed positive correlation between periodontal disease and neurodegenerative outcomes; cognitive function studies showed the strongest effects.
• Proposed periodontal pathogens and their virulence factors as novel preventive therapeutic targets for neurodegeneration.

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STUDY 18 - LtxA Therapeutic Applications

• LtxA mechanism refinement: Detailed the two-step interaction - LtxA first binds cholesterol in LFA-1-rich lipid rafts, then inserts into the membrane forming transmembrane pores.
• Cell type-specific outcomes: LtxA kills all LFA-1+ white blood cells but through distinct pathways - lymphocyte oncosis, macrophage pyroptosis (via NLRP3), neutrophil IL-1β release.
• Hematological malignancy applications: LtxA selectively kills leukemia and lymphoma cells (which overexpress LFA-1) while sparing red blood cells and platelets. Phase I/II investigations ongoing.
• Autoimmune disease applications: LtxA-mediated depletion of autoreactive lymphocytes in models of psoriasis, lupus, and rheumatoid arthritis.
• Reviewed the genetic regulation of LtxA expression: ltxCABD operon; LtxC is an acyltransferase for LtxA post-translational activation; LtxB and LtxD are the secretion apparatus (type I secretion).
• Discussed why the JP2 promoter deletion causes the LtxA overproduction that characterizes highly virulent clones.

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QUICK REFERENCE TABLE

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MASTER VIRULENCE FACTOR CHEAT SHEET

Porphyromonas gingivalis

Aggregatibacter actinomycetemcomitans

Tannerella forsythia

Treponema denticola

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Note on T. denticola and neurodegeneration (2026): A 2026 systematic review (PMID: 42119956) documented T. denticola neuroinvasion pathways and its role in amyloidogenesis and neuroinflammation, extending the clinical relevance of its dentilisin protease beyond periodontal disease to potential Alzheimer's mechanisms.