HOST-MICROBE INTERACTION IN PERIODONTOLOGY

COMPREHENSIVE THEORY EXAMINATION NOTES

Postgraduate Periodontology - University Examination Level

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ABBREVIATIONS USED

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SECTION 1: INTRODUCTION AND CONCEPTUAL OVERVIEW

1.1 Fundamental Concept

• The oral cavity is an important interface with the external environment and is in constant interaction with a great number and variety of microorganisms, mostly organized in plaque biofilms.
• Periodontal disease is currently thought to be driven by a dysbiotic microbiome, in which the nature and dynamic status of host-microbial interactions ultimately dictate the onset and progression of tissue damage observed clinically and radiographically.
• Preclinical research in gnotobiotic animals demonstrates that microorganisms are fundamental for the occurrence of periodontal disease and clinical observations and experimental studies support the rationale for antimicrobial (mechanical and chemical) therapy for the correction of the dysbiotic status and restoring health.
• Substantial experimental evidence supports the concept that host immune response is the primary mediator of periodontal tissue destruction.

1.2 Recent Conceptual Shift - EXAMINER KEYWORD

• Periodontitis is not, strictly speaking, a bacterial infection. In the classical sense of the term, infections are caused by specific exogenous pathogens and conform, broadly, to Koch's postulates. Periodontitis, on the other hand, is not caused by a single or even a select few bacterial species (traditionally known as periopathogens, the pathogenic role of which could not be confirmed as per Koch's postulates). Rather, the microbial etiology of periodontitis entails synergistic interactions between different indigenous species with distinct roles in a dysbiotic microbial community.
• Destructive inflammation and dysbiosis positively reinforce each other in a self-sustained vicious circle that may contribute to the chronicity of periodontitis.

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SECTION 2: MODELS OF HOST-MICROBE INTERACTION / ETIOLOGIC HYPOTHESES

2.1 Evolution of Plaque Hypotheses (Examiner-Important Sequence)

NON-SPECIFIC PLAQUE HYPOTHESIS (Theilade, 1986)
     ↓
"Total amount of plaque = disease; no species specificity"
     ↓
SPECIFIC PLAQUE HYPOTHESIS (Loesche, 1976)
     ↓
"Certain specific bacteria (not bulk of plaque) determine disease"
     ↓
ECOLOGICAL PLAQUE HYPOTHESIS (Marsh, 1994)
     ↓
"Health-associated microbiota + host factors + environmental perturbations
→ Ecological shift → Disease-associated dysbiotic microbiota"
     ↓
POLYMICROBIAL SYNERGY & DYSBIOSIS MODEL (Hajishengallis & Lamont, 2012)
     ↓
"Community-wide dysbiosis driven by keystone pathogens; host-microbe
homeostasis disrupted → destructive inflammation → more dysbiosis"

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SECTION 3: THE MICROBIAL COMPONENT - VIRULENCE AND PATHOGENICITY

3.1 Criteria for Periodontal Pathogenicity ("Socransky Criteria")

3.2 Microbial Complexes (Socransky et al. 1998)

3.3 Recent Terminology - Pathobiont Classification

3.4 Virulence Factors of Key Periodontal Pathogens

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SECTION 4: THE KEYSTONE PATHOGEN HYPOTHESIS

4.1 Definition and Mechanism

• P. gingivalis is considered by some authors as the primary keystone pathogen in periodontitis.
• The keystone pathogen hypothesis has been extended to include the concept of "polymicrobial synergy" and "dysbiosis model" of disease. In this model, interspecies communication between keystone pathogens and other members of the community allows the whole microbial community to become more virulent than its individual parts.
• Keystone pathogens (such as P. gingivalis) play a disproportionately large role in subverting host immunity, thereby promoting dysbiosis and allowing the overgrowth of communities of otherwise relatively non-pathogenic species.

Low-abundance KEYSTONE PATHOGEN (e.g., P. gingivalis)
     ↓
Subverts host innate/complement immunity
     ↓
Creates permissive environment for entire microbial community
     ↓
Community-wide DYSBIOSIS (overgrowth of otherwise low-virulence species)
     ↓
Dysbiotic microbiota provokes dysregulated/excessive HOST INFLAMMATION
     ↓
Tissue destruction (bone loss, attachment loss)
     ↓
Tissue breakdown products (peptides, haem) = nutrients → fuel dysbiotic bacteria
     ↓
VICIOUS CYCLE (dysbiosis ↔ inflammation ↔ more destruction)

4.2 Mechanisms of Immune Subversion by P. gingivalis

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SECTION 5: DYSBIOSIS AND ECOLOGICAL PLAQUE HYPOTHESIS

5.1 The Ecological Plaque Hypothesis (Marsh, 1994)

• The principles of this hypothesis are that both the total amount of dental biofilm and the specific microbial composition of biofilms may contribute to the transition from health to disease.
• The health-associated dental microbiota is considered to be relatively stable over time and in a state of dynamic equilibrium or "microbial homeostasis."
• The host controls subgingival biofilms to some extent by a tempered immune response and low levels of GCF flow. Perturbations to the host response may be brought about by:
  1. Excessive accumulation of nonspecific dental biofilm leading to inflammation
  2. Biofilm-independent host factors (e.g., immune disorder, hormonal changes, pregnancy)
  3. Environmental factors (e.g., smoking, diet)
• Changes in host status (inflammation, tissue degradation, high GCF flow) may lead to a shift in the microbial population. This gradual shift in the entire microbial community is known as dysbiosis, which may result in a chronic disease state such as periodontitis.

5.2 Factors That Disrupt Periodontal Homeostasis

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SECTION 6: INNATE IMMUNE RESPONSE IN THE PERIODONTIUM

6.1 First Line - Epithelial Barrier

• The epithelium provides a physical barrier to impede the ingress of bacteria and their products; disruption of the epithelial barrier can lead to further bacterial invasion and inflammation.
• Antimicrobial Peptides (AMPs), also called defensins, are expressed by epithelial cells:
  • Human β-defensins: hBD-1 and hBD-2 (expressed by gingival epithelial cells)
  • Cathelicidin: LL-37 (found in lysosomes of neutrophils; also expressed in gingiva)
  • These AMPs are important for determining the outcomes of the host-pathogen interactions at the epithelial barrier.
• The epithelium is, therefore, more than simply a passive barrier: it also has an active role in innate immunity.
• Epithelial cells in the junctional and sulcular epithelia are in constant contact with bacterial products and respond by secreting chemokines (e.g., IL-8/CXCL8) to attract neutrophils, which migrate up the chemotactic gradient toward the pocket.

6.2 Pattern Recognition Receptors (PRRs) and MAMPs/PAMPs

6.3 Toll-Like Receptors (TLRs) in Periodontitis

• Functional PRRs are known to be expressed by "non-professional" immune cells (e.g., epithelial cells, fibroblasts, osteoblasts) that have a role in innate immunity, and also by adaptive immune cells.
• TLR signaling pathway:
  • Ligand binding → recruitment of adaptor proteins and kinases
  • MyD88 is an adaptor protein required for signaling by all TLRs except TLR3 (which signals via TRIF)
  • Intracellular (endosome) TLR4 may also signal in a MyD88-independent manner via TRIF
  • Downstream of MyD88: IRAK1 → TRAF6 → TAK1 → activation of NF-κB and MAP kinases → induction of pro-inflammatory mediators
  • Also activates Inflammasome components (NLRP3) as the "first inflammasome-activating signal"

MICROBIAL LIGAND (LPS, LTA, CpG DNA)
         ↓
    TLR activation (membrane-bound or endosomal)
         ↓
Adaptor recruitment: MyD88 (all TLRs) / TRIF (TLR3; TLR4 in endosome)
         ↓
        MyD88 pathway              TRIF pathway
             ↓                          ↓
   IRAK1/4 → TRAF6 → TAK1         IRF3 / IRF7
             ↓                          ↓
   IKKβ → NF-κB activation     Type I IFN (IFNα/β) genes
             ↓
   MKK → p38, JNK, ERK (MAPKs)
             ↓
   AP-1 activation
             ↓
Expression of:
• Cytokines (TNF-α, IL-1, IL-6)
• Chemokines (CCL2, CXCL8)
• Endothelial adhesion molecules (E-selectin)
• Costimulatory molecules (CD80, CD86)
• Inflammasome components (NLRP3)

6.4 NOD-Like Receptors (NLRs) in Periodontitis

• Cytosolic PRRs that recognize peptidoglycan fragments of the bacterial cell wall
• Can also activate NF-κB and MAPK, signaling via Rip2
• NOD1 recognizes iE-DAP (Gram-negative bacteria); NOD2 recognizes MDP (muramyl dipeptide; broad bacterial recognition)

• NLRP3 Inflammasome activation can be induced by various ligands (DAMPs, ROS, ATP, cathepsins)
• Results in assembly of a multi-protein complex: oligomerization of NLRP3, ASC, and pro-Caspase-1
• Autoproteolytic cleavage of oligomerized pro-caspase-1 → cleaved (active) Caspase-1
• Active Caspase-1 cleaves pro-IL-1β → mature IL-1β (and pro-IL-18 → IL-18)
• Active caspase-1 also cleaves Gasdermin D (GSDMD) → N-GSDMD → membrane pore formation → pyroptosis
• Activated NLRP3 inflammasome is expressed in periodontitis tissues and contributes to IL-1β-mediated alveolar bone resorption

6.5 Role of Complement System

• All three pathways converge at C3 (the third component), activated by pathway-specific C3 convertases.
• C3 activation leads to:
  1. Recruitment and activation of inflammatory cells via anaphylatoxins C3a and C5a (interact with G-protein-coupled receptors on leukocytes)
  2. Microbial opsonization through opsonins such as C3b to facilitate phagocytosis
  3. Direct lysis of targeted susceptible bacteria by means of the C5b-9 Membrane Attack Complex (MAC)

• The GCF in the gingival crevice contains a functional complement system. Therefore, the subgingival bacteria likely constantly encounter complement and the generated complement-dependent host defense mechanisms may contribute to host-microbe homeostasis in the healthy periodontium.
• Clinical evidence: Complement activation products are readily detected in chronically inflamed gingivae and the GCF of patients, whereas they are undetectable or present at lower levels in control samples from periodontally healthy individuals.
• Experimental gingivitis in human volunteers causes progressive complement activation (as determined by increased C3 cleavage in GCF), which in turn correlates with increased clinical indices of inflammation. Conversely, successful periodontal therapy leads to decreased C3 activation.
• A cause-and-effect relationship between complement and periodontitis was supported by interventional and mechanistic studies in preclinical models. Specifically, genetic and pharmacological studies in rodents have shown that complement is involved in both the dysbiotic transformation of the periodontal microbiota and in the inflammatory process that leads to the destruction of alveolar bone. The critical involvement of central complement component C3 in periodontal pathogenesis was confirmed in non-human primates, where local administration of a C3 inhibitor drug blocked naturally occurring periodontitis.

• The concomitant activation of C5aR1 and TLR2 in the gingival tissue of mice after local micro-injection of specific ligands (C5a anaphylatoxin and Pam3Cys lipopeptide, respectively) causes the production of significantly higher levels of pro-inflammatory and pro-osteoclastogenic cytokines (IL-1β, IL-6, IL-17, and TNF) in the gingiva than stimulation of either receptor alone.
• The signaling pathways activated downstream of complement receptors (C3aR or C5aR1) and TLRs (TLR2 or TLR4) converge at MAPKs (ERK1/2 and JNK), which in turn enhance the activation of AP-1 and NF-κB, key transcription factors involved in the induction of inflammatory cytokines.

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SECTION 7: CELLS OF THE INNATE IMMUNE RESPONSE

7.1 Neutrophils (PMNs)

• Neutrophils are key components of the innate immune system and play a fundamental role in maintaining periodontal health, despite the constant challenge presented by the plaque biofilm.
• Transmigrating neutrophils are evident in clinically healthy gingival tissues moving toward the sulcus for the purpose of eliminating bacteria.
• The migration of leukocytes into the crevice occurs through the junctional epithelium, which, under inflammatory conditions, is largely occupied (by about 60%) by trafficking neutrophils.
• Neutrophils are protective leukocytes that phagocytose and kill bacteria; deficiencies in neutrophil functioning result in increased susceptibility to infections in general, as well as periodontal disease.
• Dual role of neutrophils (Pathway #1):
  • Beneficial: Phagocytosis, ROS production, degranulation killing bacteria
  • Harmful: Release of large quantities of destructive enzymes (MMPs - particularly MMP-8 and MMP-9) as they migrate through the tissues → breakdown of structural components → collagen-depleted areas
  • Also release lysosomal enzymes, cytokines, and ROS extracellularly → further collagen depletion and tissue damage
• Patients with periodontitis have neutrophils that demonstrate enhanced enzymatic activity and produce increased levels of ROS.
• Neutrophils can also activate the adaptive immune defense via secretion of CCL2 and CCL20 and in a limited sense increase osteoclastogenesis with membrane-bound RANKL.
• Neutrophil phenotypes:

• Salivary PMNs (orogranulocytes): PMNs in saliva have interacted with multiple antigens from the microbiome (unlike naive blood PMNs). The orogranulocytic migratory rate positively correlates with the severity of gingival inflammation and is a reliable index for assessment of gingivitis.

7.2 Macrophages

• Participate in host response to microorganisms by phagocytosing, microbial killing, and antigen presentation via MHC-II.
• These cells express functional PRRs and produce inflammatory mediators that stimulate other immune cells.
• Similarly to osteocytes, may support osteoclastic resorption of dental root and alveolar bone.
• In diabetes: Increased M1 macrophages along with additive secretion of TNFα are observed, contributing to immunological dysregulation resulting in elevated inflammation, heightened susceptibility to infection, delayed wound healing, and increased osteoclastogenesis/bone loss/attachment loss.

7.3 Dendritic Cells (DCs) and Langerhans Cells

• Dendritic cells and Langerhans cells internalize bacterial cells, breaking down the component proteins into discrete antigens for presentation to T cells and stimulation of an adaptive immune response.
• Clinically healthy human gingivae contain a network of APCs (dendritic cells and macrophages) that orchestrate local immunity.
• DC-SIGN (C-type lectin on dendritic cells) serves as a receptor for P. gingivalis streptococcal surface proteins SspA/B, mediating adhesion to and invasion of the host.

7.4 Osteoblasts and Osteocytes in Innate Immunity

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SECTION 8: HISTOPATHOLOGICAL STAGES OF THE GINGIVAL LESION

Page and Schroeder Classification (1976) - EXAMINER CLASSIC

• An inverse relationship appears to exist between the number of intact collagen bundles and the number of inflammatory cells.
• Established lesions of two types appear to exist: some remain stable and do not progress for months or years; others convert to progressively destructive lesions.
• Established lesions appear to be reversible; the sequence of events during successful periodontal therapy is essentially the reverse of the sequence observed as gingivitis develops.

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SECTION 9: INFLAMMATORY MEDIATORS AND BONE DESTRUCTION

9.1 Key Cytokines in Periodontal Inflammation

9.2 RANKL/OPG Axis - Bone Destruction Mechanism

Bacteria/MAMPs
     ↓
Innate Immune Cells (macrophages, DCs, γδ T cells) activated
     ↓
Release: TNF-α, IL-6, IL-17
     ↓
Adaptive Immune Cells activated (Th1, Th17, B cells)
     ↓
↑ RANKL expression on:
   - Osteoblasts & osteocytes (Pathway #1: direct MMP secretion + RANKL)
   - Th17 cells (direct RANKL expression)
   - Fibroblasts
     ↓
RANKL binds RANK on osteoclast precursors
     ↓
↑ RANKL/OPG ratio
     ↓
Osteoclast differentiation, activation, and survival
     ↓
ALVEOLAR BONE RESORPTION → Attachment loss → Periodontal pocket deepening

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SECTION 10: ADAPTIVE IMMUNE RESPONSE

10.1 Overview

• The adaptive immune response involves antigen-specific B cells, plasma cells, and T cell subsets (Th1, Th2, Th17, Treg, CD8+).
• Clinically healthy human gingivae contain a predominantly T-cell-rich inflammatory infiltrate and a network of APCs. In contrast, minimal numbers of B cells and plasma cells are present in a healthy periodontium.
• Adaptive immunity becomes involved when innate immune responses fail to eliminate infection; innate and adaptive immunity then work together toward a common purpose.

10.2 T Cell Subsets and Their Role

• Induced Tregs and Th17 cells share a reciprocal developmental pathway.
• Inflammatory cytokines such as IL-6, which inhibit the expression of FoxP3, tip the balance away from Treg generation and favor Th17 differentiation.
• In periodontitis: IL-17 neutralization (antibody-mediated) in different murine periodontitis models was shown to block periodontal inflammation and bone loss.

10.3 B Lymphocytes and Plasma Cells

• Plasma cells dominate in the established and advanced lesion.
• The predominance of plasma cells is thought to be a primary characteristic of established lesions.
• An increase in the proportion of plasma cells was evident with longstanding gingivitis; the time for development of classic "established lesions" may exceed 6 months.
• The junctional epithelium reveals widened intercellular spaces filled with granular cellular debris including lysosomes derived from disrupted neutrophils, lymphocytes, and monocytes. The lysosomes contain acid hydrolases that can destroy tissue components.

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SECTION 11: DYNAMICS OF HOST RESPONSE IN PERIODONTAL DISEASE

DYSBIOTIC BACTERIA & BIOFILM SECRETORY PRODUCTS
                    ↓
        PENETRATE ORAL EPITHELIUM
                    ↓
        ┌──────────────────────────────┐
        │                              │
  PATHWAY #1                    PATHWAY #2
  Neutrophil activation         Macrophage, DC, γδ T cell activation
        │                              │
  Direct neutrophil effects     Release: TNF, IL-6, IL-17
  • MMP secretion                      │
  • Membrane-bound RANKL               │
        │                              │
        └────────────┬─────────────────┘
                     │
        ACTIVATION OF ADAPTIVE IMMUNE DEFENSE
                     │
           B Cells, Th1 Cells, Th17 Cells
                     │
        RANKL upregulation (multiple sources)
                     │
        OSTEOCLAST PROGENITOR DIFFERENTIATION
                     │
        BONE RESORPTION + ATTACHMENT LOSS

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SECTION 12: MICROBIAL EVASION STRATEGIES

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SECTION 13: SALIVARY COMPONENTS IN HOST DEFENSE

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SECTION 14: SPECIFIC PERIODONTAL ENVIRONMENTS

14.1 The Gingival Crevice Microenvironment

• In the gingival crevice, the recruited inflammatory cells, the flowing GCF, and the immediate surroundings (oral sulcular epithelium, junctional epithelium, subgingival tooth surface with the attached biofilm) form a complex network of immune players.
• The innate immune players include:
  • Phagocytes (neutrophils, macrophages)
  • Epithelial cells
  • Soluble inflammatory mediators: cytokines and complement activation products
• As adaptive immunity becomes involved, other factors entering the picture include antibodies and lymphocytes (T cells, B cells, and plasma cells), which, however, are largely confined to the underlying connective tissue.
• A developing immune and inflammatory response involving elements of both innate and adaptive immunity may not necessarily result in clinically evident inflammation. A controlled and effective host response can maintain periodontal tissue homeostasis.

14.2 Host-Microbe Homeostasis vs. Dysbiosis

PERIODONTAL HEALTH (Symbiosis)
     ↕
Proactive homeostatic immunity to local microbiota
     ↕
Controlled immune surveillance by T cells, APCs, phagocytes
     ↕
Low GCF flow → Limited nutrient supply to subgingival biofilm
     ↕
Health-associated microbiota (predominantly gram-positive, facultative)
     ↕
PERTURBATION (keystone pathogens / host susceptibility / environmental factors)
     ↕
DYSBIOSIS
     ↕
Disrupted homeostatic balance → dysbiotic community → destructive inflammation
     ↕
Increased GCF flow → Rich nutrient supply → Gram-negative anaerobes flourish
     ↕
PERIODONTITIS (irreversible tissue destruction without therapy)

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SECTION 15: BIOMARKERS OF HOST-MICROBE INTERACTION

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SECTION 16: COMPARATIVE TABLE - VIEWPOINTS FROM DIFFERENT REFERENCES

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SECTION 17: RECENT TERMINOLOGY CHANGES

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SECTION 18: SUMMARY INTEGRATION FLOWCHART

ORAL MICROBIOME IN HEALTH
(Predominantly gram-positive, health-associated microbiota)
                    ↓
INITIATING FACTORS:
• Plaque biofilm accumulation (supragingival → subgingival)
• Keystone pathogen introduction (P. gingivalis)
• Host susceptibility factors (genetics, diabetes, smoking, aging)
                    ↓
MICROBIAL DYSBIOSIS:
• Ecological shift: gram-positive → gram-negative anaerobes
• Interspecies polymicrobial synergy
• Virulence factor production: fimbriae, gingipains, LPS, leukotoxin
• IMMUNE EVASION: complement inactivation, TLR manipulation, anti-phagocytosis
                    ↓
INNATE IMMUNE ACTIVATION:
• PRRs (TLR2, TLR4, TLR9, NOD1/2, NLRP3) on epithelial cells, phagocytes, osteoblasts
• NF-κB and MAPK activation → pro-inflammatory cytokines (IL-1β, TNF-α, IL-6, IL-8)
• NLRP3 inflammasome → mature IL-1β → pyroptosis
• Complement activation (all 3 pathways) → C3a, C5a, MAC
• Complement-TLR cross-talk (C5aR1 + TLR2 → amplified inflammation)
• Neutrophil recruitment (via IL-8/CXCL8 from epithelial cells)
    → Phagocytosis + ROS + MMP-8/-9 + NET formation
    → Dual role: bactericidal AND tissue destructive
                    ↓
ADAPTIVE IMMUNE ACTIVATION (via APCs/DCs):
• Th1 cells → IFN-γ → macrophage activation
• Th17 cells → IL-17 → RANKL↑ on osteoblasts → MMP↑ in fibroblasts → OSTEOCLASTOGENESIS
• Treg cells → IL-10, TGF-β → immunoregulation (deficient in periodontitis)
• B cells → Plasma cells → IgG, IgM → complement fixation, opsonization
• IL-6 → Th17↑ / Treg↓ (reciprocal imbalance in disease)
                    ↓
TISSUE DESTRUCTION:
• MMP-mediated collagen degradation (MMP-8, MMP-9, MMP-13)
• Collagen depletion in connective tissue
• Junctional epithelium proliferation → apical migration → pocket formation
• ↑ RANKL / ↓ OPG → Osteoclast differentiation and activation → ALVEOLAR BONE RESORPTION
• Periodontal ligament destruction → Attachment loss → Tooth mobility → Tooth loss
                    ↓
PERPETUATING CYCLE:
• Tissue breakdown products (haem, peptides) = nutrients for inflammophilic bacteria
• More dysbiosis → More inflammation → More destruction
• Chronicity of periodontitis maintained by self-sustaining vicious circle

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SECTION 19: HIGH-YIELD EXAMINATION KEYWORDS (BOLD = MUST KNOW)

1. Polymicrobial synergy and dysbiosis model (Hajishengallis & Lamont, 2012)
2. Keystone pathogen - P. gingivalis as prototype; disproportionate influence at low abundance
3. Pathobiont - commensal that becomes pathogenic under altered conditions
4. Inflammophilic bacteria - bacteria enriched by inflammatory products
5. MAMP vs. PAMP vs. DAMP - recognize the difference
6. TLR4-LPS / TLR2-LTA / TLR9-CpG DNA - critical receptor-ligand pairs
7. MyD88 adaptor / TRIF adaptor - signal transduction; TLR3 uses TRIF not MyD88
8. NLRP3 inflammasome - assembly, activation, IL-1β maturation, pyroptosis (GSDMD)
9. Complement C3 - central convergence point; C3a/C5a anaphylatoxins; MAC; C3 inhibitor blocking periodontitis
10. Complement-TLR cross-talk - C5aR1 + TLR2 → amplified pro-osteoclastogenic cytokines
11. hBD-1, hBD-2, LL-37 - antimicrobial peptides (defensins and cathelicidins)
12. Page & Schroeder 4 stages (Initial, Early, Established, Advanced) - dominant cells per stage
13. N1/N2 neutrophil phenotypes - pro-inflammatory vs. tolerogenic
14. M1/M2 macrophage polarization - pro-inflammatory vs. alternative activation
15. Th17/Treg balance - IL-6 tips balance; FoxP3 inhibition; IL-17 → RANKL → bone resorption
16. RANKL/OPG ratio - key determinant of alveolar bone resorption
17. Orogranulocytic migratory rate - correlates with severity of gingival inflammation
18. Dysbiosis - disruption of host-microbe homeostasis → periodontitis
19. Ecological plaque hypothesis (Marsh) - environmental changes shift gram-positive → gram-negative microbiota
20. Gingipains (P. gingivalis) - Arg-X and Lys-X proteases; degrade complement, immunoglobulins, cytokines
21. Leukotoxin (A. actinomycetemcomitans) - selectively kills LFA-1-bearing leukocytes
22. Vicious circle of dysbiosis ↔ destructive inflammation ↔ more dysbiosis

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SOURCES CITED

1. Newman and Carranza's Clinical Periodontology and Implantology, 14th Edition - Chapters 10, 11, 15, 27 (Kirkwood, Rossa Jr., Hajishengallis, Decker, Hernandez-Kapila)
2. Lamont RJ, Hajishengallis GN, Koo H, Jenkinson HF - Oral Microbiology and Immunology, 3rd Edition (ASM Press, 2019) - Chapters 14, 15
3. Carranza's Clinical Periodontology, 10th Edition (Newman, Takei, Klokkevold, Carranza - Saunders/Elsevier, 2006)
4. Clinical Periodontology and Implant Dentistry, 6th Edition (Lang & Lindhe - Wiley-Blackwell, 2015)
5. Periodontics: Medicine, Surgery, and Implants (Rose, Mealey, Genco, Cohen - Elsevier Mosby, 2004)
6. Essential Microbiology for Dentistry, 5th Edition (Samaranayake - Elsevier, 2018) - Chapters 17, 33
7. Essentials of Clinical Periodontology and Periodontics, 5th Edition (Reddy - Jaypee, 2018)
8. Biomarkers in Periodontal Health and Disease (Buduneli - Springer Nature, 2020) - Chapter 2
9. Janeway's Immunobiology, 9th Edition (2017) - (referenced for general immunology principles)

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1. Introduction & Conceptual Overview - Fundamental host-microbe interaction concept; periodontitis as a dysbiotic disease
2. Models / Etiologic Hypotheses - Evolution from non-specific → specific → ecological plaque → polymicrobial synergy/dysbiosis model (flowchart)
3. Microbial Component - Socransky criteria, microbial complexes table, pathobiont classification table
4. Keystone Pathogen Hypothesis - Definition, mechanism, vicious cycle flowchart
5. Dysbiosis and Ecological Plaque Hypothesis - Factors disrupting homeostasis table
6. Innate Immunity - Epithelial barrier, PRRs, MAMPs/PAMPs/DAMPs, TLR cascade flowchart, NLRs/NLRP3 inflammasome, complement (all 3 pathways), complement-TLR cross-talk
7. Innate Immune Cells - Neutrophil dual role (N1/N2), macrophages (M1/M2), DCs, osteoblasts/osteocytes/osteoclasts
8. Page & Schroeder Stages - Full 4-stage table with dominant cells
9. Inflammatory Mediators and Bone Destruction - Full cytokine table; RANKL/OPG axis flowchart
10. Adaptive Immunity - T cell subsets table; Th17/Treg reciprocal balance
11. Dynamics Flowchart - Two innate immune pathways (Fig. 11.4 based)
12. Microbial Evasion Strategies - Complete table
13. Salivary Host Defense - Full table of salivary components
14. Gingival Crevice Microenvironment - Health vs. dysbiosis flowchart
15. Biomarkers - GCF/salivary/serum biomarker table
16. Comparative Table - Viewpoints across all 5 major references
17. Recent Terminology Changes - Complete table
18. Summary Integration Flowchart - Complete pathogenesis from health → bone loss
19. High-Yield Examination Keywords - 22 must-know terms