4.2 Microbiology, Immunology, and Infection Pathogenesis

1.4 Microbiology, Immunology, and Infection Pathogenesis

Oral Microbiome Composition & Pathogenic Complexes

The oral cavity harbors a complex polymicrobial biofilm. Specific bacterial complexes are etiologically linked to dental caries and periodontal disease.

Cariogenic Bacteria

• Streptococcus mutans: The primary initiator of dental caries. Essential virulence factors:
  1. Acidogenicity: Ferments dietary carbohydrates (sucrose) into lactic acid.
  2. Aciduricity: Survives and functions in low pH environments.
  3. Extracellular Polysaccharide Synthesis: Produces the enzyme Glucosyltransferase (GTF), which cleaves sucrose into glucose and fructose, polymerizing glucose into insoluble sticky glucans (dextrans) that mediate firm adherence to the enamel pellicle.
• Lactobacillus species: Secondary invaders that proliferate in acidic environments, driving progression of deep dentinal caries.
• Actinomyces viscosus / naeslundii: Filamentous organisms strongly associated with root surface caries.

Periodontopathogenic Bacterial Complexes (Socransky Complexes)

ORAL BIOFILM SUCCESSION:
Early Colonizers (Gram-Positive Streptococci/Actinomyces) 
  ↓
Orange Complex (Fusobacterium nucleatum - Key Bridging Organism)
  ↓
Red Complex (Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola)

1. Red Complex: Strongly associated with severe, advanced periodontitis, deep pocketing, and active bone loss:
   • Porphyromonas gingivalis: Obligate anaerobic Gram-negative rod. Expresses gingipains (cysteine proteinases that degrade collagen and immunoglobulins), fimbriae, and endotoxin LPS.
   • Tannerella forsythia: Anaerobic Gram-negative rod.
   • Treponema denticola: Anaerobic Gram-negative spirochete producing dentilisin.
2. Orange Complex: Species such as Fusobacterium nucleatum act as essential bridging organisms, possessing surface adhesins that co-aggregate early Gram-positive colonizers with late Gram-negative anaerobes.
3. Aggregatibacter actinomycetemcomitans (Aa): Gram-negative facultative capnophilic rod that produces Leukotoxin (kills PMNs and monocytes). Etiologic agent of Localized Aggressive Periodontitis (Stage III/IV Grade C periodontitis).

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Bacterial Cell Wall Architecture & Endotoxin Action

Endotoxin (LPS) Structure & Mechanism of Septic Shock

Lipopolysaccharide (LPS) consists of three covalently linked regions:

1. O-Antigen Polysaccharide: Outer region; variable carbohydrate chains used for serotyping.
2. Core Polysaccharide: Connects O-antigen to Lipid A.
3. Lipid A (Toxic Component): Embedded in outer membrane.

Mechanism of Endotoxicity: Upon cell lysis, Lipid A binds to CD14 / Toll-Like Receptor 4 (TLR4) on macrophages, triggering massive systemic release of pro-inflammatory cytokines (TNF-alpha, IL-1, IL-6). This leads to septic shock, systemic fever, profound hypotension, and Disseminated Intravascular Coagulation (DIC).

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Viral Pathogens: Herpesviruses, Hepatitis & HIV

Human Herpesviruses (Enveloped Double-Stranded DNA Viruses)

• HSV-1: Causes Primary Herpetic Gingivostomatitis (in children: painful, diffuse oral vesicles and ulcers on both keratinized and non-keratinized mucosa, high fever, cervical lymphadenopathy) and Recurrent Herpes Labialis (cold sores: latent in Trigeminal Ganglion, recurring at vermilion border as clustered vesicles on keratinized mucosa).
• Varicella-Zoster Virus (VZV / HHV-3): Primary infection causes Varicella (chickenpox). Reactivation causes Herpes Zoster (Shingles), manifesting as painful crops of vesicles strictly limited to a unilateral dermatome along CN V1, V2, or V3.
• Epstein-Barr Virus (EBV / HHV-4): Latent in B lymphocytes. Causes Infectious Mononucleosis, Burkitt Lymphoma, and Oral Hairy Leukoplakia (OHL) (white, corrugated, non-wipeable hyperkeratotic plaque on the lateral borders of the tongue in immunocompromised/HIV patients).
• Kaposi Sarcoma-Associated Herpesvirus (HHV-8): Causes Kaposi Sarcoma, a vascular tumor presenting as red, purple, or bluish macules/nodules on the hard palate or gingiva in AIDS patients.

Hepatitis Viruses & Dental Implications

• Hepatitis B Virus (HBV): Enveloped dsDNA virus transmitted parenterally, sexually, or via blood contact. HBsAg indicates active or chronic infection; Anti-HBs indicates immunity (vaccination or recovery). Healthcare providers are required to receive the HBV vaccine series.
• Hepatitis C Virus (HCV): Enveloped ssRNA virus transmitted via blood. Possesses the highest rate of progression to chronic hepatitis (80%), cirrhosis, and hepatocellular carcinoma.

Human Immunodeficiency Virus (HIV / AIDS)

• Retrovirus carrying Reverse Transcriptase that infects CD4+ T helper cells.
• AIDS Diagnosis: CD4+ T cell count < 200 cells/µL or presence of an AIDS-defining opportunistic condition.
• Oral Markers of HIV: Pseudomembranous Candidiasis, Oral Hairy Leukoplakia, Kaposi Sarcoma, Linear Gingival Erythema (LGE), and Necrotizing Ulcerative Periodontitis (NUP).

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Fungal Infections: Candida Albicans Variants & Antifungal Therapy

Candida albicans is a dimorphic commensal fungal organism (yeast form at 37°C, invading tissue via hyphae and pseudohyphae).

Clinical Variants of Oral Candidiasis

1. Pseudomembranous Candidiasis (Thrush): Creamy white, curd-like plaques that can be wiped off with gauze, leaving an underlying erythematous, raw, bleeding mucosal surface.
2. Erythematous Candidiasis: Red, depapillated mucosal macules; common on hard palate under dentures (Denture Stomatitis) or following broad-spectrum antibiotic therapy.
3. Chronic Hyperplastic Candidiasis: White, firm plaque typically at buccal commissures that CANNOT be wiped off.
4. Angular Cheilitis: Erythema, cracking, and fissuring at the labial commissures; frequently a co-infection of Candida albicans and Staphylococcus aureus.

• Pharmacotherapy: Topical Nystatin suspension or Clotrimazole troches for localized oral infection; oral Fluconazole for systemic or refractory cases.

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Host Immune Response & Hypersensitivity Reactions

Immunoglobulin Classes & Functions

• IgG: Most abundant serum antibody (~80%). Only antibody that crosses the placenta. Mediates secondary immune response, opsonization, and complement activation.
• IgA: Dimeric structure in secretions (saliva, tears, milk). Contains a secretory component that resists proteolytic enzymatic cleavage. Primary mucosal defense antibody.
• IgM: Pentameric structure with 10 antigen-binding sites. First antibody produced during primary immune response. Most efficient activator of the classical complement pathway.
• IgE: Binds Fc-epsilon receptors on mast cells and basophils. Mediates Type I hypersensitivity reactions and immunity against parasitic helminths.

Classification of Hypersensitivity Reactions

Memory Aid: "ACID" → Anaphylactic (Type I), Cytotoxic (Type II), Immune Complex (Type III), Delayed (Type IV).

Type I   --> IgE-Mediated (Anaphylaxis, Urticaria, Penicillin Allergy)
Type II  --> Cytotoxic IgG/IgM (Pemphigus Vulgaris, Mucous Membrane Pemphigoid)
Type III --> Immune Complex Deposition (Systemic Lupus Erythematosus)
Type IV  --> T-Cell Mediated / Delayed (Contact Dermatitis, Tuberculin Skin Test)

1. Type I (Anaphylactic / Immediate): IgE-mediated. Antigen cross-links IgE on mast cells, causing degranulation of histamine, leukotrienes, and prostaglandins. Examples: Anaphylaxis, allergic asthma, penicillin allergy.
2. Type II (Cytotoxic / Antibody-Mediated): IgG or IgM antibodies target specific cell-surface or tissue antigens, triggering complement lysis or phagocytosis.
   • Pemphigus Vulgaris: Autoantibodies against Desmoglein 3 (desmosomes). Causes intraepithelial suprabasal acantholysis and a positive Nikolsky sign.
   • Mucous Membrane Pemphigoid: Autoantibodies against BP180 / Hemidesmosomes. Causes subepithelial basement membrane split.
3. Type III (Immune Complex-Mediated): Circulating antigen-antibody complexes deposit in blood vessel walls, activating complement and recruiting neutrophils. Examples: Systemic Lupus Erythematosus (SLE), Serum Sickness.
4. Type IV (Delayed / Cell-Mediated): Mediated by sensitized T-lymphocytes (CD4+ or CD8+); NO antibodies involved. Peak reaction occurs 48 to 72 hours post-exposure. Examples: Contact dermatitis (latex, poison ivy, nickel), Mantoux tuberculin skin test.