13- Pathogenicity

Pathogenicity of Human Pathogens

  • Human Pathogens Overview   - 1,400 known species of human pathogens (viruses, bacteria, fungi, protozoa, helminths).   - Represents much less than 1% of all microbial species on Earth.   - Key Concept: "Bacteria live not to cause disease but to grow and divide."

Case Study

  • Florida Man Incident (August 2019)   - Incident involving a man having 25% of his skin stripped due to a fight against flesh-eating bacteria.

Learning Objectives

  • Understand and describe key concepts in microbial pathogenicity:   - Bacterial virulence factors enhancing adherence, colonization, and invasiveness.   - Comparison of exotoxins and endotoxins.   - Role of MAMPs and toll-like receptors in endotoxin toxicity.   - Mechanisms of specific bacterial toxins: diphtheria, botulinum, tetanus, anthrax, and cholera.   - Definition and significance of pathogenicity islands in virulence.

Vocabulary

  • Key Terms:   - Pathogenicity: Ability to cause disease.   - Opportunistic vs True Pathogen: Differentiation between microbes that cause disease in healthy individuals versus those that cause disease primarily in immunocompromised individuals.   - Virulence Factor: Any characteristic or component of a pathogen that facilitates its ability to cause disease.   - Adhesin: Molecules that allow bacteria to adhere to host tissues.   - Invasiveness Factors: Components enabling pathogens to invade host tissues.   - Collagenase: Enzyme breaking down collagen; aids in spreading.   - Hyaluronidase: Enzyme that degrades hyaluronic acid, aiding bacterial spread.   - Lecithinase: Enzyme that breaks down phospholipids, potentially causing host cell lysis.   - Fibrinolysin: Enzyme that dissolves fibrin clots, releasing bacteria.   - Streptokinase: Enzyme produced by certain bacteria to dissolve blood clots.   - Leukocidin: Substance that kills white blood cells, evading immune response.   - Staph A Protein: Protein from Staphylococcus aureus that contributes to its virulence.   - Coagulase: Enzyme that coagulates blood, creating barriers against the immune system.   - Siderophore: Molecule that binds iron, essential for bacterial growth.   - Endotoxin: Part of the Gram-negative bacterial cell wall; elicits immune responses.   - Exotoxin: Toxins secreted by bacteria that can alter host cell functions.   - Toll-like receptor (TLR): Receptors on host cells that recognize pathogens.   - MAMP: Microbe-Associated Molecular Pattern, recognized by the immune system.   - Superantigen: Class of antigens that cause excessive stimulation of the immune system.   - Pathogenicity Island: Distinct genetic regions containing virulence factors in pathogens.

Pathogenicity Defined

  • Pathogenicity Explanation:   - The ability of an organism to cause disease requires:     - Survival against host defenses.     - Inducing damage to the host.

Factors Affecting Disease

  • Key Factors:   - Portal of Entry: The site where pathogens enter the host, critical for infection.   - Infectious Dose (ID):     - Anthrax Endospores:       - Skin: 10-50 spores       - Inhalation: 10,000-20,000 spores       - Ingestion: 250,000-1,000,000 spores

Microbial Adherence and Invasiveness

  • Importance of Adherence:   - Often a critical feature; pathogens must "stick" to host tissues.   - Adhesins: Attachment molecules facilitating adherence.

  • Biofilms: Often involved in infections, aiding microbial colonization and persistence.

  • Intracellular Life: Some microbes establish themselves as intracellular parasites.

Mechanisms of Invasiveness

  • Invasiveness Factors:   - Microbes can move throughout the body from their site of infection by employing various methods:     - Circulation within host cells.     - Weakening cellular junctions for passage.     - Induced phagocytosis by either professional phagocytes or non-phagocytic cells.     - Example: Salmonella entering gut cells.

Enzymatic Spread Factors

  • Spreading Factors: Enzymes produced by pathogens that help in the invasion of host tissues:   - Collagenase, Hyaluronidase, Fibrinolysin, Streptokinase, Lecithinase, among others.

  • Consequences: Enable microbial movement through tissues and enhancement of pathogenicity.

Antiphagocytic Strategies

  • Host Defense Evasion:   - Antiphagocytic Factors: Mechanisms to resist phagocytosis:     - Capsules and cell wall proteins (e.g., Staph A protein, Mycolic acid).     - Leukocidin production.     - Coagulase, which forms clots around pathogens for protection.     - Survival within white blood cells as intracellular parasites.

Immune Evasion Tactics

  • Antigenic Variation: Ability of pathogens to frequently change their surface proteins, evading immune detection.

  • Mimicking Host Structures: Some microbes camouflage themselves using similar polysaccharides to those on host cells.

  • Example: Capsules of E. coli and N. meningitidis resembling mammalian surface polysaccharides.

Pathogen-Induced Damage to Hosts

  • Mechanisms of Damage:   - Nutrient Theft: Pathogens steal essential nutrients from hosts.   - Direct Damage: Caused by pathogens or their metabolic wastes.   - Intracellular Growth: Can lead to the rupture of host cells.   - Toxin Production: Pathogens can produce toxins that damage host tissues.   - Modulating Immune Responses: Inducing harmful reactions that can potentially lead to host cell death.

Nutritional Immunity Mechanism

  • Competition for Nutrients: Iron is vital for both pathogens and hosts.   - Hosts sequester iron, causing "nutritional immunity".   - Siderophores: Molecules produced by pathogens to capture host iron, often regulated during infection.

Prevention Strategies Against Iron-Dependent Pathogens

  • Research Interventions: Scientists have attempted to prevent the absorption of iron by pathogens like anthrax by neutralizing their siderophores, presenting a potential treatment method.

Ferrosome Characteristics

  • Ferrosome: A lipid-like membrane-bound structure that stores iron in some non-pathogenic microorganisms.   - Reported in Clostridium difficile (November 2023).   - Required for pathogenicity to counteract host iron sequestration.

Overview of Toxins

  • Exotoxins vs. Endotoxins:   - Exotoxin Characteristics:     - Source: Mostly from Gram-positive bacteria.     - Composition: Proteins (often with A-B structures).     - Effects: Specific actions targeting particular host cell structures, mainly affecting cellular functions.     - Stability: Unstable, often destroyed at 60-80°C (exception: staphylococcal enterotoxin).     - Immunogenic: Yes, can be neutralized by antitoxin.   - Endotoxin Characteristics:     - Source: Found in Gram-negative bacteria; part of the Lipopolysaccharide (LPS) component of their outer membrane.     - Effects: General responses like fever and shock.     - Stability: Stable, can withstand autoclaving.     - Immunogenic: Mostly not neutralized by antitoxin.

Toll-like Receptors (TLRs) and MAMPs

  • Mechanism of Action:   - TLRs on white blood cells recognize lipid A, a component of endotoxins, which triggers signaling pathways leading to cytokine production.

Toxin Action Comparison

  • Toxins and Their Modes of Action:   - Diphtheria toxin, Botulism toxin, Tetanus toxin, Cholera toxin, Gas gangrene toxin, etc.

Contemporary Public Health Issues

  • Diphtheria Resurgence:   - Outbreaks in conflict zones (e.g., Somalia, Sudan, Yemen), highlighted low vaccination rates and monitoring.   - U.S. vaccination rates have declined, with only 92% of children fully vaccinated compared to 95% in preceding years.

Botulism and Tetanus Details

  • Toxin Potency:   - Botulinum toxin remarkably potent; only 400 grams would be lethal to the entire global population.

Mechanisms and Examples of Pathogenic Effects

  • Pathogen Interaction with Host Tissues:   - Coagulase induces blood clotting around pathogens, facilitating invasion.   - Pathogens utilize molecules such as hyaluronidase and streptokinase to break down tissue barriers and spread further.

Cholera Impact Analysis

  • Cholera Toxin Mechanism:   - The cholera toxin leads to massive water loss and dehydration due to disrupted ion movement in intestinal epithelial cells.

Anthrax Pathogen Dynamics

  • Anthrax Mechanism:   - Kills phagocytic macrophages and releases lethal factor (LF) and edema factor (EF) upon endocytosis.

Superantigen Toxicity

  • Superantigens:   - Induce strong immune responses, causing symptoms like toxic shock and food poisoning.

Staphylococcus Virulence Factors

  • Virulence Factors:   - Includes capsule formation, cell wall components like protein A, and various toxins that damage host tissues.

Pathogenicity Islands

  • In Genome:   - Virulence factors are often clustered in regions called pathogenicity islands and are often regulated by two-component systems responding to host signals.   - Disruption of these systems decreases virulence.

Evolutionary Mechanisms of Pathogenesis

  • Evolution of Group A Streptococcus:   - Genomic sequencing reveals evolutionary progress leading to contemporary epidemic clones associated with diseases like pharyngitis and necrotizing fasciitis through lysogenic phages and horizontal gene transfer.   - One significant event involved acquiring virulence-related genes, enhancing pathogenicity.

Microbiome and Disease Interrelationships

  • Microbiome Effects:   - Observed connections between gastrointestinal microbial compounds and anxious behaviors in mouse models, potentially linking to conditions such as dementia, Parkinson’s, obesity, and inflammatory bowel diseases (IBD/Crohn’s).