Microbial Mechanisms of Pathogenicity Flashcards

Fundamental Concepts of Microbial Pathogenicity

• Pathogenicity is defined as the ability of a microorganism to cause disease in a host. This ability is dictated by various virulence factors.

• Virulence factors are specific traits or mechanisms that determine the extent and severity of pathogenicity.

• An example of a virulence factor is a capsule, which is a slippery outer layer that prevents white blood cells (leukocytes) from attaching to, consuming, and destroying the pathogen through phagocytosis.

• The lifecycle of a pathogen within a host follows a specific progression: entry into the host, avoidance or activation of host defenses, replication, damaging the host, and exiting to find new hosts.

• Disease represents an altered status of the host resulting from the damage caused by the infection.

Portals of Entry into the Host

• Pathogens must enter a host through a specific portal of entry to initiate infection. Common portals include the respiratory tract, gastrointestinal tract, genitourinary tract, skin, and the placenta.

• Respiratory Tract: This is the easiest and most common portal of entry because humans must breathe continuously to survive.

  • Organism size is a critical factor; smaller organisms can penetrate deeper into the lungs, making them harder for the body to expel.

  • Examples of pathogens entering through this route include viruses causing the common cold, influenza, and COVID-19, as well as bacteria causing pneumonia and tuberculosis.

• Skin: Intact skin serves as the primary barrier against infection. Most pathogens can only breach this barrier if there is damage, such as a cut or abrasion.

  • Some organisms, like the Schistosoma fluke or certain parasitic worms, produce enzymes to burrow directly through intact skin.

  • Vector-mediated entry occurs when arthropods like mosquitoes (transmitting Plasmodium which causes malaria) or ticks (transmitting various bacteria) bite through the skin into the bloodstream.

  • Some organisms colonize specialized skin structures like hair follicles or sweat glands. Certain fungi and bacteria are adapted to the harsh environment on the skin's surface.

• Parenteral Route: This occurs when organisms are deposited directly into deep tissues via injury, such as knife wounds, bullet wounds, or accidents involving contaminated metal.

• Gastrointestinal (GI) Tract: Pathogens enter through the ingestion of contaminated food or water or via contaminated fingers (the fecal-oral route).

  • The low pH (acidity) of the stomach serves as a defensive buffer; pathogens must survive this environment to reach the rest of the GI tract.

  • Pathogens using this route include the bacteria causing cholera, protozoans like Giardia and those causing amoebic dysentery, and viruses such as Poliovirus and Hepatitis A.

• Genitourinary Tract: This is the entry point for sexually transmitted infections (STIs). Organisms enter through the mucosal membranes of the penis, vagina, cervix, or urethra.

  • STIs account for approximately $4\%$ of all infections globally.

  • Common bacterial STIs include syphilis ($Treponema\,pallidum$), gonorrhea ($Neisseria\,gonorrhoeae$), and chlamydia. Viral examples include Herpes, Hepatitis B, and Human Papillomavirus (HPV). Trichomonas vaginalis is a protozoan pathogen in this category.

• Placenta: While the placenta is designed to protect the fetus, certain pathogens can cross it, often leading to congenital abnormalities, stillbirth, or spontaneous abortion.

  • The acronym STORCH identifies these pathogens: Syphilis ($Treponema\,pallidum$), Toxoplasmosis ($Toxoplasma\,gondii$, often transmitted via cat feces), Other (HIV, Chlamydia), Rubella (measles), Cytomegalovirus, and Herpes simplex virus.

Quantifying Virulence: $ID_{50}$ and $LD_{50}$

• $ID_{50}$ (Infectious Dose 50): The number of pathogens required to cause infection in $50\%$ of a test population.

  • A lower $ID_{50}$ indicates higher virulence, as fewer organisms are needed to cause disease. For example, $Coxiella$ has a very low $ID_{50}$ of $1$ cell (causing Q fever).

  • $Giardia$ requires approximately $10$ cysts, while $Vibrio\,cholerae$ requires about $10^9$ (one billion) bacteria. Notably, one gram of feces can contain a billion bacteria, making even high infectious doses easy to encounter.

• $ID_{50}$ varies by the portal of entry for the same organism. For $Bacillus\,anthracis$ (anthrax):

  • Cutaneous (skin): $10$ to $50$ endospores.

  • Inhalation (respiratory): $10,000$ to $20,000$ endospores.

  • Ingestion (GI): Up to $1,000,000$ endospores.

• $LD_{50}$ (Lethal Dose 50): The amount of a toxin or pathogen required to kill $50\%$ of a test population.

Mechanisms of Adhesion

• Adhesion is the process by which microorganisms attach themselves to host tissues after entry, preventing them from being flushed out of the body.

• Fimbriae: $Escherichia\,coli$ use these to embed in the cilia of the digestive tract.

• Axial Filaments: $Treponema$ uses these as anchors to attach to the epithelial walls of the urethra.

• Extracellular Matrix/Capsules: Pathogens in the mouth use these as a biological glue to attach to teeth, gums, and the tongue.

• Scolices and Barbs: Tapeworms use a scolex, composed of hooks and suckers, to embed in the intestinal wall. Hookworms utilize barbs for similar purposes.

Evasion of Host Defenses

• Virulence factors allow pathogens to bypass or neutralize the immune system.

• Cell Wall Components: Mycolic acid (found in $Mycobacterium\,tuberculosis$) creates a waxy, hydrophobic layer that resists digestion by host white blood cells.

• Enzymatic Defense:

  • Coagulases: Enzymes that trigger blood clot formation (fibrinogen to fibrin). The pathogen hides inside the host-derived clot to avoid the immune system.

  • Kinases: Enzymes that break down blood clots, allowing the pathogen to spread through the host.

  • Hyaluronidase and Collagenase: Enzymes that digest molecules in the skin, weakening barriers to allow penetration.

  • IgA Proteases: Enzymes that destroy IgA antibodies, which are proteins meant to defend mucosal surfaces.

• Antigenic Variation:

  • Antigenic Drift: Found in Influenza A, B, and C. Random mutations in the RNA genome change the antigens, requiring new annual vaccines.

  • Antigenic Shift: Found only in Influenza A. Two different viral variants combine to create a brand-new subtype with unique antigens.

• Intracellular Invasion:

  • Membrane Ruffling: Pathogens like $Salmonella$ use invasins to alter host actin fibers, forcing the host cell to take the pathogen inside.

  • Listeria uses host actin to move directly from the cytoplasm of one cell to another, completely avoiding the extracellular immune response.

• Leukocidins: Toxins produced by pathogens that specifically kill white blood cells.

Damage to the Host: Toxins and Metabolism

• Siderophores: Compounds produced by pathogens to scavenge iron from host cells. This deprives the host of a vital micronutrient and fuels the pathogen's metabolism.

• Toxigenicity: The ability of a microorganism to produce toxins.

• Toxemia: The presence of toxins in the host's blood.

• Antitoxins: Antibodies produced by the host to bind and inactivate specific toxins.

• Toxoids: Inactivated toxins used in vaccines to stimulate an immune response (e.g., the tetanus vaccine).

Exotoxins vs. Endotoxins

• Exotoxins:

  • Composition: Proteins.

  • Source: Mostly Gram-positive bacteria (some Gram-negative).

  • Release: Secreted by living, metabolizing cells.

  • Heat Stability: Unstable (denatured by heat).

  • Toxicity: Extremely high (Low $LD_{50}$).

  • Fever: Usually do not cause fever (except superantigens).

  • Immunology: Strong antigens; toxoids and antitoxins are available.

• Endotoxins:

  • Composition: Lipids (Lipid A portion of lipopolysaccharides).

  • Source: Exclusively Gram-negative bacteria.

  • Release: Released only when the cell dies and the cell wall lyses.

  • Heat Stability: Stable (resistant to heat).

  • Toxicity: Relatively low, though can be fatal in large amounts (High $LD_{50}$).

  • Fever: Always cause fever.

  • Immunology: Poor antigens; no toxoids or antitoxins available.

Types of Exotoxins

• AB Toxins: Consist of an Active (A) component and a Binding (B) component. The B component attaches to the host receptor, allowing the whole toxin to enter the cell, where the A component is released to perform its toxic function.

• Membrane-Disrupting Toxins: Target the phospholipid bilayer of host cells. Examples include leukocidins (kill white blood cells) and hemolysins (kill red blood cells/erythrocytes).

• Superantigens: Trigger an intense, overwhelming immune response known as a cytokine storm. This leads to fever, nausea, vomiting, diarrhea, shock, and potentially death.

Specific Bacterial Toxins

• Botulinum Toxin: An AB neurotoxin that causes flaccid paralysis by preventing muscle contraction. Can be fatal if it affects the heart or diaphragm.

• Tetanus Toxin: An AB neurotoxin that causes permanent muscle contraction (inability to relax).

• Staphylococcal Enterotoxin: A superantigen produced by $Staphylococcus\,aureus$ associated with Toxic Shock Syndrome.

• Erythrogenic Toxin: A membrane-disrupting toxin produced by $Streptococcus\,pyogenes$ that targets red blood cells.

Pathogenic Effects of Other Microbes

• Viruses:

  • Inclusion Bodies: Sites of viral protein synthesis in the host cytoplasm. Negri bodies are specific inclusion bodies used to diagnose Rabies.

  • Cytoplasmic Masses: Sites of viral biosynthesis and assembly that disrupt normal host metabolism.

• Fungi:

  • They release metabolic waste that is toxic to the host and can cause chronic allergic responses.

  • Mycotoxins: Secondary compounds produced for defense. Ergot toxin (from $Claviceps$) is a neurotoxin and a precursor to LSD.

  • Some fungi produce proteases (like $Candida$) or capsules (like $Cryptococcus$).

• Protozoans and Helminths (Worms):

  • Cause damage through nutrient and energy depletion, toxic waste production, and physical interference with organ function.

  • Some protozoans are obligate intracellular parasites that replicate inside white blood cells.

• Algae:

  • Dinoflagellates can cause algal blooms (red tides). They produce toxins that accumulate in filter-feeding shellfish. Consuming these leads to paralytic shellfish poisoning, characterized by nausea, vomiting, and potential death.

Portals of Exit

• Pathogens must exit the host to continue their life cycle; the portal of exit usually corresponds to the portal of entry.

• Respiratory: Coughing, sneezing, or even talking expels mucus droplets containing pathogens (e.g., COVID-19, Tuberculosis).

• Gastrointestinal: Pathogens are expelled through saliva or feces (fecal-oral route). COVID-19 has been detected in feces up to a month after patient recovery.

• Genitourinary: Transmission via urine, vaginal secretions, or seminal fluids.

• Skin: Spread through oils, pimples, or lesions (like the chancre in syphilis).

• Blood-borne: Requires a fomite (e.g., dirty needles) or a biological vector (e.g., ticks, mosquitoes) to move from the blood of one host to another.