Microbiology Chapter 15: Mechanisms of Pathogenicity

Study Notes

Pathogenicity: The ability of a pathogen to cause disease by overcoming the host's defenses.
Virulence: The degree of pathogenicity.

Portals of Entry: The routes by which pathogens enter the host, including:
- Mucous membranes (e.g., conjunctiva, respiratory tract, GI tract, genitourinary tract)
- Skin (e.g., through hair follicles and sweat ducts)
- Parenteral route (e.g., trauma, arthropods, injections)
Portals of Exit: The routes by which pathogens exit the host, including:
- Respiratory tract (e.g., coughing, sneezing)
- Gastrointestinal tract (e.g., feces, saliva)
- Genitourinary tract (e.g., urine, vaginal secretions)
- Skin
- Blood (e.g., through biting arthropods, needles, or syringes)

Adhesins: Surface projections on pathogens that help them adhere to host cells, made of glycoproteins or lipoproteins.
Adherence mechanisms: Include glycocalyx, fimbriae (pili), and flagella.
Host cell receptors: Often sugars (e.g., mannose for E. coli).
Biofilms: Provide attachment and resistance to antimicrobial agents.

Capsules: Inhibit or prevent phagocytosis.
Cell wall proteins: Contribute to pathogenicity, e.g., M protein of S. pyogenes.
Antigenic variation: Avoid immune system recognition, e.g., Trypanosoma and Neisseria.
Penetration into host cells: Invasins, proteins that cause actin to form a basket that carries the bacteria into the cell, e.g., Salmonella and E. coli.

Coagulase: Causes blood clot formation, protecting the pathogen from phagocytosis.
Kinase: Dissolves blood clots, e.g., streptokinase.
Hyaluronidase: Breaks down "intercellular cement" for tissue penetration.
Collagenase: Hydrolyzes collagen.
IgA protease: Destroys IgA.

Exotoxins: Proteins produced by Gram-positive and Gram-negative bacteria, e.g., diphtheria toxin, erythrogenic toxin, botulinum toxin.
Endotoxins: Lipid A component of LPS in Gram-negative bacteria, e.g., E. coli, Salmonella.
A-B toxins: Exotoxins with two components, e.g., cholera toxin.
Membrane-disrupting toxins: Lyse host cells, e.g., leukocidin, hemolysin.
Superantigens: Stimulate T-cells, causing intense immune response, e.g., staphylococcal enterotoxin.

Viruses: Evasion of immune system by growing inside cells, mimicking Ach, and hiding attachment sites.
Fungi: Produce waste products that cause symptoms, e.g., ergot toxin, aflatoxin.
Protozoa: Produce waste products that cause symptoms, avoid host defenses by growing in phagocytes, and undergo antigenic variation.
Helminths: Interfere with host function, produce metabolic waste that causes symptoms, and avoid host defenses by growing in phagocytes.