Definitions Pathogenicity The ability of a pathogen to produce a disease by overcoming the defenses of the host. Virulence The degree of pathogenicity Portal of Entry The specific route by which a particular pathogen gains access to the body.
Preferred Portal of Entry Many microorganisms can cause infections only when they gain access through their specific portal of entry. If they enter via another portal they cause no response. Example: Flu is specific for respiratory tissues and cannot cause disease if it enters a different portal.
Virulence Can Be Expressed By the Numbers of Invading Microbes LD50 - The number of microbes in a dose that will kill 50% of inoculated test animals.
ID50 - The dose required to produce a demonstratable infection in 50% of the test animals
Question? Agent A has a LD50 of 3. Agent B has a LD50 of 3000. Which is the more virulent?
Capsules Aid in Colonizing a Host Resist Phagocytosis Increase Adherence –Streptococcus pneumoniae –Klebsiella pneumonia –Baillus anthracis –Yersinia pestis Remember: The Likelihood of Disease Increases as the Number of Invading Microbes Increases
Proteins in the Cell Wall Can Facilitate Adherence or Prevent a Pathogen From being Phagocytized These Are Called M-Proteins
Bacteria Also Produce Enzymes To Aid in Colonization Leukocidins / Destroy WBC - Staphylococci and Streptococci Hemolysins / Lyse RBC - C. perfringens, Staphylococci, Streptococci Fibrin Clot / Formed By Coagulase - Staphylococci Kinases / Destroy Blood Clots - S. pyogenes, S. aureus
Microbial Colonization Can Result In Cell Damage and Death Direct Damage Host cells can be destroyed via pathogens multiplying and being released.
Production of Toxins by Microbes Toxins Poisonous Substances Toxemia Presence of Toxins in the Blood Toxigenicity Ability to Produce Toxins
Exotoxins Exotoxins Produced Inside the Bacteria as Part of their Growth and Released into the Surrounding Medium
Streptococcal pyrogenic exotoxins A (SPE- A) and B (SPE-B) have been implicated in the pathogenesis of serious group A streptococcal infections including streptococcal toxic shock-syndrome. Clostridium difficile produces large oval subterminal spores and two different toxins; toxin A (an exotoxin causing fluid accumulation in the intestine) and toxin B (a cytopathic agent).
Endotoxins A Structural Component in the Bacteria That is Toxic and Released When the Bacterial Cell Dies and in Lysed For example, the endotoxin of Bacillus thuringiensis makes crystal-like inclusion bodies next to the endospore inside the bacteria. It is toxic to larvae of insects feeding on plants, but is harmless to humans. Endotoxins
Endotoxins Result In Fever Chills Weakness Aches Miscarriage Shock Death
Antitoxins Antibodies Produced Against Exotoxins An antitoxin is an antibody with the ability to neutralize a specific toxin. Antitoxins are produced by certain animals, plants, and bacteria. Although they are most effective in neutralizing toxins, they can kill bacteria and other microorganisms.
Cytotoxins Work by Destroying Particular Parts of the Hosts Cell or by Inhibiting Metabolic Functions (Ulcers) Neurotoxins Inhibit or Prevent Nerve Transmission (Tetanus) Enterotoxins Induce Fluid and Electrolyte Loss from Host Cells (Cholera)
Viruses Avoid the Host’s Immune Response by Growing Inside Cells.
Visible Indications of Viral Infections are Called Cytopathic Effects Stopping Mitosis Lysis Formation of Inclusion Bodies Cell Fusion Chromosomal Changes Cytocidal Effects (Cell Death) Noncytocidal Effects (Cell Damage but not Death)
Fungi, Protozoa, Helminthes, & Algae Symptoms Caused by 1. Capsules 2. Toxins 3. Allergic Reactions Resulting In 1. Damage to Host Tissue 2. Poisoning by Metabolic Wastes