Chapter 27HostmicrobeMed micro13th ed2


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Unformatted text preview: patient Clostridium perfringens Hyaluronidase & Collagenase Clostridia:Food poisoning → Gas gangrene (death and putrefaction of tissue, bubbles of gas in decomposing tissue) Widespread & destroys tissue. (Contrast with later C. tetani) Virulence Factors: One Example Virulence Factors: One Example Colonization and growth in the human host Physical characteristics—move, attach, hide Toxins—poison Read Sidebar page 810 (3 types) Siderophores: Iron chelating & sequesters Fe to aid growth Salmonella typhimurium Measuring Virulence Measuring Virulence • • • • The virulence of a pathogen can be estimated from experimental studies of the LD50 (lethal dose50) The LD50 is the dose of an agent that kills 50%of the animals in a test group Invasiveness is the ability of an organism to grow in host tissue Toxicity is the ability of an organism to cause disease by means of a preformed toxin that inhibits host cell function or kills host. Sample Question Sample Question Imagine that you receive an LD50 report about two bacteria, A & B. LD50 [A] = 50 cells; LD50 [B] = 150 cells. Which of the following could be concluded? A. Organism B must have an exotoxin B. Organism B is more virulent than A C. Organism A is more virulent than B D. Organism B has a capsule but A does not E. A & B Table 21.5 Properties of exotoxins and endotoxins Property Exotoxins Endotoxins Chemical Properties Proteins, excreted by certain gram­positive or gram negative Bacteria ; generally heat­labile Lipopolysaccharide­ lipoprotein complexes; released on cell lysis as part of the outer membrane of gram­ negative Bacteria ; extremely heat­ stable Mode of action; symptoms Specific; usually binds to specific cell receptors or structures; either cytotoxin, enterotoxin, or neurotoxin with defined specific action on cells or tissues General; fever, diarrhea, vomiting Toxicity Often highly toxic; sometimes fatal Weakly toxic, rarely fatal Immunogenicity Highly immunogenic; stimulate the production of neutralizing antibody (antitoxin) Relatively poor immunogen; immune response not sufficient to neutralize toxin Toxiod potential Treatment of toxin with formaldehyde will destroy toxicity, but treated toxin (toxoid) remains immunogenic None Fever potential Does not produce fever in host Pyrogenic, often induces fever in host Toxoid Toxoid Toxin Toxoid Antitoxin Heat or chemical treatment Vaccine! inject into animal EXOTOXINS: GENERAL TYPES EXOTOXINS: GENERAL TYPES Three categories based on types: Cytolytic toxins (include hemolysins): damage cell membranes, causing cell lysis & death (also are cytotoxins, next slide). A­B toxins: B promotes specific binding of toxin to host cell receptor (allows transfer of A (toxic part) across targeted cell membrane). Superantigen toxins: Stimulate large numbers of immune lymphocytes and causes systemic as well as inflammatory responses. Extensive inflammation & tissue damage. Later: simultaneously bind to VB domain of T cell receptor & alpha chain of Class II MHC: crosslink T cell & MHC Leads to cytokine overproduction, leading to systemic toxicity Food poisoning induced by Staph enterotoxins and toxic sho...
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This document was uploaded on 09/17/2013.

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