Chapter 15 - Chapter 15 Mechanisms of Pathogenicity...

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Chapter 15 Mechanisms of Pathogenicity
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Establishment of Infection In order to cause disease pathogen must follow a series of steps Gain entrance to host Adherence Colonization Avoid Host Defenses Cause host damage
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How Microorganisms Enter a Host Portals of entry 1. Mucus membranes Respiratory tract Gastrointestinal tract Genitourinary tract 2. Skin 3. Parenteral route
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Most microbes have a preferred portal of entry May be a prerequisite for pathogenicity Streptococci when inhaled may cause pneumonia; when ingested they do not A few microbes cause illness no matter how they enter May cause different illness based on portal Plague has 2 forms; bubonic and pneumonic
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Adherence Pathogen must adhere to host cells to establish infection Bacteria use adhesins Often located at the top of fimbriae Binding of adhesins to host cells receptors is highly specific
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Colonization Organism must multiply in order to colonize New organisms must compete with established organisms for nutrients and space
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Virulence factors Structural or physiological characteristics that help organisms colonize Aid in penetrating or avoiding host defenses Fimbrae, adhesins, enzymes, capsule, cell wall
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Capsules Avoid phagocytosis; Prevents dessication; aids in attachment Fimbrae Attachment Opa protein of Neisseria Components of cell wall M proteins of Streptococcus Mycolic acid of Mycobacteria and Norcardia Outer membrane of Gram- bacteria
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Extra-cellular enzymes (exoenzymes) Coagulases Kinases Hyaluronidase Collagenase IgA proteases Leukocidins
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Antigenic variation Avoid antibodies by altering surface antigens Neisseria varies pili type Penetration into host cytoskeleton major component of cytoskeleton is actin protein Some bacteria use actin to penetrate cells and to move between cells Surface invasins rearrange actin filaments Some pathogens induce non-phagocytic cells into endocytosis Disruption of cytoskeleton may cause ruffling in cell membrane
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Some pathogens survive within a phagocyte and hide from the immune system Escape the phagosome Prevent fusion with lysozome Survive inside phagolysozome
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