marcie 25 - Chapter 25 Microbial Pathogenesis Host-Pathogen...

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Chapter 25 Microbial Pathogenesis Host-Pathogen Interactions Infection = entry of pathogen or parasite Infection doesn’t always cause disease Most infections removed by immune system Primary pathogens Have ability to penetrate host defenses Opportunistic pathogens Cause disease only in compromised hosts Immune system defective Break in tissue allows organism access to new site Loss of other microflora allows organism to bloom Pathogenicity of organism Measure of ability to cause disease Determined by genetic makeup of organism Infectious dose= ID 50 Number of organisms to colonize 50% of hosts Virulence Rate of lethal infections Lethal dose= LD 50 Number of organisms to kill 50% of hosts Infection Cycle Spread by direct contact Spread by indirect contact Contact with fomites
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Inanimate object that can carry infectious microbes Horizontal transmission via vectors (organism) Mosquitoes—yellow fever, malaria Reservoir for disease organism May not show disease symptoms Mode of entry depends on pathogen Mucosal surfaces, wounds, insect bites Virulence Factors Virulence genes Encode factors allowing pathogen to invade host Toxins, attachment proteins, capsules Pathogenicity Section of genome Contain multiple virulence genes Often encode related functions E.g., protein secretion system, toxin production Transferred as a block from other organisms
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Virulence Factors
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