lecture 3

lecture 3 - 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 (don’t just prey on the immunocompromised: have virulence factors or toxins, etc.) Opportunistic pathogens Cause disease only in compromised hosts Immune system defective Breaks 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 sub 50 (ID50) Number of organisms to colonize 50% of hosts (low #= high pathogenicity, high#= low pathogenicity) Virulence Rate of lethal infection Lethal dose = LD50 Number of organisms to kill 50% of hosts Infection Cycle Spread by direct contact Spread by i ndirect contact
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Contact with fomites Inanimate objects that can carry infectious microbes Horizontal transmission via vectors (organism) Mosquitoes— Yellow fever, malaria Reservoir for disease organism May not show disease symptoms (birds with west nile) 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 Islands 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 Structures & characteristics of
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lecture 3 - Chapter 25 Microbial Pathogenesis Host-Pathogen...

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