chapter+25 - Chapter 25 Microbial Pathogenesis...

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Chapter 25 Microbial Pathogenesis Host-Pathogen Interactions Infection = entry of a pathogen or parasite into a host 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 tissue allows organism access to new site Loss of other microflora (normal flora) allows organism to bloom Pathogenicity of organism Measure of ability to cause disease (how likely to cause disease and colonize) Determined by genetic make-up of organism Infectious dose = ID 50 Number of organisms to colonize 50% of hosts Virulence (how likely to kill) Rate of lethal infection Lethal dose = LD 50 Number of organisms Infection Cycle Spread by direct contact Spread by indirect contact Contact with formites In animate 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
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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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