2 Host Pathogen Interaction Origin of Microbial FloraSymbiosis: association of 2 organisms living togetherCommensalism: organism benefit with no benefit or harm to the hostParasitism: microbe gains at host expense
3 Host-Pathogen Interaction (cont’d) TermsPathogen – microbe that can cause disease in a susceptible hostOpportunistic Pathogen – microbe that can cause disease only if a significant change occurs in host resistance or within the organism itselfOpportunistic infections- infections caused by opportunistic pathogens
4 Host-Pathogen Interaction (cont’d) TermsIatrogenic Infections – resulting from medical treatment or proceduresHospital-acquired Infections- acquired in the hospital or another health care setting
5 Host Pathogen Interaction Characteristics of Normal FloraResident flora vs. transient floraCarriers?
6 Host Pathogen Interaction Factors that determine normal floraAvailability of nutrientsMoisture of anatomical sitePresence of bile, lysozyme, fatty acidspH
7 Host Pathogen Interaction ColonizationPersistent survival of a microbe on a surface of the human body.Dictated by the defenses of the bodyDictated by the microbes ability to survive
14 Host-Pathogen Interaction (cont’d) Virulence – relative ability of a microorganism to cause disease, or the degree of pathogenicity
15 Host-Pathogen Interaction (cont’d) Infectious Agent StepsAdherence – most infectious agents must attach to host cells before infection occursProliferation – pathogens must be able to replicate after attachment to host cells (overcome host resistance factors)Tissue Damage – makes the infection visible; results from toxins or from host inflammatory substances
16 Host-Pathogen Interaction (cont’d) Invasion – all pathogens have the ability to penetrate and grow in tissuesDisseminationSpread of organisms to distant sitesSome pathogens stay at site (C. diphtheriae); others spread (Salmonella ssp.)
17 Host-Pathogen Interaction (cont’d) Virulence factors – factors such as capsules, toxins, enzymes, cell wall receptors, pili, etc. that allow pathogens to evade or overcome host defenses & enable them to cause disease
18 Host-Pathogen Interaction (cont’d) Virulence factorsAttachmentFimbriae/piliResist phagocytosisCapsulesProtein ALeukocidinsAbility to MoveFlagella
20 Host-Pathogen Interaction (cont’d) Host Resistance Factors: First Line of DefensePhysical barriers – skin, mucous membranesCleansing mechanismsDesquamation (shedding of skin)Fluids of the eye (IgA and lysozyme)Respiratory, digestive, urinary, and genital tracts have fluids(mucous) and movements( cilia/ peristalsis) to cleanse the surfaces
21 Host-Pathogen Interaction (cont’d) Host Resistance Factors: Second Line of DefenseInflammatory responseVasodilationIncreased permeability of capillariesArrival of leukocytesChemotaxisPhagocytosisImmune Responses: innate
26 Host-Pathogen Interaction (cont’d) Routes of TransmissionAirborneCoughing, sneezing, talkingDroplet nucleiAirborne pathogens must be resistant to drying and inactivation by ultraviolet lightExamples: Strep throat, otitis media, diphtheria, rhinoviruses (colds)
27 Host-Pathogen Interaction (cont’d) Transmission by Food and WaterIngestion of contaminated food or waterSometimes oral-fecal routePathogens must be able to survive stomach conditions and compete with normal flora of the gutPre-formed toxins (Clostridium botulinum, S. aureus) vs. toxins produced after infection (C. difficile, V. cholerae)
28 Host-Pathogen Interaction (cont’d) Close ContactPassage of organisms by salivary, skin, and genital contactExamples: Infectious mononucleosis, STDsCuts and BitesArthropods ( ticks, fleas)Zoonoses – diseases of animals accidentally transmitted to humans; examples: plague, rabies, tularemia
29 ReferencesEngelkirk, P., & Duben-Engelkirk, J. (2008). Laboratory Diagnosis of Infectious Diseases: Essentials of Diagnostic Microbiology . Baltimore, MD: Lippincott Williams and Wilkins.Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook of Diagnostic Microbiology (4th ed.). Maryland Heights, MO: Saunders.
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