MLAB 2434 –
MICROBIOLOGY
KERI BROPHY-MARTINEZ

Host Pathogen Interaction
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Symbiosis: association of 2 organisms living together
Commensalism: organism benefit with no benefit or harm to the host
Parasitism: microbe gains at host expense

Host-Pathogen Interaction
(cont’d)
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Terms
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Pathogen – microbe that can cause disease in a susceptible host
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Opportunistic Pathogen – microbe that can cause disease only if a significant change occurs in host resistance or within the organism itself
Opportunistic infections- infections caused by opportunistic pathogens

Host-Pathogen Interaction
(cont’d)
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Terms
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Iatrogenic Infections – resulting from medical treatment or procedures
Hospital-acquired Infections- acquired in the hospital or another health care setting

Host Pathogen Interaction
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Resident flora vs. transient flora
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Carriers?

Host Pathogen Interaction
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Availability of nutrients
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Moisture of anatomical site
Presence of bile, lysozyme, fatty acids pH

Host Pathogen Interaction
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Persistent survival of a microbe on a surface of the human body.
Dictated by the defenses of the body
Dictated by the microbes ability to survive

Host Pathogen Interaction

Host Pathogen Interaction

Host Pathogen Interaction

Host Pathogen Interaction

Host-Pathogen Interaction
(cont’d)
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Virulence – relative ability of a microorganism to cause disease, or the degree of pathogenicity

Host-Pathogen Interaction
(cont’d)
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Infectious Agent Steps
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Adherence – most infectious agents must attach to host cells before infection occurs
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Proliferation – 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

Host-Pathogen Interaction
(cont’d)
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Invasion – all pathogens have the ability to penetrate and grow in tissues
Dissemination
• Spread of organisms to distant sites
• Some pathogens stay at site (C.
diphtheriae); others spread (Salmonella ssp.)

Host-Pathogen Interaction
(cont’d)
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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

Host-Pathogen Interaction
(cont’d)
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Attachment
• Fimbriae/pili
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Resist phagocytosis
• Capsules
• Protein A
• Leukocidins
Ability to Move
• Flagella

Host-Pathogen Interaction
(cont’d)
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Virulence factors (con’t)
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IgA protease
Toxin production
• Endotoxin
• Exotoxin
Exoenzyme production
• Necrotizing enzymes
• Coagulase
• Kinases
• Hyaluronidase
• Hemolysins

Host-Pathogen Interaction
(cont’d)
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Host Resistance Factors: First Line of Defense
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Physical barriers – skin, mucous membranes
Cleansing mechanisms
• Desquamation (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

Host-Pathogen Interaction
(cont’d)
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Host Resistance Factors: Second Line of Defense
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Inflammatory response
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• Vasodilation
• Increased permeability of capillaries
• Arrival of leukocytes
• Chemotaxis
• Phagocytosis
Immune Responses: innate

Inflammatory process

Host-Pathogen Interaction
(cont’d)

Host-Pathogen Interaction
(cont’d)
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Adaptive/specific immunity

Host-Pathogen Interaction
(cont’d)
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Routes of Transmission
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Airborne
• Coughing, sneezing, talking
• Droplet nuclei
• Airborne pathogens must be resistant to drying and inactivation by ultraviolet light
• Examples: Strep throat, otitis media, diphtheria, rhinoviruses (colds)

Host-Pathogen Interaction
(cont’d)
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Transmission by Food and Water
• Ingestion of contaminated food or water
• Sometimes oral-fecal route
• Pathogens must be able to survive stomach conditions and compete with normal flora of the gut
• Pre-formed toxins (Clostridium
botulinum, S. aureus) vs. toxins produced after infection (C. difficile,
V. cholerae)

Host-Pathogen Interaction
(cont’d)
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Close Contact
• Passage of organisms by salivary, skin, and genital contact
• Examples: Infectious mononucleosis,
STDs
Cuts and Bites
Arthropods ( ticks, fleas)
Zoonoses – diseases of animals accidentally transmitted to humans; examples: plague, rabies, tularemia

References
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Engelkirk, P., & Duben-Engelkirk, J. (2008). Laboratory
Diagnosis of Infectious Diseases: Essentials of Diagnostic
Microbiology . Baltimore, MD: Lippincott Williams and
Wilkins.
http://garrisonfive.blogspot.com/2009/05/home-sweethome.html http://www.5thguy.com/billboards.htm
http://www.thefreedictionary.com/vagrant
Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook
of Diagnostic Microbiology (4th ed.). Maryland Heights, MO:
Saunders.