1. MLAB 2434 – MICROBIOLOGY KERI BROPHY-MARTINEZ
Host-Pathogen Interaction

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

3. Host-Pathogen Interaction (cont’d)
Terms
Pathogen – microbe that can cause disease in a susceptible host
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

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

5. Host Pathogen Interaction
Characteristics of Normal Flora
Resident flora vs. transient flora
Carriers?

6. Host Pathogen Interaction
Factors that determine normal flora
Availability of nutrients
Moisture of anatomical site
Presence of bile, lysozyme, fatty acids pH

7. Host Pathogen Interaction
Colonization
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

8. Host Pathogen Interaction

9. Host Pathogen Interaction

10. Host Pathogen Interaction

11. Host Pathogen Interaction

12. Host Pathogen Interaction

13. Host Pathogen Interaction

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 Steps
Adherence – most infectious agents must attach to host cells before infection occurs
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

16. Host-Pathogen Interaction (cont’d)
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.)

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 factors
Attachment
Fimbriae/pili
Resist phagocytosis
Capsules
Protein A
Leukocidins
Ability to Move
Flagella

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

20. Host-Pathogen Interaction (cont’d)
Host Resistance Factors: First Line of Defense
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

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

22. Inflammatory process

23. Host-Pathogen Interaction (cont’d)

24. Host-Pathogen Interaction (cont’d)
Host Resistance Factors: Third Line of Defense
Adaptive/specific immunity

26. Host-Pathogen Interaction (cont’d)
Routes of Transmission
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)

27. Host-Pathogen Interaction (cont’d)
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)

28. Host-Pathogen Interaction (cont’d)
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

29. References
Engelkirk, 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.