MLAB 2434 – MICROBIOLOGYKERI BROPHY-MARTINEZ

Host-Pathogen Interaction

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

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

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

Host Pathogen Interaction Characteristics of Normal Flora

Resident flora vs. transient floraCarriers?

Host Pathogen Interaction Factors that determine normal

floraAvailability of nutrientsMoisture of anatomical sitePresence of bile, lysozyme, fatty

acidspH

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

Host Pathogen Interaction

Host Pathogen Interaction

Host Pathogen Interaction

Host Pathogen Interaction

Host Pathogen Interaction

Host Pathogen Interaction

Host-Pathogen Interaction (cont’d)

Virulence – relative ability of a microorganism to cause disease, or the degree of pathogenicity

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

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.)

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

Host-Pathogen Interaction (cont’d) Virulence factors

Attachment• Fimbriae/pili

Resist phagocytosis• Capsules• Protein A• Leukocidins

Ability to Move• Flagella

Host-Pathogen Interaction (cont’d) Virulence factors (con’t)

IgA protease Toxin production

• Endotoxin• Exotoxin

Exoenzyme production• Necrotizing enzymes• Coagulase• Kinases• Hyaluronidase• Hemolysins

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

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

Inflammatory process

Host-Pathogen Interaction (cont’d)

Host-Pathogen Interaction (cont’d) Host Resistance Factors:

Third Line of DefenseAdaptive/specific immunity

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)

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)

Host-Pathogen Interaction (cont’d)

Close Contact• Passage of organisms by salivary,

skin, and genital contact• Examples: Infectious mononucleosis,

STDsCuts and BitesArthropods ( ticks, fleas)Zoonoses – diseases of animals

accidentally transmitted to humans; examples: plague, rabies, tularemia

References

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-sweet-home.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.