Host-Parasite Interactions


            Commensalism--indigenous microbes in body (majority)

                        benefit from the association with host, host is not affected

            Mutualism--benefit the host while thriving from the relationship

            Parasitism--live at the expense of the host (perhaps disease)

            Infectious disease--host / parasite survival

                        Recovery of Host dependent upon eradication of microbe

Pathogenicity, Virulence, and Infection

            Pathogenicity--capability of organisms to cause disease

            Disease--response of the host to infection, impairment of body function

            Pathogen--any microbe or macrobe capable of producing disease

            Pahogenicity is in influenced by:

1)      properties inherent in the microbe (virulence factors)

2)      host’s ability to resist infection

Virulence--degree of pahogenicity

Conditions of pathogen infection

1)      must enter the host

2)      must metabolize and multiply in host tissues

3)      must resist host defenses

4)      must damage the host


Microbial Virulence Factors


                        Exotoxins (excreted) protein

                                                Clostridium botulinum

                                                Clostridium perfringens

                                                Clostridium tetani

                                                Corynebacterium diphtheriae

                                    Toxoids—heat instability—chemical reactivity

                                                Phenol, formaldehyde, etc.

                                                Stimulate antitoxins

                        Endotoxins (liberated upon lysis) lipopolysaccharide

                                    Popular in Gram negative organisms

                                    Toxin is resident in cell wall


1)      heat stable

2)      do not from toxoids

3)      less toxic than exotoxins

Extracellular enzymes

            Hyaluronidase--hyaluronic acid hydrolysis

            Lecithinase--lysis of RBC’s

            Collagenase--collagen network dissolved

            Coagulase--fibrinogen>fibrin--walling of process

            Leukocidin--lysis of WBC’s

            Hemolysin--RBC lysis

Capsules--pneumococci (virulent / avirulent)

            capsule prevents phagocytosis

Pili--host / parasite adherence


Other Factors Influencing Infections

            Tissue affinity

                        brucellae--placenta (erythritol)

            Portal of entry

1)      suitable number of pathogens must enter host

2)      many must enter by certain routes

based upon ability to attack tissues and organs

                        Toxins may have specific portals of entry


                        Epidemics--unusual prevalence of a disease over the normal incidence

                                    Self-limiting--survival of parasitic relation

                        Dependent factors:

1)      escape of pathogen form host

2)      new host susceptibility


1)      airborne

2)      contact

3)      foodbrone and waterborne

4)      arthropod-borne


Success of pathogen is based on its ability to escape host, survive interhost periods, and find new susceptible host.

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