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4: How to be a Good Pathogen

  • Reading assignment: skim/review chapter 18 in Belk’s Biology p466-477


  • zoonoses
  • adhesins
  • reservoir
  • host receptors
  • transmission
  • antibodies                                            
  • vehicles
  • toxins
  • arthropod vectors
  • enzymes
  • fomites
  • pyogenic
  • portals of entry


To be a good pathogen, one must follow some basic steps: (determinants of infectious disease)

  1. maintain a reservoir
  2. transmission to new host
    • enter host->portals of entry
  3. attach to host cells
  4. “outsmart”/evade host defenses
  5. cause damage to host
  6. leave host to find new hosts

1. Maintain a reservoir

                3 types of reservoirs

                                a. humans: ex HIV, cold virus

                                b. animals

-human diseases caused by pathogens which use animals as a reservoir are called “zoonoses”

-ex rabies, “Bird Flu”

                                c. environment

  • water, soil
  • ex anthrax, fungal infections, cholera


2. Transmission: leave reservoir and find new host  (see Belk’s Biology p476-477)

                a. Direct: infected host contacts new host and passes pathogen to new host

                                                i. vertical: pathogen passed from mom to baby

                                                                ex. HIV, rubella virus, syphilis

                                                ii. horizontal: spread among members of a population

                                                                -sexual contact: HIV, HPV, herpes, chlamydia infections

                                                                -fluid exchange ex saliva and kissing

                                                                                ex meningitis, “strept throat”

                                                                -bites: rabies

b. Indirect: infected host need NOT be present for pathogen to be passed to new host

                -vehicles: water, soil, food

 Ex. anthrax, botulism, cholera, polio, prions, hepatitis A

                -fomites: inanimate objects contaminated with pathogens

ex smallpox

                -blood supplies/donations

ex HIV, hepatitis B/C

                -contaminated needles

HIV, hepatitis B/C

                                -arthropod vectors: West Nile virus, malaria, Lyme Disease

Note: the term “contagious”  is defined: a communicable disease that is easily transmitted from a reservoir or patient. (source: Bauman’s Microbiology)    


c. “Portals of entry”: how to “get into” host

                                -broken or damaged skin

                                - mucous membranes:

                                                gastrointestinal, respiratory, urinary, genital tracts, conjunctiva



3. Attaching to host cell

  1. Adhesins: surface molecules on pathogen bind to host receptors
  2. Host receptors: surface molecules on host cells. Pathogen adhesins bind to “complementary” host cell surface receptors (like a lock and key)
  3. Antibodies against pathogen adhesins: antibodies bind to adhesins and block ability of pathogen to bind to host cells, thus stops disease!


4. “Outsmarting”/evading host defenses

                a. capsules: prevent phagocytosis by protective host cells

                b. toxins (leukocidins) to kill protective cells  -triggers “pus” production

                                pus= fluid containing dead tissue cells, leukocytes and pathogens

                                -found in abscesses. Pimples, boils and pustules are examples of pus-filled abscesses

                                -microbes which trigger pus production are called “pyogenic”= pus-makers

- 2 common pyogenic bacterial pathogens are Staphylococcus aureus and Streptococcus pyogenes

                c. changing pathogen surface molecules outwits antibodies

                d. superantigens “short circuit” immune system


5. Damage to host-a few examples

                a. enzymes: examples of some enzymes produced by Staphylococcus aureus 

1. enzymes which permit pathogen to invade tissues =hyaluronidase breaks down intercellular “cement” , permits pathogen to spread between cells

2. enzymes which help pathogen “hide” ex coagulase triggers fibrin clot formation around pathogen; camouflage, impedes access of phagocytes

3. enzymes which permit escape from fibrin clots: ex staphylokinase, breakdown fibrin clots, permit spread of pathogen

b. membrane pore-forming substances examples

                1. hemolysins lyse red blood cells

                2. leukocidins: destroy defensive leukocytes/white blood cells

c. toxins

-example: endotoxins of gram-negative bacteria: trigger circulatory collapse, shock, death

-ex exotoxins such as tetanus, botulinum toxins, diphtheria toxins (disrupt normal cell function)

6. Leave host and find another host!

Study guide How to be a good pathogen

1. What is a “reservoir”?

            Provide one specific example of a pathogen which uses each of the following       reservoirs

            a. humans

            b. (non-human) animals

            c. environment

2. What is a “zoonosis”? Specific examples?

3. What are “fomites”?

4. What does “transmission” mean?

            Describe the following and provide one specific example of a pathogen which is   transmitted in the following  ways:

  1. direct, contact transmission
  2. indirect transmission
  3. horizontal transmission
  4. vertical transmission

e. arthropod-vector transmission

5. Why is important that a pathogen attach to host cells?

            -What do the following terms mean?

            a. adhesin

            b. host surface receptor

            -draw and label a diagram showing how a pathogen may attach to a host cell

6. How can antibodies against pathogen adhesins protect hosts from colonization by a pathogen?   Draw and label a diagram to help illustrate your answer.


7. Describe the function of the following microbial virulence factors by filling-in the table:

Virulence factor
















8. What does “pyogenic” mean?

  • name 2 pyogenic bacterial pathogens.
  • what is “pus”?
  • what are: abscesses, pimples, pustules, boils?

9. How can pathogens evade/avoid defenses/immune responses of the host?