Skip to main content
Biology LibreTexts

4: How to be a Good Pathogen

  • Page ID
    • 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?