- Reading assignment: skim/review chapter 18 in Belk’s Biology p466-477
- host receptors
- arthropod vectors
- portals of entry
To be a good pathogen, one must follow some basic steps: (determinants of infectious disease)
- maintain a reservoir
- transmission to new host
- enter host->portals of entry
- attach to host cells
- “outsmart”/evade host defenses
- cause damage to host
- leave host to find new hosts
1. Maintain a reservoir
3 types of reservoirs
a. humans: ex HIV, cold virus
-human diseases caused by pathogens which use animals as a reservoir are called “zoonoses”
-ex rabies, “Bird Flu”
- 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”
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 HIV, hepatitis B/C
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
- Adhesins: surface molecules on pathogen bind to host receptors
- Host receptors: surface molecules on host cells. Pathogen adhesins bind to “complementary” host cell surface receptors (like a lock and key)
- 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
-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
b. (non-human) animals
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:
- direct, contact transmission
- indirect transmission
- horizontal transmission
- vertical transmission
e. arthropod-vector transmission
5. Why is important that a pathogen attach to host cells?
-What do the following terms mean?
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:
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?