The weather was cool after the previous night's monsoon storm, and Dr. J was feeling
ambitious, so she went out early Saturday morning to cut back some prickly pear
cactus pads encroaching on the driveway. As she worked, she noticed a cactus
spine in her finger, but by the time she quit work and went to wash her hands
the spine was no longer visible. A few days later, she noticed pain and redness,
which over time spread up the finger. She visited her physician, who asked the
lab to culture some of the pus from the infection site. The lab report diagnosed
Staphylococcus aureus, a Gram positive coccus commonly found on skin.
S. aureus is a gram positive, nonmotile, catalase positive, coccus found on the outside of the body, especially around the nose. About 30% of the population carries S. aureus at any particular time, and about 2/3 of people are at least occasional carriers. The bacteria induce localized inflammation that causes capillary endothelial cell damage and gives the bacteria access to the circulation. S. aureus produces an antiphagocytic capsule and surface adhesins. In addition to catalase, it also secretes protease, lipase, and hyaluronidase that destroy tissue, and coagulase that converts fibrinogen to a fibrin clot inside which the bacteria can grow. Many strains of S. aureus produce exotoxins and some strains are antibiotic resistant. Fortunately for Dr. J, the S. aureus infecting her finger did not produce toxic shock syndrome toxin and her infection responded quickly to antibiotics.
1. List as bullets with brief descriptions and in approximate chronological order
the steps in the innate response to a bacterial infection. Include both the cells
and the molecules that are involved in the response to the bacteria and what
each does.
Example:
2. How do phagocytes recognize pathogens? What are TLR and what molecules do they bind? What cytokines do macrophages secrete in response to signals through their TLR and what do those cytokines do?
3. How do phagocytes eliminate pathogens? Which phagocytes do most of the pathogen destruction?
4. What is inflammation? What are the four signs/symptoms and what causes them? What is the purpose of inflammation? How are leukocytes (neutrophils and macrophages) signaled where to leave the circulation and enter the infection site?
5. What is complement? In what three ways is complement activation initiated and what are the three functions of complement?
6. Of the virulence factors of S. aureus listed below, which promote innate immunity and which protect the bacterium from the immune system? Include in your answer the elements of innate immunity from your list above that are affected by each factor and how. (For example: X stimulates or inhibits phagocytosis, Y activates or blocks a certain activity of complement.)
7. What are acute phase proteins and how do they enhance innate immunity?
8. What is endotoxin shock? How does it differ from inflammation? Was Dr. J at risk of endotoxin shock with her Staph infection?
9. Suppose that instead of being
stuck with a cactus spine,Dr. J had been bitten by an animal infected with rabies
virus. Rabies virus binds to and enters skin cells, where it replicates to produce
more virions. Infected cells have rabies virus peptides on their membrane Class
I MHC. Compare the innate response to rabies virus at the site of the bite with
the innate response to S. aureus. What factors would be the same and what
would be specific to virus infections?
Supplementary Material : Innate Immunity, Complement, Cytokines, Infectious Disease, Immunity Rules, Antigen