Staphylococcus aureus Flashcards
What is SA the most common cause of?
Food poisoning
What is the relative incidence of SA in NZ?
SA is a very common disease in NZ and most New Zealanders will get it at some point in their lifetime.
Is SA more common in the North or South Island? Why is this?
SA related infections are more common in the North than South as the population density is greater, there is higher temperatures and there are more poverty stricken areas - all of which enhance the spread of disease.
Where is SA found on our body and is it always harmful?
Staphylococci (‘staph’) are a common type of bacteria that live on the skin and mucous membranes (eg nostrils) of humans.
A lot of the time it is not harmful - About 15–40 per cent of healthy humans are carriers of S aureus, that is, they have the bacteria on their skin without any active infection or disease (colonisation).
What diseases can SA infection cause?
SA mainly causes infections of the skin.
Staph infections may cause disease due to direct infection or due to the production of toxins by the bacteria.
Boils, impetigo, food poisoning, cellulitis, and toxic shock syndrome are all examples of diseases that can be caused by Staphylococcus.
How are SA infections diagnosed and treated?
Diagnoses is mostly clinical but if there is difficulty diagnosing then it can be confirmed with a positive swab culture sent to the laboratory.
The treatment depends on the infection but it mostly antibiotics to disrupt the cell wall (eg amoxicillan) or drainage (if it is an absess) or surgical removal of the necrotic site.
In severe cases it can cause death.
What are the 5 factors of SA virulence?
- Adherence
- Invasion
- Immune evasion
- Toxins
- Superantigens
When does the peak incidence of SA infection occur?
During the first year of life
Why does SA love colonising in the nose?
It loves the salty environment
Summary of the effects of SA.
800,000 new zealanders are infected at any one time
75,000 develop skin or soft tissue infections (infected wound) each year
600 develop an invasive disease each year
350 develop bacteraemia each year (prescence of bacteria in the blood)
50 die each year.
How do you treat someone with an SA infection?
- Stabilise
- Drain absess or wound of puss
- Antibiotics
How might SA cause food poisoning?
If someone is making food and flicks SA into the food and then the food is left uncovered out for a while the SA will replicate.
Some of this SA will produce toxic shock syndrome toxins. If someone ingests these toxins they will start vomitting about 6 hours later.
What is the most common two antibiotics prescribed to treat SA infection and what is the problem with them?
Penicillin is the most commonly prescirbed. It acts by inhibiting the transpepetidase enzyme produced by SA. (this enbables the cross links of peptidoglycan to form in the bacterial wall which provide strength and structure to the cell wall so when these arent present the bacterial will explode).
However due to miss and over prescribing within a few years the SA bacteria had evolved to produce an enzyome that dissolved this antbiotic.
This resulted in a new antibiotic being prodcuced - methicillin. Soon enough strains of SA also become resistant to this (MRSA = methicillin resistant staphloccocus aureus).
Now there is a need for a new drug.
How does SA get from the skin surface to inside the skin and cause a skin infection?
- The bacteria attempts to get into the hair follicle. Some of these SA bacteria have the ability to interact with the epithelial cells by releasing toxins. Note: most bacteria on the skin cannot damage our cells
- The damaged cells released cytokines which recruit immune cells to the site of infection (chemotaxis)
- As more cytokines are relased the nearby capillaries open up and allow more WBCs to move out (diapedis)
Capillaries also produce adhesion molecules casuing the WBCs to move slower and adhere to the vessel wall. - Cytokines also irritate the nearby nerves causing pain
- The neutrophils ingest the bacteria and enzymes are released that kill the bacteria. Within the neutrophil toxic granules bind to the phagosome (phagocytosed bacteria) and form a lysozome.
How does SA avoid the immune system?
Protein A binds to the heavy chain of the antibody which masks it from the immune system (normally the antibody binds to the light chain)
The SA uses clumping factor to coat itself in fibronogen so it isnt recognised by the immune system.
SA can release chemicals (CHIPS) to prevent chemotaxis
SA can produce an enzyme that breaks down H202 in the neutrophils.
SA can also release a toxin that destroys neutrophils.
