St. aureus and food poisoning
St. aureus causes gastro-enteritis
Food poisoning is not caused by the organism but by the toxin that the organism secretes
St. aureus food poisoning is the most common form of food poisoning in the US
Properties of Staphylococcus aureus
Gram positive cocci arranged in grape like clusters
Colonies on TSA are small, creamy and golden colored
Colonies on MSA are yellow and turns the media yellow
Properties of St. aureus that make it persistent in nature
Relatively heat resistant
Resistant to high concentrations of salt
Can survive long periods on dry inanimate objects
2 Case Studies
What were the symptoms of food poisoning?
How did the chefs contaminate the food?
How did the chef get a staph infection?
How did the chef get a staph infection?
Staph is found on any inanimate surface
Staph is often found associated with the external nares of 30% of the human population
Staph is often found on skin surfaces because they can tolerate the low moisture and high salt content of skin
Staph can easily spread from person to person via hand to hand contact
Staph can penetrate the deep tissues of skin damaged by
skin diseases—acne, psoriasis, eczema
What happens when Staph enters a wound and how does this relate to food poisoning ?
Localized staph infection leading to an abscess (collection of pus)
boils=abscesses in the skin
Rupture of the abscess leads to the release of live bacteria and associated toxin
How do abscesses and boils form?
Chef cuts arm and Staph enters deeper skin layer
St. aureus is surrounded by a capsule
thick slime layer that prevents an immediate immune response
Bacteria multiply at the site surrounded by the capsule
St. aureus establishes intimate contact with skin cells via bacterial techoic acids and fibronectin skin cell receptors
Abscess and boil formation (cont’d)
St. aureus produces coagulase which converts soluble fibrinogen in plasma to insoluble matrix fibrin
There are two types of coagulase
bound coagulaseà on the surface of the bacteria causes the bacteria to clump together
free coagulaseàsecreted from the bacteria into the environment
Why produce coagulase
Bound coagulase causes bacteria to clump together. Why?
the more bacteria in a given location the more effective they are in 1. shielding each other from an immune response and in 2. excreting toxic factors in high quantities
Free coagulase causes a protective fibrin clot to form around bacteria. Why?
bacteria can grow and divide in protective environment; most immune cells have been denied entry to the region
Pus formation is due to an immune response inside the fibrin clot
Many bacteria are found in fibrin clot
Also some immune cells did get trapped in fibrin clot
Immune cells want to kill St. aureus
St. aureus wants to kill immune cells
The war that ensues leads to pus formation
Pus consists of dead and living St. aureus, dead neutophils and plasma inside a fibrin clot
Pus formation continued
The immune cells killing St. aureus
neutrophils surround bacteria, ingest them and produce lysosomal enzymes that kill bacteria.
This releases bacterial components that lead to a greater inflammatory response which kills host cells.
St. aureus killing immune cells
when neutrophils ingest bacteria the lysosome fuses with the phagosome
St. aureus produces catalase that converts hydrogen peroxide into water and oxygen
St. aureus produces cytotoxins that kill the neutorphils
The dead neutrophils release lysosomal campartment enzymes that will may kill St. aureus but will kill adjacent host cells
St. aureus and food
Staph grows and divides in food and produces an enterotoxin
The Staph doesn’t cause food poisoning, the enterotoxin does
Enterotoxin is stable to heating at 100oC for 30 minutes.
Enterotoxin is resistant to degradation by stomach gastric acids
Staph enterotoxin causes gastro-enteritis in two ways
VOMITINGàtoxin works on the vomiting control center of the brain this leads to reversal of peristalsis and vomiting
DIARRHEAàenterotoxin is a superantigen and elicits a strong immune response in the region where the toxin is most concentrated. Immune response causes a loss of brush borders in intestinal epithelial cells; these cells cannot absorb water from the gut.