Staphylococcus aureus Food Poisoning

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 Non-sporulating 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?

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 burns cuts insect bites 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 carbuncle=interconnected abscesses 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 coagulase  on the surface of the bacteria causes the bacteria to clump together free coagulase  secreted 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 WAR 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 100 o C for 30 minutes. Enterotoxin is resistant to degradation by stomach gastric acids

Staph enterotoxin causes gastro- enteritis in two ways VOMITING  toxin works on the vomiting control center of the brain this leads to reversal of peristalsis and vomiting DIARRHEA  enterotoxin 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.