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?

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

byburnscutsinsect bitesskin 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 coagulasebound coagulase on the surface of the bacteria causes the bacteria to

clump togetherfree coagulasesecreted 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 continuedThe WAR

• The immune cells killing St. aureusneutrophils 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 cellswhen neutrophils ingest bacteria the lysosome fuses with the phagosomeSt. aureus produces catalase that converts hydrogen peroxide into water and oxygenSt. aureus produces cytotoxins that kill the neutorphilsThe 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

• VOMITINGtoxin works on the vomiting control center of the brain this leads to reversal of peristalsis and vomiting

• DIARRHEAenterotoxin 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.