Staphylococcus species are Gram-positive, spherical bacteria (cocci) that grow in grape-like clusters. The most important pathogen is Staphylococcus aureus.S. aureus lives harmlessly on the skin and in the nose of many healthy people (roughly 30%). It becomes a problem when it gets into the body through a cut, a surgical wound or a medical device such as a catheter, or when it contaminates food.Diseases caused by S. aureus include:
• skin infections such as boils, abscesses and impetigo;
• wound infections, and more serious infections of the blood (septicaemia), bones and lungs;
• food poisoning, with sudden vomiting 1 to 6 hours after eating;
• toxic shock syndrome.Exotoxins. S. aureus causes damage mainly by secreting exotoxins:
• some form pores in host cell membranes, causing lysis of cells, including red and white blood cells;
• enterotoxins act on the gut to cause food poisoning. They are heat-stable, so reheating food does not destroy them;
• toxic shock syndrome toxin acts as a superantigen, activating huge numbers of T cells so that massive amounts of cytokines are released. This causes high fever, a rash, a sharp fall in blood pressure and possibly organ failure.Food poisoning comes on quickly because the toxin, not the bacterium, causes it: the toxin is already in the food, so no time is needed for the bacteria to multiply in the gut. Some strains of S. aureus are resistant to many antibiotics; these are covered in the MRSA lesson.
• skin infections such as boils, abscesses and impetigo;
• wound infections, and more serious infections of the blood (septicaemia), bones and lungs;
• food poisoning, with sudden vomiting 1 to 6 hours after eating;
• toxic shock syndrome.Exotoxins. S. aureus causes damage mainly by secreting exotoxins:
• some form pores in host cell membranes, causing lysis of cells, including red and white blood cells;
• enterotoxins act on the gut to cause food poisoning. They are heat-stable, so reheating food does not destroy them;
• toxic shock syndrome toxin acts as a superantigen, activating huge numbers of T cells so that massive amounts of cytokines are released. This causes high fever, a rash, a sharp fall in blood pressure and possibly organ failure.Food poisoning comes on quickly because the toxin, not the bacterium, causes it: the toxin is already in the food, so no time is needed for the bacteria to multiply in the gut. Some strains of S. aureus are resistant to many antibiotics; these are covered in the MRSA lesson.