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Staphylococcus aureus is a bacterium that causes illnesses ranging from minor skin infections and abscesses, to life-threatening diseases such as pneumonia, meningitis and septicemia.
Each year some 500,000 patients in American hospitals contract a staphylococcal infection. By changing its chemical makeup slightly to evade attack, S. aureus has become resistant to many commonly used antibiotics. In 1997, physicians were alarmed to encounter staph strains that resist even vancomycin, which used to work when all else failed.
Staphylococcus aureus appears as a gram-postive coccus, in grape-like clusters when viewed through a microscope. More characteristic is its appearance when grown out on agar plates. It appears as large, round golden-yellow (which is where the name aureus comes from) colonies, with beta-haemolysis of blood agar.