Staphylococcus aureus

bacterium, frequently living on the skin or in the nose of a healthy person, that can cause illnesses ranging from minor skin infections (such as pimples, boils, and cellulitis) 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 (see discussion about MRSA). 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.

Staph infections can be spread through contact with pus from an infected wound, skin to skin contact with an infected person, and contact with objects such as towels, sheets, clothing, or athletic equipment used by an infected person.

In the hospital laboratory, Staphylococcus aureus is differentiated from most other staphylcocci by the coagulase test. Staphylococcus aureus is coagulase-positive.