Staphylococcus aureus

Staphylococcus aureus is a round, gram-positive bacterium that grows in grape-like clusters and often forms golden-yellow colonies, the source of its species name. It tolerates salt and drying, grows with or without oxygen, and is catalase-positive and coagulase-positive, the two tests that separate it from streptococci and from milder staphylococci such as S. epidermidis. About 30 percent of people carry it in the nose at any time without harm, and it spreads by skin contact and shared objects.

Once it gets through broken skin or into the body, S. aureus can produce many different illnesses. Common forms are skin and soft-tissue infections such as boils, abscesses, impetigo and infected wounds. It also causes pneumonia, bone and joint infections, infections of heart valves and implanted devices, and bloodstream infections that can lead to sepsis. Its weapons include coagulase, which clots plasma around the bacteria; toxins that kill white blood cells; protein A, which grabs antibodies by the wrong end and blunts phagocytosis; and, in some strains, toxins behind food poisoning, scalded skin syndrome and toxic shock syndrome.

Laboratories identify it by culture and biochemical or molecular tests. Treatment depends on susceptibility testing, because many strains resist common antibiotics, the best known being MRSA.