staphylococcus, any of the pathogenic bacteria, parasitic to humans, that belong to the genus Staphylococcus. The spherical bacterial cells (cocci) typically occur in irregular clusters [Gr. staphyle=bunch of grapes]. The term staphylococcus is also sometimes used loosely for the cluster arrangement itself and, broadly, for any bacteria with such a growth pattern. The pigments produced by staphylococci are the basis of the names given to the various strains—those with colors ranging from orange to yellow are designated S. aureus; white strains are known as S. albus.

Staphylococci cause abscesses, boils, and other infections of the skin, such as impetigo. They can also produce infection in any organ of the body (e.g., staphylococcal pneumonia of the lungs). The most common form of food poisoning is brought on by staphylococcus-contaminated food. The staphylococcus organisms also generate toxins and enzymes that can destroy both red and white blood cells.

Unlike some other types of bacteria, staphylococci are generally partly or wholly resistant to antibiotic action; this raises serious problems in the treatment and control of staphylococcus infections (see drug resistance). The rise of drug-resistant virulent strains of S. aureus, particularly methicillin-resistant S. aureus (MRSA), has led increasing concern in the medical community. Although sick patients with compromised immune systems and children are most susceptible to the strains, which most typically are contracted in hospital, nursing home, and other health-care settings, healthy persons have also been infected. Pharmaceutical companies are working to develop new antibiotics to kill drug-resistant strains of staphylococcus and other bacteria, and a vaccine for S. aureus has been developed.

Any of the spherical bacteria that make up the genus Staphylococcus. The best-known species are present in great numbers on the mucous membranes and skin of all humans and other warm-blooded animals. The cells characteristically group together in grapelike clusters. Staphylococci are gram-positive (see gram stain) and stationary and do not require oxygen. Of significance to humans is the species S. aureus, an important agent of wound infections, boils, and other human skin infections, and one of the most common causes of food poisoning. It also causes udder inflammation in domestic animals and breast infections in women. The largest cause of hospital infections (accounting for almost 15percnt), “staph” is often difficult to treat because of its increasing resistance to antibiotics.

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