Staphylococci are common bacteria which can cause many forms of infection. Staphylococcus aureus is of most general concern because of its varied action and increasing resistance to antibiotics. Certain genetic strains are a major cause of hospital infections, sometimes extremely virulent, yet those same strains are also common among healthy people.
S. aureus causes superficial skin lesions (boils, styes) and localized abscesses in other sites; it also causes deep-seated infections, such as osteomyelitis and endocarditis, pneumonia and meningitis, and more serious skin infections (furunculosis). S. aureus is responsible for certain types of food poisoning by releasing enterotoxins into food. It also causes toxic shock syndrome by release of superantigens into the blood stream. S. aureus is a major cause of hospital acquired (nosocomial) infection of surgical wounds and, with S. epidermidis, causes infections associated with indwelling medical devices. S. saprophiticus causes urinary tract infections, especially in girls. Other species of staphylococci (S. lugdunensis, S. haemolyticus, S. warneri, S. schleiferi, S. intermedius) are infrequent pathogens.
Staphylococcus aureus is one of the most common causes of serious infections that patients acquire while hospitalized. About 60% of people in the population carry the bacteria in their nose at one time or another and this is usually symptomless. An US analysis of 1,278 hospital patients known to have S. aureus living in the nose, found that 12 people eventually developed serious blood infections with S. aureus that exactly matched the type of bacteria seen in their nose. Another US study of 219 hospital patients with infections involving S. aureus found that in 82% of cases, the bacteria invading the blood were identical to the bacteria in the patient's nose. Simple infections are believed to occur when such nasal bacteria enter the bloodstream. With more invasive infections, the bacteria were likely to be introduced in different ways, including the use of a catheter or the development of skin or respiratory infections.