Staphylococcus aureus is easily the most important species of the staphylococci. It is found in the environment and is frequently seen as normal flora bacteria in people, and according to Elmer Koneman, M.D., 20 to 40 percent of adults have S. aureus colonized in the nares. It can also colonize without disease in the armpit area, the perineum, skin fold and the vagina. However, S.aureus is a major opportunistic pathogen that causes a myriad of diseases in humans.
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The microscopic appearance of Staphylococcus aureus is round and resembles that of a sphere (cocci). Because of the way the bacteria divide and multiply, it will appear in clusters or tetrads. In Greek, staphylococcus means “clusters of grapes.” The use of a common bacteriological stain, the Gram stain, helps to identify S. aureus. The organism will appears purple using this staining technique and is called gram-positive.
When grown on bacteriological media, Staphylococcus aureus appears as a large white to golden colony. The majority of the time the colony of Staphylococcus aureus produces a zone of hemolysis surrounding the colony.
It is not very fastidious and grows well, either aerobically or under anaerobic conditions and produces good growth within 24 hours.
Key Identification Characteristics
In addition to its microscopic appearance, S. aureus reacts to certain laboratory tests very characteristically. All staphylococci produces the enzyme catalase when introduced to hydrogen peroxide. This test easily differentiates the staphylococci from the streptococci. It also produces the enzyme coagulase which allows the organism to produce a clot in rabbit plasma. This is a key test to differentiate S. aureus from other staphylococci.
Staphylococcus aureus produce a wide variety of virulence factors that allow it to produce many different types of disease. The production of capsules around the bacterium helps prevent the phagocytosis by macrophages and leukocytes. The production of coagulase can also protect the bacteria from phagocytosis by coating the cell with fibrin. S. aureus also produces hemolysins that can cause the lysis of certain cells. Different strains produce different toxins that can cause a myriad of disease from mild to life-threatening.
The spectrum of disease caused by Staphylococcus aureus ranges from mild skin infections to serious systemic disease. The different skin diseases that are a result of S. aureus infection include boils, impetigo and carbuncles.
Systemic infections can include any part of the body, but is most commonly seen in pneumonia, bacteremia, wounds and osteomyelitis (infection of the bone).
Staphylococcus aureus produce certain enterotoxins that cannot be killed by cooking that cause a very common type of food poisoning. The exfoliatin toxin can cause scalded skin syndrome in newborns and infants and the toxin that causes toxic shock syndrome is extremely serious and a multi-system disease. There is an association with tampon use and toxic shock syndrome.