Analysing for Staphylococcus aureus

Staphylococcus aureus is facultative anaerobic, gram-positive cocci. It was named as such because of its grape-like (Greek: staphyl) forming clusters of cells and golden (aureus) colour of colonies. S. aureus can cause diseases through direct infection or indirectly by the production of toxins. This bacterium is known as a leading cause of bloodstream infections with consequences such as infective endocarditis, septic arthritis, and osteomyelitis.

Strains of S. aureus produce several groups of exotoxins. One group is the superantigens which induce toxic shock syndrome (TSS). Another group is the exfoliative toxins which cause the staphylococcal scalded-skin syndrome (SSSS). There are also other toxins which act on the level of cell membranes such as the alpha, beta, and delta toxin.

To detect and identify S. aureus, its production of coagulase is the characteristic used by analysts. The standardised procedure comprises a horizontal method for the enumeration of coagulase-positive staphylococci using Baird-Parker agar (ISO 6888-1), rabbit plasma fibrinogen agar (ISO 6888-2) and MPN technique for low numbers (ISO 6888-3). After 24 to 48 hours of incubation on culture media at 35-37ºC., colonies of S. aureus will appear black, convex, shiny, and 1-1.5 mm in diameter.

In clinical microbiology, a major focus is placed on a highly resistant strain of S. aureus named MRSA (methicillin-resistant S. aureus). This strain induces heavy infections of the skin and lung inflammation. Thus, it represents a major risk for hospital patients. MRSA has developed resistance first to penicillin and later on to methicillin and the other members of beta-lactam antibiotics.

To detect and identify MRSA, standardised laboratory and susceptibility tests have been recommended by Clinical and Laboratory Standards Institute (CLSI), Centers for Disease Control and Prevention (CDC), and other agencies.

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