7.4.1 Isolation of S. aureus

S. aureus is a fast-growing bacterium that grows well in basal medium but is usually cultured on sheep blood agar plates that are more widely used as primary media. Occasionally, clinical specimens from non-sterile sites may need an initial incubation into selective enrichment broth (see Section 4.4.1). This can be achieved in a basal nutrient broth supplemented with 7.5% NaCl (S. aureus is halotolerant and grows well in this medium).

The characteristic colony appearance, coupled with the Gram-stain reaction and a positive rapid catalase test, allow its tentative identification by the experienced microbiologist (Figure 16). Regardless of whether a preliminary identification is made based on the above, a comprehensive procedure can be used to identify S. aureus (Figure 17).

Figure 16 S. aureus produces typically round, flat to convex opaque colonies that may show white, golden or lemon pigmentation. Many strains produce haemolysis and bovine strains may produce double zone haemolysis on blood agar. In the figure, this is indicated by a zone of complete haemolysis surrounded by a zone of partial haemolysis

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A zoomed-in image of a petri dish depicting Staphylococcus aureus. There is a circle zooming in on a section. The image says ‘double zone of beta hemolysis (indicates activity of at least two separate hemolysins)’.

Figure 16 S. aureus produces typically round, flat to convex opaque colonies that may show white, golden or lemon pigmentation. Many ...

Figure 17 Isolation and identification schemes for S. aureus

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A flowchart depicting the isolation and identification schemes for S. aureus.

Figure 17 Isolation and identification schemes for S. aureus

S. aureus can be stored frozen, freeze dried or refrigerated, and may remain viable for several weeks on storage media slants in refrigeration, or even at room temperature.