Sometimes additional diagnostic features may be required to confirm S. aureus colonies because the inhibitors used may not completely prevent growth of other organisms, such as bacilli, micrococci, streptococci, and some yeasts. Microscopic morphology helps to differentiate bacilli, streptococci, and yeasts from staphylococci, which form irregular or grape-like clusters of cocci. Staphylococci may be further differentiated from streptococci on the basis of the catalase test, with the former being positive. Additional features are needed to differentiate staphylococci further from micrococci. Usually staphylococci are lysed by lysostaphin but not by lysozyme, and they can grow in the presence of 0.4 |g/mL of erythromycin. Micrococci are not lysed by lysostaphin, may be lysed by lysozyme, and will not grow in the presence of erythromycin. In a deep stab culture, micrococci will grow at the surface, whereas most staphylococci grow throughout the agar. Staphylococci will grow and produce acid from glucose and mannitol anaerobically, whereas micrococci do not. Staphy-lococcal cells contain teichoic acids in the cell wall and do not contain aliphatic hydrocarbons in the cell membrane, whereas the reverse in true with micrococci. Further, the G + C content (mole percentage) of staphylococci is 30-40% and 66-75% for micrococci. Testing for some of these features is difficult, time consuming, and expensive and usually not required for routine detection and enumeration procedures. Several commercially available miniaturized systems have been developed to speciate staphylococci. A number of commercially available nucleic acid and serological-based assays for the detection and confirmation of S. aureus are listed in Table 3.
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