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In growing cultures, the cells of S. aureus are uniformly Gram-positive and regular in size, fitting together in clusters with the precision of pool balls. In older cultures, in re- solving lesions, and in the presence of some antibiotics, the cells often become more vari- able in size, and many lose their Gram positivity.
The cell wall of S. aureus consists of a typical Gram-positive peptidoglycan interspersed with molecules of a ribitol-teichoic acid, which is antigenic and relatively specific for S. aureus.
In most strains, the peptidoglycan of the cell wall is over-laid with surface proteins; one protein, protein A, is unique in that it binds the Fc portionof IgG molecules, leaving the antigen-reacting Fab portion directed externally. This phe-nomenon has been exploited in test systems for detecting free antigens . It probably contributes to the virulence of S. aureus by interfering with opsonization.
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