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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Staphylococci

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Morphology and Structure - Staphylococcus aureus

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.

Staphylococcus aureus

BACTERIOLOGY

MORPHOLOGY AND STRUCTURE

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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