Staphylococcus Aureus

Morphology and culturing. Fig. 4.1a shows the appearance of Gram-stained S. aureus. This is a facultative anaerobe that is readily cultured on normal nutrient mediums at 37 °C. Colonies as in Fig. 4.1b develop after 24 hours of incubation. Hemolytic zones are frequently observed around the colonies.

Fine structure. The cell wall consists of a thick layer of murein. Linear teichoic acids and polysaccharides are covalently coupled to the murein polysaccharide (Fig. 3.10, p. 154). The lipoteichoic acids permeating the entire murein layer are anchored in the cell membrane. Teichoic and lipoteichoic acids can trigger activation of complement by the alternative pathway and stimulate macrophages to secrete cytokines. Cell wall-associated proteins are bound to the peptide components of the murein. Clumping factor, fibronec-tin-binding protein, and collagen-binding protein bind specifically to fibrino-gen, fibronectin, and collagen, respectively, and are instrumental in adhesion to tissues and foreign bodies covered with the appropriate matrix protein. Protein A binds to the Fc portion of immunoglobulins (IgG). It is assumed that "false" binding of immunoglobulins by protein A prevents "correct" binding of opsonizing antibodies, thus hindering phagocytosis.

Staphylococcus aureus

— Staphylococcus aureus

Staph Aureas Surface Proteins
Fig. 4.1 a Gram staining of a pus preparation: Gram-positive cocci, some in grapelike clusters. Clinical diagnosis: furunculosis. b Culture on blood agar: convex colonies with yellowish pigment and porcelainlike surface.
Staph Aureas Surface Proteins

Extracellular toxins and enzymes. S. aureus secretes numerous enzymes and toxins that determine, together with the fine structures described above, the pathogenesis of the attendant infections. The most important are:

■ Plasma coagulase is an enzyme that functions like thrombin to convert fibrinogen into fibrin. Tissue microcolonies surrounded by fibrin walls are difficult to phagocytose.

■ a-toxin can have lethal CNS effects, damages membranes (resulting in, among other things, hemolysis), and is responsible for a form of dermone-crosis.

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Essentials of Human Physiology

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