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
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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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.