staphylococcus

Staphylococcus: Dr.Jawad Kadhim Tarrad
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Gram-positive cocci
Two genera of Gram-positive cocci are major medical important pathogens for human: Staphylococcus and Streptococcus.
Staphylococcus species are catalase-positive and appear in cluster cells under microscope.
Streptococcus species are catalase-negative and appear in chains cells under microscope.

Staphylococcus aureus:
Important properties of species:
• Gram-positive cocci usually arranged in grape-like clusters .
• Usually, it is uncapsulated, but some encapsulated.
• It is non-motile.
• It produces ?-hemolysis and coagulase.
• Golden yellow colonies.



Antigenic structures and virulence factors:
Structural components:
• Capsular polysaccharides; usually staphylococci are non-capsulated, but most pathogenic strains coated with microcapsule. It acts as antiphagocytic factor and make S.aureus more virulent.
• Protein-A: surface major protein of cell wall, binds strongly to IgG molecule at complement-binding site, thereby preventing the activation of complement. Moreover protein A may interfere with phagocytosis then protecting the organism from humoral immune clearance.
Enzymes:
• Coagulase: It causes clot through activating prothrombin to form thrombin by coagulase-reacting factor(CRF). Thrombin then catalyzes the activation of fibrinogen to form fibrin clot around bacteria and prevent the bacteria from phagocytosis.