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Staphylococcus

Staphylococcus is a genus of gram-positive bacteria. Under the microscope they appear round (cocci), and form in grape-like clusters (staphyl is Greek for bunch of grapes).

There are many species of staphylococci, most are completely harmless, and reside normally on the skin.

Staphylococcus aureus

One harmful species is Staphylococcus aureus, which can infect wounds.

Morphology

Staphylococcus aureus is a bacterium that causes illnesses ranging from minor skin infections and abscesses, to life-threatening diseases such as pneumonia, meningitis and septicemia.

Cultural characters

More characteristic is its appearance when grown out on agar plates. It appears as large, round golden-yellow (which is where the name aureus comes from) colonies, with beta-haemolysis of blood agar. They are facultative anaerobes.

Antigenic composition

Antigens are one of the mechanisms by which staphylococci can cause disease (pathogenesis). Determination of antigenic composition also helps in the classification of staphylococcus strains.

Important cell wall components and antigens include:

  1. Peptidoglycan
  2. Teichoic acid
  3. Protein A

Pathogenesis

Staphylococci can produce disease (pathogenesis) by their ability to multiply and invade tissues and also through the production of extracellular enzymes and toxins which include:

Coagulase: An enzyme which produces coagulation of plasma around the surface of the staphylococcus and fibrin deposition around the lesions of infection. This protects the staph bacteria from body defense mechanisms such as phagocyctosis (a process by which the body's immune cells 'eat up' the bacterium).

Haemolysins: They cause lysis of red blood cells in can be demonstrated in blood agar.

Exfoliative toxin: The toxin responsible for scalded skin syndrome. The toxin causes desquamation especially in children.

Toxic shock syndrome toxin: A superantigen that causes toxic shock syndrome by release of large amounts of cytokines from immun T cells and macrophages.

Enterotoxins: They are responsible for causing staphylococcal food poisoning.

Others: Catalase, leucocidine, hyaluronidase, staphylokinase, proteinase and lipase.

Diseases caused by staphylococcus

Each year some 500,000 patients in American hospitals contract a staphylococcal infection. By changing its chemical makeup slightly to evade attack, S. aureus has become resistant to many commonly used antibiotics. In 1997, physicians were alarmed to encounter staph strains that resist even vancomycin, which used to work when all else failed.

Focal suppuration

  • Superficial infections (folliculitis, carbuncles, boils, abscess formation).
  • Deep seated infections (osteomyelitis, bronchopneumonia, empyema, endocarditis, meningitis).
  • Bacteremia
  • Hospital acquired infections

Toxigenic disease

  • Food poisoning
  • Toxic shock syndrome
  • Scalded skin syndrome

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