Cultural characters

CULTURAL CHARACTERS
  • Colonies on nutrient agar are circular, convex, glistening, having entire edges and pigmented.
  • The colour varies from white to lemon yellow.
  • On Sheep / Ox blood agar staphylococci produces a , b and d haemolysins.
    • a - Haemolysin produces a medium sized zone of clear haemolysis around the colonies in rabbit and sheep blood agar.
    • The margins are not sharply demarcated and the haemolysis is not being increased on refrigeration.
    • b - Haemolysin produces a larger zone of incomplete haemolysis at 37°C, with sharply demarcated edges. After storing the blood plate under refrigeration at 4°C, the hemolytic zone becomes clear in sheep, goat and cattle blood agar. This phenomenon is called as hot cold lysis.
    • d - haemolysin acts on the blood of all species and produces a narrow zone of clear haemolysis around the colonies.

Mannitol Salt Agar

  • A common medium used for the isolation of pathogenic staphylococci is the Mannitol Salt Agar.
  • The high salt concentration of this medium inhibits the growth of most other organisms.
  • Pathogenic staphylococci not only grow on the medium, but they also produce acid from it.
  • This acid production turns the pH indicator from red to yellow. Non-pathogenic staphylococci can grow on the medium but produce no acid from it.
    • Staphylococcus aureus rapidly ferment maltose and the acid metabolic product cause the pH indicator (bromocresol purple) to change the medium and colonies to yellow.
    • Staphyloccous intermedius gives weak and delayed reaction.
    • Staphylococcus hyicus does not ferment maltose but attacks peptone and produces alkaline reaction (a deeper purple) around the colonies.
      • On Baird-Parker medium , Staphylococcus aureus produces black colonies due to the reduction of tellurite and halo formation due to proteolysis and lipolysis.
      • This medium contains lithium chloride and tellurite to inhibit contaminating flora and a suspension of yolk to demonstrate the presence of lipases, especially the yolk factor.
      • Staphylococcus aureus forms (black reduction of the tellurite) shining colonies, which are surrounded by clear areas (2-5mm wide).
Last modified: Monday, 4 June 2012, 6:20 AM