This is a sterile emulsion of Egg Yolk in Saline containing Potassium Tellurite and is generally used as a selective differential agent in Baird Parker Medium. The complete medium is selective for Staphylococcus aureus as the Potassium Tellurite inhibits most coliform organisms and is also reduced by Staphylococcus aureus to tellurite giving typical black colonies on the Baird Parker Medium.