Mannitol Salt Agar Mannitol Salt Agar is a medium used for isolating and differentiating both pathogenic and non-pathogenic Staphylococci. The high concentration of Sodium Chloride makes this medium selective for Gram-positive bacteria. D-Mannitol is the carbohydrate source, while Beef Extract and the enzymatic digest of Casein and Animal Tissue supply amino acids, peptides, and other nitrogenous sources. On this medium, Staphylococcus aureus strains produce large yellow colonies surrounded by yellow zones while non-pathogenic, mannitol-fermenting Staphylococci form smaller, red colonies due to the color shift of the pH indicator Phenol Red.  M13-114 Product Information Sheet M13-114 Safety Data Sheet