What is mannitol salt test for and whats a positive test result?

Answer 1

Mannitol Salts agar is a selective and differential media used to identify Staphylococcus sp. The media is selective due to the high salt concentration which prevents all but halophiles from growing. The media is differential due to the presence of mannitol and the indicator Phenol Red. Staphylococcus aureus can ferment mannitol and produce lactic acid as a result. The media contains an indicator called Phenol Red which turns yellow in low pH environments. This results in a yellow halo around the S.aureus colonies. Staphylococcus epidermidis cannot ferment mannitol and so the colour of the media around these colonies does not change. Certain vibro species and other halophiles may grow on these plates some of these appear pink and some may be able to ferment mannitol leading to false positives. S.epidermidis is a normal commensal organism and grows on the skin. S.aureus is a potential pathogen, antibiotic strains exist and can be hospital accquired infections. Search for "MRSA" for more information.