甘露醇盐琼脂（Mannitol salt agar）简称MSA，为繁殖细菌的一种营养份，一般容于培养皿，此营养份含较高的盐分（7.5～10% NaCl），因此对于喜好盐成份的细菌例如葡萄球菌（Staphlococcus）的繁殖最佳。而葡萄球菌之外的细菌的生长多半被抑制。

甘露醇盐琼脂内含指示剂酚红（Phenol red），会因酸碱值（pH）而改变颜色。可以酵解甘露醇（Mannitol）成分的细菌，其菌落周围将形成黄色。其中金黄色葡萄球菌（S. aureus）能酵解甘露醇，培养基不发生改变﹝如图﹞。

Mannitol salt agar or MSA is a commonly used selective and differential growth medium in microbiology. It encourages the growth of a group of certain bacteria while inhibiting the growth of others. This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time. It contains a high concentration (~7.5%-10%) of salt (NaCl), making it selective for Gram positive bacterium Staphylococci (and Micrococcaceae) since this level of NaCl is inhibitory to most other bacteria. It is also a differential medium for mannitol-fermenting staphylococci, containing carbohydrate mannitol and the indicator phenol red, a pH indicator for detecting acid produced by mannitol-fermenting Staphylococci. Staphylococcus aureus produce yellow colonies with yellow zones, whereas other Staphylococci produce small pink or red colonies with no colour change to the medium. If an organism can ferment mannitol, an acidic byproduct is formed that will cause the phenol red in the agar to turn yellow. It is used for the selective isolation of presumptive pathogen (pp) Staphylococci.