Mannitol salt agar or MSA is a commonly used growth medium

Growth medium

A growth medium or culture medium is a liquid or gel designed to support the growth of microorganisms or cells, or small plants like the moss Physcomitrella patens.There are different types of media for growing different types of cells....

 in microbiology

Microbiology

Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...

. 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

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 (NaCl), making it selective for Staphylococci

Staphylococcus

Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....

(and Micrococcaceae

Micrococcaceae

The family Micrococcaceae includes bacterial genera of Gram positive cocci that inhabit the air and skin such as Micrococcus luteus.-External links:* * J.P. Euzéby's...

) since this level of NaCl is inhibitory to most other bacteria.