What Is Mannitol Salt Agar?

Mannitol salt agar (MSA) is a common culture medium used in laboratories for culturing certain types of bacteria, particularly members of the genus Staphylococcus. It is a sterile, jelly-like substance and is usually put in small, shallow, disk-shaped containers called petri dishes before a bacteria is introduced. Mannitol salt agar contains several ingredients, most of which promote the development and growth of some bacteria but retard the growth of others. This helps doctors and researchers to identify bacteria as well as to study them.

The combination of ingredients in mannitol salt agar is designed to manage the culturing of Staphylococci bacteria. Mannitol is a type of sugar, and certain types of Staphylococci will feed on and metabolize it, excreting acidic byproducts that will change the pH of the mixture. This will result in the activation of another ingredient in the mixture, a pH indicator, usually phenol red, which causes it to change color from red to yellow. This is useful because generally only pathogenic, or sickness-causing, bacteria will metabolize, or ferment, the mannitol.

Salt is added to create a medium in which only Staphylococci and certain related bacteria will grow and thrive. Mannitol salt agar has a high concentration of salt, usually 7.5% by weight. Most other bacteria cannot tolerate such conditions, or if they survive at all, will grow and multiply slowly. Staphylococci bacteria thrive in salty conditions and multiply quickly.

Agar, which is the base of the mixture, is a jelly-like substance derived from certain species of algae that grow in the world's oceans. Chemically it is a carbohydrate and serves primarily as a growth medium or home for the bacteria and gives the mixture its jelly-like, semi-solid consistency. The mixture also contains other ingredients like casein, beef extracts and treated animal protein. These, along with the mannitol, feed the bacteria. MSA is typically sold as a powder which is mixed with distilled, purified water in the laboratory and then sterilized.

Doctors use mannitol salt agar primarily to culture a sample from a patient or equipment to determine if dangerous Staphylococci bacteria are present. The sample is introduced to the MSA and the petri dish is kept under conditions of temperature and humidity that favor growth. Staphylococci bacteria, if present, will grow quickly and a color change from red to yellow indicates that a pathogenic, or disease-causing, strain is present. Researchers and microbiologists also use MSA to culture these kinds of bacteria to study them.