Mannitol salt agar (MSA) is a growth medium used to culture Staphylococcus bacteria. It contains mannitol, salt, agar, and other ingredients that promote growth and change color when pathogenic strains are present. MSA is used by doctors and researchers to identify and study bacteria.
Mannitol salt agar (MSA) is a common growth medium used in laboratories for culturing certain types of bacteria, especially members of the genus Staphylococcus. It is a sterile, gelatinous substance and is usually placed in small, shallow, disc-shaped containers called petri dishes before a bacterium 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 identify the bacteria and study them.
The combination of ingredients in mannitol salt agar is designed to manage the culture of Staphylococcus bacteria. Mannitol is a type of sugar and some types of staph will feed and metabolize it, excreting acidic by-products which will change the pH of the mixture. This will involve activating another ingredient in the mixture, a pH indicator, usually phenol red, causing it to change color from red to yellow. This is helpful because generally only pathogenic or disease-causing bacteria will metabolize or ferment mannitol.
Salt is added to create a medium in which only staph and some related bacteria will grow and thrive. Mannitol salt agar has a high salt concentration, usually 7.5% by weight. Most other bacteria cannot tolerate such conditions or, if they survive, will slowly grow and multiply. Staph bacteria thrive in salty conditions and multiply rapidly.
Agar, which is the basis of the mixture, is a gelatinous 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 bacteria and gives the mixture its gel-like, semi-solid consistency. The blend also contains other ingredients such as casein, beef extracts and processed animal proteins. These, together with mannitol, feed the bacteria. MSA is typically sold as a powder that is mixed with laboratory-purified, distilled water and then sterilized.
Doctors use mannitol salt agar primarily to grow a sample from a patient or equipment to determine if dangerous Staphylococcus bacteria are present. The sample is introduced into the MSA and the Petri dish is stored in conditions of temperature and humidity that favor its growth. Staph bacteria, if present, grow rapidly 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 grow these types of bacteria for study.
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