Mannitol Salt Agar

You might have heard about the different types of growth media or culture media that are used in laboratories. Such media can be both liquid or gel type and are used for cell culture as well as microbiological culture. In other words, such growth media are used for growing cells (derived from plants or animals) or microorganisms, like, bacteria, yeast, etc. Bacterial cultures are rich in nutrients that aid the growth of these microorganisms, in large colonies. These cultures are composed of the required amount of chemicals, moisture, nutrients, etc. While, some of these media are simple liquids or solids, like, peptone water, agar, etc., there are some nutrient-enriched ones, like, blood agar, chocolate agar, etc. Mannitol salt agar (MSA) is one such growth medium, which is meant for bacteria.

What is Mannitol Salt Agar

Ingredients of this growth medium include certain nutrients, agar, sodium chloride, phenol red, etc. To be more precise, this growth medium is made of enzymatic digest of casein (5 g per liter), agar (15 g per liter), D-mannitol (10 g per liter), phenol red (0.025 g per liter), sodium chloride (75 g per liter), enzymatic digest of animal tissue (5 g per liter) and beef extract (1 g per liter). The pH of this mixture is found to be 7.4 at a temperature of 25 degree Celsius.

Mannitol salt agar is selective as well as differential. It is selective because, this growth medium contains a good amount of salt, thereby inhibiting the growth of almost all bacteria, except those in genus Staphylococcus. So, it is used for isolating staphylococci bacteria. This medium is also differential, as it helps to identify the different species of staphylococci bacteria.

How Does Mannitol Salt Agar Test Work

As it has a high salt content, only staphylococci bacteria are found to grow in this medium. As stated above, mannitol salt agar is differential, as it helps in identifying the different species of staphylococci bacteria. This is done with the help of phenol red (a dye) that changes color as the organic wastes released by certain bacteria ferment mannitol. In case of staphylococci, non pathogenic species are not found to ferment mannitol and change color of the media. But, the pathogenic staph species, like, Staphylococcus aureus produce organic wastes that ferment mannitol and change the color of the medium to bright yellow. Such change of color does not happen in case of Staphylococcus epidermidis, that are normally found on the skin of humans. In case of coagulase positive staphylococci (that can coagulate blood), the mannitol salt agar results show change of color to yellow. Even though coagulase negative staphylococci are found as a part of normal flora in the human body, these bacteria may cause severe infections in immunocompromised people. Such bacteria grow as small pink or red colonies, without changing the color of the medium. Coagulase positive bacteria are found to grow as large yellow colonies and change the color of the medium to yellow.

This was only a brief overview about mannitol salt agar and its working mechanism. In general, it is said that typical pathogenic staphylococci ferment this medium and turns it yellow, whereas, non pathogenic ones form red or pink colonies.