Mannitol Salt Agar


What is this medium used for?


Some bacteria are resistant to high salt concentrations in their environment, while others are unable to grow under such conditions. Salt in the surrounding environment draws water from cells, a natural consequence of osmosis and diffusion. Those bacteria able to deal with the stress placed on the cell in a high-salt environment are able to grow under such conditions. Those unable to do so are dehydrated and cannot grow. This allows us to include salt in a medium to produce an excellent selective medium. Mannitol salt agar is selective in allowing bacteria capable of growth in 7.5% NaCl while preventing the growth of salt sensitive microbes.


Additionally, mannitol salt agar contains the sugar mannitol. As with the other sugar fermentation tests, we are able to determine the ability of a microbe to metabolize mannitol by observing color changes of the pH indicator phenol red in the medium. If the microbe metabolizes mannitol, phenol red is changed to yellow by the acidic products.


Mannitol salt agar is thus selective in prohibiting the growth of bacteria unable to tolerate 7.5% NaCl and differential in the use of mannitol by those that can grow. Most frequently, it is used to select for Gram positive cocci, particularly Staphylococci.



How is growth in 7.5% NaCl determined? 


Growth in 7.5% NaCl is easily determined by observing an inoculated mannitol salt agar plate after 24 hours of incubation for the presence of colonies.



How is mannitol fermentation on mannitol salt agar determined? 


Bacteria growing on mannitol salt agar are positive for mannitol fermentation if the culture medium under their colonies is changed from its normal red color to yellow, indicating acidic growth products. Bacteria unable to use mannitol usually change the medium to a magenta color in the vicinity of growth. This color change indicates medium ingredients other than mannitol supported growth.


Note: It is unwise to generalize that bacteria incapable of growing on mannitol salt agar are automatically negative for mannitol fermentation. Bacteria inhibited by the salt cannot grow to use the mannitol present. In the case of bacteria not growing on mannitol salt agar, determination of the ability to metabolize mannitol should be performed using phenol red mannitol broth.



What is the content of this medium?  


Mannitol salt agar is a nutrient agar to which has been added 7.5% NaCl as an inhibitor, phenol red as an indicator, and 0.5-1.0% mannitol. It is autoclaved and dispensed into sterile petri plates. Once it solidifies, it is ready for use.



How are the tests for growth on 7.5% NaCl and mannitol fermentation performed?  


For information on how mannitol salt agar is used to determine growth on 7.5% NaCl, refer to the growth on 7.5% NaCl test


For information on how mannitol salt agar is used to determine mannitol fermentation for bacteria able to grow on 7.5% NaCl, or on how mannitol fermentation is determined using phenol red mannitol broth, refer to the mannitol fermentation test.