Question

Differentiate between the terms selective and differential as they apply to mannitol salt agar

What would be the effect of removing the sodium chloride from mannitol salt agar plates?

Answer 1

Selective and differential media are used for the isolation or for the identification of the particular organisms.

Selective media allow certain types of organisms to grow and inhibit the growth of other organisms. Mannitol salt agar is a selective medium used for the isolation of pathogenic staphylococci. The medium besides mannitol, phenol red indicators contains high salt concentration (7.5 sodium chloride) which inhibits the growth of almost all other bacteria (gram-negative) other than staphylococci. So it is used to isolate staphylococci as these can tolerate high saline levels.

Differential media are used to differentiate closely related organisms or groups of organisms. Mannitol salt agar is differential as it is also used to determine if the bacteria can ferment mannitol or not. If bacteria can ferment mannitol a yellow halo is formed around the colony because of acid production indicating mannitol breakdown. So it is differential for fermentators and non-fermentators of mannitol.

The removal of NaCl from MSA would alter the sensitivity of the medium since the role of NaCl in the MSA medium is to inhibit the growth of gram-negative organisms. Therefore omitting NaCl causes non-staphylococcus genus to grow and may lead to false positives for non-staphylococcus which can ferment mannitol.