Chapter 45, Problem 1Q

Why is mannitol salt agar used as a selective medium for normal skin microbiota?

Summary Introduction

To analyze:

The reason for the usage of mannitol salt agar medium as selective medium for normal skin microbiota.

Introduction:

Many microbes lack the efficiency to reside on normal skin as its keratin containing cells making up a dry layer does not enable certain microbes to adhere and multiply. Another reason is that the oil glands secrete sebum that inhibits the growth of bacterium and there is a high concentration of salt. Bacterium that is tolerant to the salty environment only can survive on skin, hence identifying these salt tolerant bacterium requires a special medium. Mannitol salt agar comes as the answer for that.

Explanation of Solution

Mannitol salt agar comprises of majority of Sodium chloride, casein digest, animal tissue digest, beef extract, D-mannitol, phenol red and agar. As it contains high concentration of salt approximately 7.5%-10% it favours the growth of bacterium like Staphylococcus. It acts as a differential medium bacterium that ferments mannitol. Phenol red which is the pH indicator helps to identify acid producing bacterium.

Conclusion

Thus mannitol salt agar is used as selective media for skin microbes.