[Microbiology] Mannitol Salt Agar

Mannitol Salt Agar, Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory,

Purpose

Mannitol Salt Agar (MSA) is used for isolation and differentiation of pathogenic staphylococci, principally Staphylococcus aureus.

Principle

Mannitol Salt Agar contains the carbohydrate mannitol, 7.5% sodium chloride (NaCl), and the pH indicator phenol red. Phenol red is yellow below pH 6.8, red at pH 7.4 to 8.4, and pink above 8.4. The sodium chloride makes this medium selective for staphylococci since most other bacteria cannot survive in this level of salinity. The pathogenic species of Staphylococcus ferment mannitol (Figure 2-21)and produce acid, which turns the pH indicator yellow. Nonpathogenic staphylococcal species grow, but produce no color change. Refer to pages 71–73 and Figure A-5 in the Appendix for more information on fermentation. The development of yellow halos around the bacterial growth is presumptive evidence that the organism is a pathogenic Staphylococcus (usually S. aureus). Good growth that produces no color change is presumptive evidence for nonpathogenic Staphylococcus (Figures 2-22 and 2-23). With few exceptions, organisms that grow poorly on the medium are not staphylococci.

2-21 MANNITOL FERMENTATION WITH ACID END-PRODUCTS The organic acids produced lower the pH and turn the medium yellow

2-22 MANNITOL SALT AGAR MSA inoculated with

Staphylococcus aureus (top) and S. epidermidis (bottom).

(Note: Some strains of S. epidermidis are inhibited by

this medium). The yellow halo around S. aureus is due

to mannitol fermentation with acid end products.  

2-23 MANNITOL SALT AGAR STREAKED FOR ISOLATION

MSA inoculated with Staphylococcus aureus and Staphylococcus

epidermidis. The growth shown in this photo is typical of the two

species on this medium; the colonies of S. epidermidis are small

and red whereas those of S. aureus are slightly larger and yellow  

Suggested Reading

- Michael J. Leboffe & Burton E. Pierce. A Photographic Atlas for the Microbiology Laboratory 4th edition 2011