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MANNITOL SALT AGAR

Code: CM0085

A selective medium for the isolation of presumptive pathogenic staphylococci. Most other bacteria are inhibited, with the exception of a few halophilic species.

* Adjusted as required to meet performance standards

Directions
Suspend 111g in 1 litre of distilled water and bring to the boil to dissolve completely. Sterilise by autoclaving at 121°C for 15 minutes.

Description
A selective medium prepared according to the recommendations of Chapman¹ for the isolation of presumptive pathogenic staphylococci. Most other bacteria are inhibited by the high salt concentration with the exception of some halophilic marine organisms. Presumptive coagulase-positive staphylococci produce colonies surrounded by bright yellow zones whilst non- pathogenic staphylococci produce colonies with reddish purple zones.

Mannitol Salt Agar is recommended for the detection and enumeration of coagulase-positive staphylococci in milk², in food³ and other specimens⁴.

Oxoid Mannitol Salt Agar has been used for the examination of meat or fish^5,6,7,8,9.

The addition of 5% v/v Egg Yolk Emulsion SR0047 to Mannitol Salt Agar enables the lipase activity of staphylococci to be detected as well as mannitol fermentation¹⁰. The high concentration of salt in the medium clears the egg yolk emulsion and lipase production is detected as a yellow opaque zone around colonies of staphylococci which produce this enzyme.

Technique
Heavily inoculate the Mannitol Salt Agar plate and incubate for 36 hours at 35°C or for 3 days at 32°C - the latter is recommended by the APHA³.
Presumptive coagulase-positive staphylococci produce colonies with bright yellow zones whilst coagulase-negative staphylococci are surrounded by a red or purple zone. Pick off suspect colonies and subculture in a medium not containing an excess of salt (e.g. Nutrient Broth No.2 CM0067) to avoid interference with coagulase or other diagnostic tests.

Storage conditions and Shelf life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
Store the prepared plates at 2-8°C.

Appearance
Dehydrated medium: Straw-pink coloured, free-flowing powder
Prepared medium: Red coloured gel

Quality control

* This organism is available as a Culti-Loop®

Precautions
A few strains of Staphylococcus aureus may exhibit a delayed fermentation of mannitol. Negative plates should be re-incubated overnight before discarding.
Presumptive Staphylococcus aureus must be confirmed with a coagulase test (Staphylase Test DR0595).

References
1. Chapman G. H. (1945) J. Bact. 50. 201-203.
2. Davis J. G. (1959) `Milk Testing’ 2 ed., Dairy Industries Ltd., London.
3. American Public Health Association (1966) `Recommended Methods for the Microbiological Examination of Foods’ 2nd Ed., APHA Inc., New York.
4. Silverton R. E. and Anderson M. J. (1961) `Handbook of Medical Laboratory Formulae’ Butterworths, London.
5. Barnes Ella M. and Shrimpton D. H. (1957) J. Appl. Bact. 20. 273-285.
6. Thornley Margeret J. (1957) J. Appl. Bact. 20. 286-298.
7. Bain Nora, Hodgkiss W. and Shewan J. M. (1958) DSIR, Proc. 2nd Internat. Symp. Food Microbiol., 1957, HMSO, London, pp. 103-116.
8. Spencer R. (1961) J. Appl. Bact. 24. 4-11.
9. Eddy B. P. and Ingram M. (1962) J. Appl. Bact. 25. 237-247.
10. Gunn B. A., Dunkelberg W. E. and Creitz J. R. (1972) Am. J. Clin. Path. 57. 236-238.