Mannitol Salt Agar

LAB 7

Description

A medium for staphylococci which is selective because the high sodium chloride level inhibits most other species with the exception of halophilic vibrios. The majority of Staphylococcus aureus ferment mannitol producing yellow colonies, occasional strains of coagulase negative staphylococci may also ferment mannitol. It is necessary to confirm the identity of presumptive S. aureus colonies by other means e.g. coagulase, protein A, DN'ase, thermonuclease or latex agglutination. This medium can be made selective for methicillin resistant S. aureus by the addition of X207 (methicillin supplement).

Description

An osmotically controlled solution which is an alternative to, and a replacement for, 1/4 strength Ringer's LAB 100. The presence of a low level of peptone lessens the physiological shock normally experienced by bacterial cells when they are introduced to a diluent such as Ringer's Solution. The level of peptone is such that multiplication of the organisms is not possible in the time in which the sample will be present in the diluent (1-2 hours). This formula is recommended by ISO 6887: BS5763.

Method for reconstitution

Weigh 108 grams of powder, disperse in 1 litre of deionised water. Allow to soak for 10 minutes, mix by swirling. Sterilise by autoclaving for 15 minutes at 121˚C. Cool to 47˚C before pouring into Petri dishes.

Appearance: Red, clear gel.

pH: 7.4 ± 0.2

Method for reconstitution

Dissolve 9.5 grams of powder in 1 litre of deionised water, heat gently to dissolve then distribute into final containers. Sterilise by autoclaving at 121˚C for 15 minutes.

Appearance: Clear fluid.

Storage of Prepared Medium: Capped containers - up to 3 months at 15-20˚C in the dark.

References

Straka, R.P. and Stokes, J.L. (1957). Rapid destruction of bacteria in commonly used diluents and its elimination. Appl. Microbiol. 5: 21-25.

ISO 6887. (1983). Microbiology-General guidance for the preparation of dilutions for microbiological examination. BS5763 Part 6. Preparation of dilutions.