M.R.S. AGAR 500g

Description
The MRS formulation was developed by de Man, Rogosa and Sharpe¹ to replace a variable product (tomato juice) and, at the same time, provide a medium which would support good growth of lactobacilli in general, even those strains which showed poor growth in existing media. MRS medium is superior to the tomato juice medium of Briggs² and the meat extract tomato juice medium of de Man. It gives more profuse growth of all strains of lactobacilli, especially the difficult and slow growing strains of Lactobacillus brevis and Lactobacillus fermenti.

MRS Agar and Broth were designed to encourage the growth of the `lactic acid bacteria’ which includes species of the following genera: Lactobacillus, Streptococcus, Pediococcus and Leuconostoc. All these species can produce lactic acid in considerable amounts. They are Gram-positive, catalase and oxidase negative and are fastidious in their nutritional requirements. Growth is enhanced considerably by microaerobic conditions. Generally the `lactic acid bacteria’ show delayed growth and smaller colony size than other micro-organisms. They may be overgrown in non-selective media, especially if incubation is required for 2-4 days.

MRS medium is selective for lactobacilli but some growth of leuconostocs and pediococci may occur. Selectivity can be altered by pH adjustment. Lactobacilli will tolerate lower pH levels than streptococci (pH 5.0-6.5) with pediococci and leuconostocs growing best within this range. Inhibitors of the main groups of competitor microflora include thallous acetate, sodium acetate, sorbic acid, acetic acid, sodium nitrite, cycloheximide and polymyxin. These substances can be used at varying concentrations and combinations but, inevitably, a compromise has to be reached between selectivity and productivity of the organism(s) sought³. MRS Agar with sorbic acid has been described^3,4. This is MRS medium with its pH reduced to 5.7 and 0.14% sorbic acid added (equating to 0.2% potassium sorbate).

An evaluation of media for selective enumeration of Lactobacillus acidophilus and Bifidobacterium species showed that minor adjustments to the basic formula of MRS Agar can readily be made to optimise its performance for determining the content of Lactobacillus acidophilus and Bifidobacterium spp. in the presence of other lactic acid bacteria which are present in yoghurt⁵.

Lactobacilli are microaerophilic and generally require layer plates for aerobic cultivation on solid media. Submerged or surface colonies may be compact or feathery, and are small, opaque and white.

Specification