MRS agar (deMan, Rogosa, Sharpe) | Principle | Preparation | Interpretation

Ⅰ. Overview

Ⅱ. Preparation / Composition

Suspend the components, dehydrated powder, in water (70g in 1000ml of purified/distilled water). Bring the medium to a boil with constant stirring for at least 1 minute.
Sterilize in autoclave at 121°C for 15 minutes, cool to 45-50°C. Adjust pH aseptically before pouring.

◈ MRS agar can be modified (acid pH, acid pH + sorbic acid, neutral pH) and/or added different supplements depending on the flora sought (thallium acetate, sodium acetate, sorbic acid, acetic acid, sodium nitrate, cycloheximide and polymyxin.).

Ⅲ. Principle of MRS agar

Nutrition is provided by a blend of peptones, glucose, beef and yeast extracts

Polysorbate 80 provides the fatty acids necessary for the metabolism of lactobacilli.

The selectivity against streptococci and molds is provided by ammonium citrate and sodium acetate. Used at low pH, ammonium citrate allows the growth of lactobacilli while inhibiting a number of other groups of microorganisms.

Magnesium sulphate and manganese sulphate provide essential ions for the multiplication of lactobacilli.

Les phosphates assurent une bonne action tampon dans le milieu.

Ⅳ. Utilization

Put 1 ml of diluted sample in sterile Petri dishes, and add the MRS agar (15 - 20 ml) melted at 45°C, then mix well.

After solidification, add within 15 minutes, a new layer of MRS agar (15 mL), uninoculated, on the surface and allow to solidify. It is important to have an adequate humid atmosphere above the agar, so that it does not dry out during incubation (this would concentrate the selective factors on the surface and make the medium inhibitory).

Incubate for 3 days at 25°C or 2 days at 37°C, depending on the type of flora sought, under an atmosphere enriched with 5% CO2.

Ⅴ. Interpretation

◈ Lactobacilli appear as large white colonies embedded in or on MRS agar. Due to the possibility of development of germs other than lactic acid bacteria, it may be necessary, in some cases, to verify under the microscope that it is indeed Gram-positive, non-spore forming bacilli.