MRS Agar, 500g

MRS Agar is a medium for the enumeration of mesophilic lactic acid bacteria according to ISO 15214:1998, and is not intended for use in the diagnosis of disease or other conditions in humans.

MRS Agar was originally developed in 1960 by de Man, Rogosa & Sharpe for the cultivation and enumeration of Lactobacillus spp. from various sources and is intended as a substitute for Tomato Juice Agar. This original formulation has been adapted and adjusted to pH 5.7 according to ISO 15214:1998.

Nutrition is provided by enzymatic digest of casein, glucose, meat & yeast extracts whilst polyoxyethylenesorbitan monooleate, magnesium and manganese sulphates act as growth stimulants. Selectivity against streptococci & molds is provided by ammonium citrate and sodium acetate. Used at low pH, ammonium citrate allows growth of lactobacilli whilst inhibiting a number of other organism groups.

Occasionally, sterilization of this medium at 121°C can cause the pH to fall outside of the specified pH limits of 5.7 +/- 0.1. In these rare cases, adjustment of the medium using acetic acid or sodium hydroxide is recommended.

Typical Formulation

Formula Liter
Enzymatic Digest of Casein 10.0 g/L
Meat Extract 10.0 g/L
Yeast Extract 4.0 g/L
Triammonium Citrate 2.0 g/L
Sodium Acetate 5.0 g/L
Magnesium Sulphate Heptahydrate 0.2 g/L
Manganese Sulphate Tetrahydrate 0.05 g/L
Dipotassium Hydrogen Phosphate 2.0 g/L
Glucose 20.0 g/L
Polyoxyethylenesorbitan Monooleate 1.08 g/L
Agar 15.5 g/L

Preparation

Suspend 70 grams of the medium in one litre of purified water.
Heat with frequent agitation and boil for one minute to completely dissolve the medium.
Autoclave at 121 ºC for 15 minutes.
Cool to 45–50 ºC.