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 5.7 +/- 0.1. In these rare cases adjustment of the medium using acetic acid or sodium hydroxide is recommended.
