MRS Agar is a selective medium, based on the formulation developed by de Man, Rogosa and Sharpe. They wanted to provide a medium that would support the good growth of lactobacilli in general, but of those strains which showed poor growth in existing media such as L. brevis and L. fermenti, replacing a variable product (tomato juice). The medium is apt for the growth of lactic acid bacteria, including Lactobacillus, Pediococcus and Leuconostoc.

Ammonium citrate, at a low pH, inhibits most microorganisms, but allows the growth of Lactobacilli. Dipotassium phosphate and Sodium acetate are buffer agents to maintain a low pH. Tween 80 is an emulsifier. Manganese and Magnesium sulphates are sources of ions and sulphate. Bacteriological peptone and Beef extract provide nitrogen, vitamins, minerals, and amino acids essential for growth. Yeast extract is a source of vitamins, particularly the B-group. Dextrose is the fermentable carbohydrate. Bacteriological agar is the solidifying agent. Lactobacilli are microaerophilic and generally require layered plates for aerobic cultivation on solid media. Submerged or surface colonies may be compact or feathery, and are small, opaque, and white.