MRS agar

De Man–Rogosa–Sharpe agar, often abbreviated to MRS, is a selective culture medium designed to favour the luxuriant growth of Lactobacilli for lab study. Developed in 1960, this medium was named for its inventors, Johannes Cornelis de Man [Wikidata], Morrison Rogosa [Wikidata], and Margaret Elisabeth Sharpe [Wikidata]. It contains sodium acetate, which suppresses the growth of many competing bacteria (although some other Lactobacillales, like Leuconostoc and Pediococcus, may grow). This medium has a clear brown colour.^[1]

MRS agar typically contains (w/v):^[2]

• 1.0% peptone
• 1.0% beef extract
• 0.4% yeast extract
• 2.0% glucose
• 0.5% sodium acetate trihydrate
• 0.1% polysorbate 80 (also known as Tween 80)
• 0.2% dipotassium hydrogen phosphate
• 0.2% triammonium citrate
• 0.02% magnesium sulfate heptahydrate
• 0.005% manganese sulfate tetrahydrate
• 1.0% agar
• pH adjusted to 6.2 at 25 °C

The yeast/meat extracts and peptone provide sources of carbon, nitrogen, and vitamins for general bacterial growth. The yeast extract also contains vitamins and amino acids required by Lactobacilli. Polysorbate 80 is a surfactant which assists in nutrient uptake by Lactobacilli. Magnesium sulfate and manganese sulfate provide cations used in metabolism.

• MacConkey agar (culture medium designed to grow Gram-negative bacteria and differentiate them for lactose fermentation).