Mode of Action
The MRS culture media contain polysorbate, acetate, magnesium and manganese, which are known to act as special growth factors for lactobacilli, as well as a rich nutrient base. As these media exhibit a very low degree of selectivity, Pediococcus and Leuconostoc species and other secondary bacteria may grow on them.
Typical Composition (g/litre)
Peptone from casein 10.0; meat extract 10.0; yeast extract 4.0; D(+)-glucose 20.0; dipotassium hydrogen phosphate 2.0; Tween® 80 1.0; di-ammonium hydrogen citrate 2.0; sodium acetate 5.0; magnesium sulfate 0.2; manganese sulfate 0.04; agar-agar 14.0.
Suspend 68.2 g MRS Agar/litre, autoclave 15 min at 121°C (or 15 min at 118 °C). Autoclavation at 118 °C result in better growth of Bifido bacterium spp.
pH: 5.7 ± 0.2 at 25 °C.
The plates filled into tubes are clear and brown.
Experimental Procedure and Evaluation
If necessary, homogenize the sample material. Inoculate the MRS Agar with this material or with the original sample; it is best to use the pour-plate method.
Incubation: up to 3 days at 35 °C or up to 5 days at 30 °C, if possible incubate the plates in a CO2 enriched atmosphere in an anaerobic jar.
Do not allow the surface of the plates to dry as this causes the acetate concentration to increase at the surface, which inhibits the growth of lactobacilli.
Determine the bacterial count. Identify the lactobacilli by the methods proposed by SHARPE (1962) and SHARPE et al. (1966).
For further methods of differentiation and identification see ROGOSA et al. (1953), ROGOSA and SHARPE (1959) and DAVIS (1960).