I'm currently conducting a study on Lactic Acid Bacteria. I'd like to know if any other groups of bacteria other than LAB can grow on MRS agar.
Bifidobacterium spp.mostly grow on MRS medium. Actually the principle behind it is that MRS is a medium where Dextrose present in it acts as a fermentable carbohydrate, where as , Sodium Acetate and Ammonium Citrate are selective agents. When an organism is inoculated into the MRS medium, those which are able to ferment the dextrose sugar and overcome the selective agents are capable of exhibiting luxuriant growth on it.
Other microbial groups can grow on MRS agar. LAB bacteria mainly grow on MRS agar, in addition to LAB Yeast and Mold can also grow on MRS agar. Therefore in order to inhibit the the growth of other microbial groups on MRS agar you should have to add sorbic acid as a supplement while preparing MRS agar media. Because sorbic acid has the potential to inhibit the growth of Total fungus (yeast and mold) by disrupting their tRNA system.
MRS agar is an elective medium at best, not a really selective one. It favors the growth of lactic acid bacteria, but it supports the growth of many others. Many streptococci (which are not all members of the LAB), will grow in MRS. Adding organic acids and antibiotics to the original formula of the medium has been done to increase its selectivity, but the concomitant increase in selectivity comes at the price of worsened performance regarding LAB growth. Another selectivity tool is the incubation conditions (temperature, CO2-enriched atmosphere), which will decrease the chances of growth of other bacteria, while favoring LAB. Still, when reporting counts performed in MRS, one should be aware of its low selectivity.
MRS Agar and Broth were designed to encourage the growth of the `lactic acid bacteria' which includes species of the following genera: Lactobacillus, Streptococcus, Pediococcus and Leuconostoc. All these species can produce lactic acid in considerable amounts.
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