One should not expect any selective medium to be absolute. It is the responsibility of the microbiologist to confirm identiy of isolates. Lactobacilli are very easily diffrentiated from fungi. MRS may also allow the growth of some Pediococcus and Leuconostoc spp. isolates.

Hello, it usual picture when you want to isolate lactic acid bacteria from samples with yeast and mould content. The MRS is reach medium with glucose which is ideal for yeast and mould grow. Normally of you have high titre of LAB in sample they will eliminate other microorganisms grow by lactic acid formation. But in your case better to use antifugal substances When seeding samples with high probability of row any yeast and mould use Nistatin (100-200U/mL) to avoid yeast and fungus growth and it worked well or cyclohexamide which also working well.

This Answer from @Frances Ann Draughon on RG are related and introducing many factors for optimising MRS media, or even the replacement with M17 media:

https://www.researchgate.net/post/How_can_I_modify_MRS_culture_media_to_prevent_the_growth_of_yeasts_and_other_non-lab_bacteria

All the best👍

I suggest you should be using 0.5% CaCO3 added in the MRS medium to initially classify organic acid-producing bacteria.