Contamination on MRS agar?

Dear all,
my colleagues and I are trying to cultivate few different Lactobacillus strains. I worked with the same strains for few months and never had problems cultivating them in MRS broth and agar (meaning, I never had contamination). Since then, we continued our work in a different lab but with same bacteria and we constantly have a present contamination on MRS agar. Furthermore, when we made overnight cultures in MRS broth, we few times noticed CO2 sound while opening Falcons in witch they were growing in and also a lot of foam/bubbles after vortexing cultures (see picture: https://postimg.org/image/uyiaeoep7/).

I don't have much experience with other microorganisms so I'm kindly asking You if you could tell me from the pictures what the possible problem could be and what would be the further steps?
We used the first Petri dish on the right as a control, so we can confirm that Petri dish and agar are properly sterilized (https://postimg.org/image/hgbe2e2jv/, https://postimg.org/image/4eppcje5n/).

Thank you in advance!