These may be marine, violacein-producing strains of Janthinobacterium spp. or Duganella spp.

What morphology does these microbes have under a light microscope?

May be salmonella

@Pawel,

Thank you! It looks like Janthinobacterium indeed. One flagella. I have isolated recently pigmented Janthinobacterium spp. from potato tubers, and will compare them.

Alex

I can also suggest that when growing pigmented bacteria (e.g. Janthinobacterium spp.), environmental stress is an important factor "forcing" microorganisms to maintain pigment formation. In too favorable in vitro conditions, these bacteria often lose their ability of pigmentation after passaging. The presence of low concentrations of antibiotics or other stressors in culture media can induce pigmentation in these microbes.

@Pawel, I noticed that the pigmentation was weaker at plates of higher dilution and smaller number of other bacteria - pigment production was a reaction to presence of other species on the same plate.