I am conducting a study on bacterial morphotype screening, focusing on Salmonella enterica and Salmonella enterica subsp. enterica. The experimental setup involves using LB medium without NaCl, with the addition of Congo Red agar and Coomassie Brilliant Blue. The goal is to identify and characterize different morphotypes based on their growth characteristics and biofilm production.
I have used Congo Red (CR) agar plates without NaCl but with E. coli to analyze morphology of macrocolony biofilms as part of my doctoral thesis work. CR readily binds expolysaccharide cellulose and curli (which are biofilm matrix components in E. coli and Salmonella). Both of these proteins are regulated by CsgD and in both E. coli and Sallmonella it is expressed at 30C or below. So I would recommend preparing overnight culture of your strains and then spotting 5uL onto your LB CR plates, letting them dry and growing them at 28C for 5-7 days to see the difference in morphology based on the expression of curli and cellulose. For E. coli there is a visual difference in the appearance of the colony depending on whether only curli, only cellulose or both proteins are expressed. I think this will also be the case for Salmonella. You can also take a look at my thesis for detailed description of methods and images of E. coli macrocolonies (I have my thesis uploaded under my research). I also did a search on "Salmonella macrocolony congo red" and found other peoples published research, so you could also take a look at that. Hope that helps, let me know if you have other questions.