Dear Haleema,

We have used Congo red in agar plates (usually LB with no NaCl) and have never seen 'black' colonies, even for strains of Pseudomonas that we know are over-expressing cellulose. Usually a red or deep-red colour is seen (off-white - pink is sometime seen with strains expressing low amounts of cellulose as confirmed with Epifluorescent microscopy and Calcofluor stain).

However, some assays also include Coomassie blue with the Congo red (to assess the RDAR phenotype) and this should produce a deeper red if sufficient cellulose and proteins are expressed. I feel that this approach is more difficult to control as the Coomassie blue will also stain the cell contents.

If you are using a combination of stains, I would suggest trying them out one at a time in different agar plates to see which is not producing the results you expect.

Andrew

but with different strain all having no colour chnge colonies when which I streak all showed Same color as Congo red plates no other color showed .its seem only strain grow on Congo red agar plates nothing else happen