E.coli not expressing fluorescent protein in minimal media?

I’m needing to tag WT cells with eGFP then grow with mutant cells for 5 days so I can see on an agar plate how many of each are left, have just looked at agar plates of WT by itself and after being mixed with the mutant and most cells are not green (on WT only plate there are a couple green but not many). The plasmid is pUCBB-eGFP with a constitutive lac promoter. How can I make sure the protein is expressed in minimal media so I can tell the difference between the cells?