The reporter gene cat can be used to determine promoter activation and to seek genes related to pathogenicity, where the promoter becomes tagged. The estimation of promoter expression in these cases can be evaluated by three approaches: i) determination of relative quantity of the mRNA; ii) quantification of protein (chloramphenicol acetyl-transferase); or iii) determination of specific enzymatic activity. However the ultimate result for cat expression in a cell is to endow resistance against chloramphenicol. In this work we use the cell density measure with chloramphenicol antibiotic as an analysis of the promoter regulation and strength during a study of Pseudomonas syringae pv. maculicola mutant screening. We found that the promoter expression level modifies accordingly the cell density in liquid media and also the colony size in solid media at defined times. We propose the determination of cell density in liquid media supplemented with a constant concentration of chloramphenicol to estimate not only the expression conditions of a promoter tagged with cat, but also the expression intensity in those conditions.