In an experiment to optimize the culture conditions for the catalase(CAT) production by Serratia marcescens SYBCT02,an interesting phenomenon occurred that the addition of ctric acid to the culture medium significantly stimulated the accumulation of CAT.To explore the reason for this finding,an experiment was designed based on the previous reports.Three kinds of carbon source,i.e.glucose,ctric acid and a mixture constituted by glucose and citric acid,with the same amount of carbon element,were used in the culture medium for CAT production by Serratia marcescens SYBCT02.After determining the contents of cellular antioxidant substrates and the activities of cellular antioxidant enzymes,an occurrence of oxidative stress was found in the cells of Serratia marcescens SYBCT02 cultivated in the media with citric acid.Compared with the control,the increased amounts of hydrogen peroxide(H₂O₂) and hydroxyl radical(? OH ) induced the synthesis of CAT after an addition of citric acid to the culture medium. This conclusion was further confirmed by the addition of exogenous reactive oxygen(produced by menadione and paraquat) to the culture media.