Overnight cultures were subcultured at a 1:100 dilution in 50 ml of LB medium for 0 (without bacteria), 2, 4, and 6 hours. Supernatant was collected by centrifugation for glucose content using the CheKine Micro Glucose Assay Kit (KTB1300, Abbkine Scientific Co. Ltd., China). Briefly, 50 μl of supernatant was added to 1000 μl of o-toluidine reagent and vortexed. The mixture was heated in a boiling water bath for 8 min and cooled to room temperature in a cool water bath for 4 min. Then, 200 μl of each mixture was added to a 96-well plate, and absorbance at 630 nm was measured.