Measurement of fluorescence intensity of GFP

The constructed strains were cultured in BHI medium at pH 7.4 or 5.5 at 37 °C for 12 h, 24 h, 36 h, and 48 h, respectively. The cultures were collected by centrifugation, washed twice with 20 mM Tris-HCl, and resuspended. After adjusting to appropriate absorption at 600 nm (OD₆₀₀), the fluorescence intensity of GFP (excitation at 485 nm and emission at 525 nm) was measured in microtiter plates (Assay Plate, 96 wells, Black Polystyrene; Corning, New York, USA) using a SpectraMax M2 microplate reader (Molecular Devices, San Jose, CA, USA).