MycoLight? Ratiometric Bacterial Membrane Potential Kit *Red/Green Fluorescence*. AAT Bioquest's MycoLight? Ratiometric Bacterial Membrane Potential Kit uses a fluorescent sensor that exhibits green fluorescence in both gram-positive and negative bacterial cells in low concentration, but the fluorescence shifts toward red emission at higher cytosolic concentrations due to the dye molecule aggregation caused by larger membrane potentials. The magnitude of membrane potential varies with different bacterial species. For many gram-positive species, the red/green ratio tend to vary with the intensity of the proton gradient while in many gram-negative bacteria the response of the dye does not appear to be proportional to proton gradient intensity. This kit is designed to assay bacterial membrane potentials when the bacterial concentrations are in the range of 105 - 107 organisms per mL. Stained cells can be monitored fluorimetrically at 510-530 nm (FITC filter) and 600-660 nm (Texas red filter) with excitation at 488 nm, the most common excitation light source.