The FilmTracer™ LIVE⁄DEAD Biofilm Viability kit provides a two-color fluorescence assay of bacterial viability, based on membrane integrity, that has proven usefule for a diverse array of bacterial genera including those growing in biofilm communities. The LIVE⁄DEAD Biofilm Viability kit utilizes mixtures of our SYTO® 9 green fluorescent nucleic acid stain and the red-fluorescent nucleic acid stain, propidium iodide. These stains differ both in their spectral characteristics and in their ability to penetrate healthy bacterial cells. When used alone, the SYTO® 9 stain generally labels all bacteria in a population—those with intact membranes and those with damaged membranes. In contrast, propidium iodide penetrates only bacteria with damaged membranes, causing a rduction in the SYTO® 9 stain fluorescence with both dyes are present. Thus, with an appropriate mixture of the SYTO™ 9 and propdium iodide stains, bacteria with intact cell membranes stain fluorescent green, whereas bacteria with damaged membranes stain fluorescent red.