SYTO9 is a green fluorescent nucleic acid stain (Ex=485 nm, Em=498 nm (DNA)/ 501 nm (RNA)). This compound is cell-permeable; however, its permeability is limited across the dual cell membranes of Gram-negative bacteria. The quantum yield of SYTO9 significantly increases upon binding with nucleic acids. It is suitable for fluorescence microscopy and flow cytometry analysis.