Acridine orange is a dye that interposes or binds to nucleic acid (DNA or RNA) present in organisms and fluoresces to emit different colors that aid in the differentiation of cellular organelles. This binding is the result of electrostatic interactions of the acridine molecule between the base pairs of the nucleic acid. Acridine orange (AO), because of its metachromatic properties, is commonly used in fluorescence microscopy and flow cytometry for the analysis of cell physiology and cell cycle status, including fluorescence microscopic examination of microorganisms.