Flow cytometric discrimination of bacterial populations in seawater based on SYTO 13 staining of nucleic acids

Flow cytometric (FCM) counts of bacteria stained with SYTO 13, a cyanine dye, were highly correlated with DAPI epifluorescence microscopic counts in coastal seawater samples. Fluorescence intensity of stained cells appeared to depend on nucleic acid content and on the polarization of cell membranes. Right angle light scatter values of bacterial populations were clearly related to cell size. By FCM analysis of SYTO 13-stained samples from a batch mesocosm of Mediterranean seawater, several bacterial clusters having different apparent sizes and physiological status were discriminated showing that FCM analysis allows key cell categories to be followed for understanding bacterial community dynamics.