Comparison of Blue Nucleic Acid Dyes for Flow Cytometric Enumeration of Bacteria in Aquatic Systems

Seven blue nucleic acid dyes from Molecular Probes Inc. (SYTO-9, SYTO-11, SYTO-13, SYTO-16, SYTO-BC, SYBR-I and SYBR-II) were compared with the DAPI (4′,6-diamidino-2-phenylindole) method for flow cytometric enumeration of live and fixed bacteria in aquatic systems. It was shown that SYBR-II and SYTO-9 are the most appropriate dyes for bacterial enumeration in nonsaline waters and can be applied to both live and dead bacteria. The fluorescence signal/noise ratio was improved when SYTO-9 was used to stain living bacteria in nonsaline waters. Inversely, SYBR-II is more appropriate than SYTO dyes for bacterial enumeration of unfixed and fixed seawater samples.