Some observations on direct counts of freshwater bacteria obtained with a fluorescence microscope

The use of different fluorochromes for direct counts of bacteria in water was compared. The two dyes in most common use, acridine orange (AO) and fluorescein isothiocyanate (FITC) were compared with an acridine derivative, euchrysine 2GNX (E‐2GNX). Minor changes in technique could produce significant differences in counts. The acridine based dyes were easier to apply than FITC, and of these, E‐2GNX gave consistently higher counts, all at a final dye concentration of 5 mg liter −1 with a contact time of 3 min. Details of the methodology should be consulted since changes can produce opposite results.