Direct counts of aquatic bacteria by a modified epifluorescence technique1

Various excitation and emission filters, microscope lamps, membrane filters, and field storage techniques used for direct counting of bacteria by epifluorescence microscopy have been compared. A rapid, simple modification of the Acridine Orange procedure using a new light filter combination, Sartorius cellulose membrane filters, and distilled water rinse gave the brightest cell fluorescence, darkest background, least fading, and highest counts for both estuarine and freshwater samples. Field fixation with Formalin permits storage of bacterial samples for 1 to 2 weeks without changes in bacterial numbers.