THE ENUMERATION OF AQUATIC BACTERIA USING DAPI

Abstract

A comparison was made between acridine orange (AO) and 4'6-diamidino-2-phenylindole (DAPI), a highly sensitive DNA fluorescing stain, for enumerating aquatic bacteria. When water samples were processed in a darkened room and a final DAPI concentration of 0,05 μg ml ⁻¹ was used, there was no significant difference between the population estimates obtained with AO and DAPI. With DAPI, bacteria are more easily distinguished from other particulate matter than when stained with AO. The cyanobacterium, Microcystis aeruginosa, normally fluoresced the blue colour characteristic of the DNA-DAPI complex. Frequently colonies fluoresced pink and were heavily populated by bacteria. DAPI may offer a rapid method to assess the proportion of senescing colonies in a population.