Fluorescein Diacetate as a Stain of Metabolically Active Bacteria in Soil

Fluorescein diacetate (FDA) was applied as a vital stain to soil bacteria in pure cultures, sterilized soil, and natural soils. The staining ability of FDA was tested on 111 isolates of soil bacteria in pure cultures in a diluted laboratory medium and in sterilized soil. Nearly 80% of the isolates were stained. In a growth experiment in sterilized soil, FDA was compared to acridine orange (AO). At a given time, when most bacteria were considered metabolically active, the number of FDA-stained bacteria exceeded that stained by AO. The staining efficiency and the simple procedure indicate that the FDA-method is useful for estimating the number of metabolically active bacteria in soil. This was confirmed by examination of ten different soils, where the FDA-method was compared to AO-staining, fluorescein isothiocyanate (FITC)-staining, and plate counts.