A comparison between DAPI direct count (DDC) and most probable number (MPN) to quantify protozoa in infiltration systems

Abstract

An adjustment of DAPI direct count (DDC) technique for bacteria was developed for enumeration of protozoa. A comparison between the DDC technique and the MPN method was then conducted in a determination of the number of protozoa in porous media. The active protozoa enumerated by MPN constituted between 0.01–1.4% of the number of active protozoa enumerated by DDC. The number of protozoa enumerated in these experiments by the MPN technique, are probably close to the maximum number it is possible to enumerate by the culture method, because protozoa were taken from a nutrient rich environment and Escherichia coli was used as a prey bacteria. Enumeration by DDC allows observation of active protozoa only and therefore excludes quantification of cysts. This is a weakness of the DDC method compared to the MPN method. The effect of physical treatment before enumeration was also evaluated, and damaging of protozoa was significant ( P<0.05) in samples containing particles. The highest enumerable number was obtained with 100 strokes/min for 1 min on a reciprocal shaker. Fixation of protozoa with glutaraldehyde concentrations at 1% or higher gave a significantly better ( P<0.05) result compared to lower concentrations.