A COMPARISON BETWEEN DAPI DIRECT COUNT (DDC) AND MOST PROBABLE NUMBER(MPN) TO QUANTIFY PROTOZOA IN INFILTRATION SYSTEMS

An adjustment of DAPI direct count (DDC) technique for bacteria was de veloped for enumeration of protozoa. A comparison between the DDC tech nique 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 enu merated 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 tak en from a nutrient rich environment and Escherichia coli was used as a prey bacteria. Enumeration by DDC allows observation of active protoz oa only and therefore excludes quantification of cysts. This is a weak ness of the DDC method compared to the MPN method. The effect of physi cal treatment before enumeration was also evaluated, and damaging of p rotozoa 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 conce ntrations at 1% or higher gave a significantly better (P<0.05) result compared to lower concentrations. (C) 1998 Elsevier Science B.V.