An automated technique for most-probable-number (MPN) analysis of densities of phagotrophic protists with lux AB labelled bacteria as growth medium

An automated modification of the most-probable-number (MPN) technique has b een developed for enumeration of phagotrophic protozoa. The method is based on detection of prey depletion in micro titre plates rather than on presen ce of protozoa. A transconjugant Pseudomonas fluorescens DR54 labelled with a luxAB gene cassette was constructed, and used as growth medium for the p rotozoa in the micro titre plates. The transconjugant produced high amounts of luciferase which was stable and allowed detection for at least 8 weeks. Dilution series of protozoan cultures and soil suspensions were inoculated into micro titre plates amended with a suspension of the transconjugant. A fter 45 days measurement of light emission allowed detection of individual wells in the titre plates, where protozoan grazing had removed the inoculat ed bacteria. (C) 1999 Elsevier Science B.V. All rights reserved.