Use of green fluorescent protein as a marker for ecological studies of activated sludge communities

The potential of the green fluorescent protein (GFP) as a marker gene for ecological investigations of an activated sludge community was assessed. By inserting the hybrid transposon mini-Tn5 gfp into the chromosome of Pseudomonas putida KT2442 a strongly fluorescent mutant was obtained. This strain was used for in vivo tracking of individual cells after introduction into a simple sludge microcosm. It is demonstrated that the observed reduction of introduced bacteria from sewage is mainly the result of predation by protozoa. The feasibility of combining detection of GFP fluorescence with whole cell hybridization employing fluorescently labeled, rRNA-targeted oligonucleotides in paraformaldehyde fixed samples is demonstrated.