DIRECT-DETECTION OF RHIZOSPHERE-COLONIZING PSEUDOMONAS SP USING AN ESCHERICHIA-COLI RIBOSOMAL-RNA PROMOTER IN A TN7-LUX SYSTEM

A promoterless Tn7-lux system conferring bioluminescence was fused with an Escherichia coli rRNA gene promoter and compared with neo- or lac-lucCDABE analogs after introduction in Pseudomonas cells. Fusion of the ribosomal promoter with luxCDABE genes increased the bioluminescence of cells by approx. 100- to 1500-fold over the neo-lux system depending on the growth conditions and bacterial strain. When the cells were grown in suspension culture, light production and growth were strongly dependent on the nutrient composition of the medium. Root-colonizing competence was tested in nonsterile soil by autophotographic detection of bacterial bioluminescence on plant roots. The lower detection limit of the autophotographic method for roots inoculated with Pseudomonas fluorescens 2-79 was 10(5) cfu g(-1) fresh root weight. The new bioluminescence marker did not require addition of supplemental nutrients or the aldehyde substrate for the luciferase enzyme and provides a simple and highly sensitive detection method for long term in situ studies on the microbial ecology of specific bacterial strains.