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 wit h an Escherichia coli rRNA gene promoter and compared with neo- or lac -lucCDABE analogs after introduction in Pseudomonas cells. Fusion of t he ribosomal promoter with luxCDABE genes increased the bioluminescenc e of cells by approx. 100- to 1500-fold over the neo-lux system depend ing on the growth conditions and bacterial strain. When the cells were grown in suspension culture, light production and growth were strongl y dependent on the nutrient composition of the medium. Root-colonizing competence was tested in nonsterile soil by autophotographic detectio n of bacterial bioluminescence on plant roots. The lower detection lim it of the autophotographic method for roots inoculated with Pseudomona s fluorescens 2-79 was 10(5) cfu g(-1) fresh root weight. The new biol uminescence marker did not require addition of supplemental nutrients or the aldehyde substrate for the luciferase enzyme and provides a sim ple and highly sensitive detection method for long term in situ studie s on the microbial ecology of specific bacterial strains.