Use of a site‐specific recombination‐based biosensor for detecting bioavailable toluene and related compounds on roots

We constructed and characterized a plasmid‐based genetic system that reports the expression of a toluene‐responsive promoter (P_tbuA1) by effecting an irreversible, heritable change in the biosensor cell. Expression of the reporter gene gfp is strongly repressed in the absence of expression from the P_tbuA1 promoter, and high level gfp expression in the original cell and its progeny is mediated by the site‐specific recombination machinery of bacteriophage P22 to initiate removal of a repressor cassette. The reporter plasmid pTolLHB was functional in two soil saprophytes, Pseudomonas fluorescens A506 and Enterobacter cloacae JL1157, with the efficiency and sensitivity to low toluene concentrations being optimal in P. fluorescens A506. In culture, 80–100% of the A506 (pTolLHB) population expressed gfp following exposure to 0.2 µm toluene for one to three hours. Compared to the response of A506 containing a plasmid‐borne P_tbuA1‐gfp fusion, the recombination‐based biosensor was more sensitive at detecting low toluene and trichloroethylene concentrations. An A506 (pTolLHB) inoculum, which had a background of 2.5% of the cells expressing gfp, was introduced onto barley roots in soil microcosms. If toluene was introduced into the microcosms, after 24 h, 72% of the A506 (pTolLHB) cells recovered from roots expressed gfp, indicating bioavailable toluene to rhizosphere bacteria. When toluene was not introduced, 16.5% of the A506 (pTolLHB) cells recovered from the roots expressed gfp, indicating that natural inducers of the P_tbuA1 promoter were present in the barley rhizosphere. When introduced into rhizotrons containing barley plants and toluene vapours, the biosensor allowed localization of the availability of toluene along the seminal roots. In rhizotrons that were not exposed to toluene vapours, the biosensor exhibited high P_tbuA1‐promoter activity in distinct regions along the seminal roots, indicating spatial heterogeneity plant‐ or rhizosphere microbial community‐derived inducers of the P_tbuA1 promoter. This recombination‐based toluene biosensor thus was useful in identifying bacterial exposure to transient or low levels of toluene, or related compounds, directly in the environment.