Construction and Use of Broad Host Range Mercury and Arsenite Sensor Plasmids in the Soil Bacterium Pseudomonas fluorescens OS8

We have generated new sensors for the specific detection and studies of bioavailability of metals by engineering Pseudomonas fluorescens with reporter gene systems. One broad host range mercury (pTPT11) and two arsenite (pTPT21 and pTPT31) sensor plasmids that express metal presence by luminescence phenotype were constructed and transferred into Escherichia coli DH5α and Pseudomonas fluorescens OS8. The maximal induction was reached after 2 h of incubation in metal solutions at room temperature (22°C). In optimized conditions the half maximal velocity of reaction was achieved at acidic pH using a d-luciferin substrate concentration that was nearly sixfold lower for P. fluorescens OS8 than for E. coli DH5α. When using a luciferin concentration (150 μM) that was optimal for E. coli the luminescence declined rapidly in the case of Pseudomonas, for which the substrate level 25 μM gave a stable reading between about 20 min and 3 h. The ability of the strain OS8 to quantitatively detect specific heavy metals in spiked soil and soil extracts is as good, or even better in being a real-time reporter system, than that of a traditional chemical analysis. The Pseudomonas strain used is an isolate from pine rhizosphere in oil and heavy metal contaminated soil. It is also a good humus soil colonizer and is therefore a good candidate for measuring soil heavy metal bioavailability.