Construction and use of specific luminescent recombinant bacterial sensors for the assessment of bioavailable fraction of cadmium, zinc, mercury and chromium in the soil

Abstract

Recombinant luminescent bacterial sensors for the detection of zinc [MC1061(pzntRluc)] and chromate [AE104(pchrBluc)] were constructed. The sensors carry firefly luciferase gene as a reporter under the control of zinc-inducible regulatory unit from Zn resistance system in chromosomal DNA of Escherichia coli or chromate-inducible unit from Ralstonia metallidurans CH34 megaplasmid pMOL28. The response of the sensors was calibrated using Zn 2+, Cr 2O 7 2− and CrO 4 2−, respectively. The detection limit of the zinc sensor for zinc was 40 μM (2.6 mg l −1 of Zn) and that of chromate sensor for dichromate 30 nM (1.6 μg l −1 of Cr) and for chromate 50 nM (2.6 μg l −1 of Cr), respectively. The specificity of the two above-mentioned sensors constructed in this study and a mercury-inducible bacterial sensor MC1061(pmerBR BSluc) constructed by us earlier [Anal. Chem. 73 (2001) 5168] was determined by using different heavy metal compounds. The zinc and mercury sensors were not completely specific to the target metals. The zinc sensor was co-inducible with cadmium and mercury and the mercury sensor with cadmium. The chromate sensor was inducible not only by chromate but also with Cr 3+. The bacterial sensors constructed were used for the estimation of bioavailable fraction of heavy metals in soils spiked with different amounts of zinc, cadmium, mercury and chromate. Both soil–water suspensions and soil–water extracts were analyzed. The results showed that the majority of heavy metals remained adsorbed to soil particles: only 0.6% of Cd, 1.3% of Hg, 2% of Zn and 46% of Cr (VI) were available to the bacterial sensors in soil–water extracts. However, when the soil–water suspensions were analyzed, approximately 20 times more cadmium and 30 times more mercury (12 and 40%, respectively) was available for the sensor bacteria whereas the available fractions of zinc and chromate in soil–water extract and suspension were similar. Thus, this study showed that in case of cadmium and mercury (but not zinc and chromium) the particle-bound metals were also partially bioavailable.