Construction and evaluation of a self-luminescent biosensor

Determination of zinc, cadmium and lead bioavailability in contaminated soils at the single-cell level by a combination of whole-cell biosensors and flow cytometry

Zinc, lead and cadmium are metallic trace elements (MTEs) that are widespread in the environment and tend to accumulate in soils because of their low mobility and non-degradability. The purpose of this work is to evaluate the applicability of biosensors as tools able to provide data about the bioavailability of such MTEs in contaminated soils. Here, we tested the genetically-engineered strain Escherichia coli pP(ZntA)gfp as a biosensor applicable to the detection of zinc, lead and cadmium by the biosynthesis of green fluorescent protein (GFP) accumulating inside the cells. Flow cytometry was used to investigate the fluorescence induced by the MTEs. A curvilinear response to zinc between 0 and 25 mg/L and another curvilinear response to cadmium between 0 and 1.5 mg/L were highlighted in liquid media, while lead did not produce exploitable results. The response relating to a Zn2+/Cd2+ ratio of 10 was further investigated. In these conditions, E. coli pP(ZntA)gfp responded to cadmium only. Several contaminated soils with a Zn2+/Cd2+ ratio of 10 were analyzed with the biosensor, and the metallic concentrations were also measured by atomic absorption spectroscopy. Our results showed that E. coli pP(ZntA)gfp could be used as a monitoring tool for contaminated soils being processed.

Whole-cell bioreporters for the detection of bioavailable metals

Whole-cell bioreporters are living microorganisms that produce a specific, quantifiable output in response to target chemicals. Typically, whole-cell bioreporters combine a sensor element for the substance of interest and a reporter element coding for an easily detectable protein. The sensor element is responsible for recognizing the presence of an analyte. In the case of metal bioreporters, the sensor element consists of a DNA promoter region for a metal-binding transcription factor fused to a promoterless reporter gene that encodes a signal-producing protein. In this review, we provide an overview of specific whole-cell bioreporters for heavy metals. Because the sensing of metals by bioreporter microorganisms is usually based on heavy metal resistance/homeostasis mechanisms, the basis of these mechanisms will also be discussed. The goal here is not to present a comprehensive summary of individual metal-specific bioreporters that have been constructed, but rather to express views on the theory and applications of metal-specific bioreporters and identify some directions for future research and development.

Micro-algal biosensors

Fighting against water pollution requires the ability to detect pollutants for example herbicides or heavy metals. Micro-algae that live in marine and fresh water offer a versatile solution for the construction of novel biosensors. These photosynthetic microorganisms are very sensitive to changes in their environment, enabling the detection of traces of pollutants. Three groups of micro-algae are described in this paper: chlorophyta, cyanobacteria, and diatoms.

A suite of recombinant luminescent bacterial strains for the quantification of bioavailable heavy metals and toxicity testing

Background

Recombinant whole-cell sensors have already proven useful in the assessment of the bioavailability of environmental pollutants like heavy metals and organic compounds. In this work 19 recombinant bacterial strains representing various Gram-positive (Staphylococcus aureus and Bacillus subtilis) and Gram-negative (Escherichia coli, Pseudomonas fluorescens) bacteria were constructed to express the luminescence encoding genes luxCDABE (from Photorhabdus luminescens) as a response to bioavailable heavy metals ("lights-on" metal sensors containing metal-response elements, 13 strains) or in a constitutive manner ("lights-off" constructs, 6 strains).

Results

The bioluminescence of all 13 "lights-on" metal sensor strains was expressed as a function of the sub-toxic metal concentrations enabling the quantitative determination of metals bioavailable for these strains. Five sensor strains, constructed for detecting copper and mercury, proved to be target metal specific, whereas eight other sensor strains were simultaneously induced by Cd²⁺, Hg²⁺, Zn²⁺and Pb²⁺. The lowest limits of determination of the "lights-on" sensor strains for the metals tested in this study were (μg l^-1): 0.002 of CH₃HgCl, 0.03 of HgCl₂, 1.8 of CdCl₂, 33 of Pb(NO₃)₂, 1626 of ZnSO₄, 24 of CuSO₄ and 340 of AgNO₃. In general, the sensitivity of the "lights-on" sensor strains was mostly dependent on the metal-response element used while the selection of host bacterium played a relatively minor role. In contrast, toxicity of metals to the "lights-off" strains was only dependent on the bacterial host so that Gram-positive strains were remarkably more sensitive than Gram-negative ones.

Conclusion

The constructed battery of 19 recombinant luminescent bacterial strains exhibits several novel aspects as it contains i) metal sensor strains with similar metal-response elements in different host bacteria; ii) metal sensor strains with metal-response elements in different copies and iii) a "lights-off" construct (control) for every constructed recombinant metal sensor strain. To our knowledge, no Gram-positive metal sensor expressing a full bacterial bioluminescence cassette (luxCDABE) has been constructed previously.

Design of a highly specific and noninvasive biosensor suitable for real-time in vivo imaging of mercury (II) uptake

Mercury is a ubiquitous pollutant that when absorbed is extremely toxic to a wide variety of biochemical processes. Mercury (II) is a strong, "invisible" poison that is rapidly absorbed by tissues of the intestinal tract, kidneys, and liver upon ingestion. In this study, a novel fluorescence-based biosensor is presented that allows for the direct monitoring of the uptake and distribution of the metal under noninvasive in vivo conditions. With the introduction of a cysteine residue at position 205, located in close proximity to the chromophore, the green fluorescent protein (GFP) from Aequorea victoria was converted into a highly specific biosensor for this metal ion. The mutant protein exhibits a dramatic absorbance and fluorescence change upon mercuration at neutral pH. Absorbance and fluorescence properties with respect to the metal concentration exhibit sigmoidal binding behavior with a detection limit in the low nanomolar range. Time-resolved binding studies indicate rapid subsecond binding of the metal to the protein. The crystal structures obtained of mutant eGFP205C indicate a possible access route of the metal into the core of the protein. To our knowledge, this engineered protein is a first example of a biosensor that allows for noninvasive and real-time imaging of mercury uptake in a living cell. A major advantage is that its expression can be genetically controlled in many organisms to enable unprecedented studies of tissue specific mercury uptake.

Recombinant luminescent bacterial sensors for the measurement of bioavailability of cadmium and lead in soils polluted by metal smelters

Environmental hazard of heavy metals in soils depends to a large extent on their bioavailability. The approach used in this study enables the determination of bioavailable metals in solid-phase samples. Two recombinant bacterial sensors, one responding specifically to cadmium and the other to lead and cadmium by increase of luminescence (firefly luciferase was used as a reporter) were used to determine the bioavailability of these metals in soil-water suspensions (a contact assay) and respective particle-free extracts. Fifty agricultural soils sampled near zinc and lead smelters in the Northern France containing up to (mg/kg) 20.1 of Cd, 1050 of Pb and 1390 of Zn were analysed. As the soil matrix interferes with the assay, recombinant luminescent control bacteria lacking the metal recognizing protein and corresponding promoter (thus, being not metal-inducible) but otherwise comparable to the sensor bacteria (the same host bacterium and plasmid encoding luciferase) were used in parallel to take into account the possible quenching and/or stimulating effects of the sample on the luminescence of the sensor bacteria. Both, chemical and sensor analysis showed that only microg/l levels of metals were extracted from the soil into the water phase (0.1% of the total Cd, 0.07% of Pb and 0.5% of Zn). However, 115-fold more Cd and 40-fold more Pb proved bioavailable if the sensor bacteria were incubated in soil suspensions (i.e., in the contact assay). The bioavailability of metals in different soils varied (depending probably on soil type) ranging from 0.5% to 56% for cadmium and from 0.2% to 8.6% for lead.

Measurement of firefly luciferase reporter gene activity from cells and lysates using Escherichia coli arsenite and mercury sensors

The structural gene encoding firefly luciferase from Photinus pyralis is a widely used reporter both in traditional monitoring of gene expression and in bacterial sensors. Its activity can be detected from living cells (in vivo) without disruption or from cell-free lysate (in vitro). We compared the two measurement methods by using an overall toxicity detecting strain Escherichia coli MC1061(pCSS810), a mercury-sensing strain E. coli MC1061(pTOO11), and two new arsenic sensor strains MC1061(pTOO31) and AW3110(pTOO31) which were constructed for this study. Plasmid pTOO31 was constructed by inserting the ars promoter and the arsR gene from plasmid R773 to control firefly luciferase gene expression. Both in vivo and in vitro methods correlated well with the strains tested [correlation coefficients R = 0.99484 and 0.99834] and gave highly comparable results with standard solutions of arsenite or mercury ions and from six environmental water samples spiked with the ions. Use of the in vivo method resulted in lower variation between replicates of the same sample (CVs ranging from 3.9 to 7.2%) and also between different samples (from 8.6 to 25.9%) compared to the in vitro method (CVs ranging from 8.6 to 17.8% for replicates and from 13.1 to 36.3% for different samples).

Cyanobacteria carrying an smt-lux transcriptional fusion as biosensors for the detection of heavy metal cations

The metal-responsive smt operator/promoter region of Synechococcus PCC7942 was fused to the luxCDABE genes of Vibrio fischeri. Plasmid DNA (pJLE23) carrying this fusion conferred metal ion-inducible luminescence to transformed cyanobacteria. Synechococcus PCC7942 (pJLE23) was sensitive to ZnCl2 concentrations within a range of 0.5-4 microM as demonstrated by induction of luminescence. Trace levels of CuSO24 and CdCl2 were also detected.

Development of biosensors for the detection of mercury and copper ions

The development of genetically engineered biosensors for copper and mercury ions is described. The biosensors have been constructed by fusing thelux or light emitting genes fromVibrio fischeri with genetic regulating elements that respond to copper ions or mercury ions, derived respectively fromEscherichia coli andSerratia marcescens. The fusions were placed intoE. coli cells which then emitted light in response to copper or mercury ions. Data is presented describing the sensitivity, specificity, and dynamic range of the biosensors to their respective target metal ions. A preliminary description of experiments is provided indicating how these biosensors might be used to investigate the bioavailability of mercury and copper ions in environmental samples.

Immobilized cells used for detection and analysis

Many kinds of biosensor have now been developed and utilized for various types of analysis including clinical, medical and environmental monitoring, industrial process control, as well as many other applications. The microbial biosensor has advantages, such as longer lifetime and lower cost, over other types of biosensor. Recently, photobacteria and recombinant bacteria have been employed in biosensors both for determining biochemical oxygen demand and for detecting heavy metals and other toxic compounds.