Oxidative stress detection with Escherichia coli harboring a katG'::lux fusion

Microbial whole-cell arrays

The coming of age of whole‐cell biosensors, combined with the continuing advances in array technologies, has prepared the ground for the next step in the evolution of both disciplines – the whole‐cell array. In the present review, we highlight the state‐of‐the‐art in the different disciplines essential for a functional bacterial array. These include the genetic engineering of the biological components, their immobilization in different polymers, technologies for live cell deposition and patterning on different types of solid surfaces, and cellular viability maintenance. Also reviewed are the types of signals emitted by the reporter cell arrays, some of the transduction methodologies for reading these signals and the mathematical approaches proposed for their analysis. Finally, we review some of the potential applications for bacterial cell arrays, and list the future needs for their maturation: a richer arsenal of high‐performance reporter strains, better methodologies for their incorporation into hardware platforms, design of appropriate detection circuits, the continuing development of dedicated algorithms for multiplex signal analysis and – most importantly – enhanced long‐term maintenance of viability and activity on the fabricated biochips.

Are luminescent bacteria suitable for online detection and monitoring of toxic compounds in drinking water and its sources?

Biosensors based on luminescent bacteria may be valuable tools to monitor the chemical quality and safety of surface and drinking water. In this review, an overview is presented of the recombinant strains available that harbour the bacterial luciferase genes luxCDABE, and which may be used in an online biosensor for water quality monitoring. Many bacterial strains have been described for the detection of a broad range of toxicity parameters, including DNA damage, protein damage, membrane damage, oxidative stress, organic pollutants, and heavy metals. Most lux strains have sensitivities with detection limits ranging from milligrams per litre to micrograms per litre, usually with higher sensitivities in compound-specific strains. Although the sensitivity of lux strains can be enhanced by various molecular manipulations, most reported detection thresholds are still too high to detect levels of individual contaminants as they occur nowadays in European drinking waters. However, lux strains sensing specific toxic effects have the advantage of being able to respond to mixtures of contaminants inducing the same effect, and thus could be used as a sensor for the sum effect, including the effect of compounds that are as yet not identified by chemical analysis. An evaluation of the suitability of lux strains for monitoring surface and drinking water is therefore provided.

Randomly distributed arrays of optically coded functional microbeads for toxicity screening and monitoring

We have successfully developed optically coded functional microbeads by co-encapsulating both bioluminescent reporter bacterial cells and fluorescent microspheres within a common alginate microbead. These microbeads harboring an individual self-identification code using fluorescent microspheres could be randomly scattered on any multi-well chip plate as long as the size of the microbeads are made to fit on it with the result that, since cell types are identified on the basis of fluorescent color, microbead arrays were fabricated without pre-designation of an individual well. As an example of this method, five different stress specific bioluminescent bacterial strains, each with a different optical code, were successfully implemented to make five different types of optically coded functional microbeads, with a speed of about 30 microbeads/min. Each functional microbead has a specific stress-specific bacterial strain and, as an identification optical code, one of five optical codes generated from fluorescence microspheres such as yellow, green, red, yellow + green, or no fluorescence. This final randomly scattered functional microbeads array biochip, with a fast fabrication of each chip at every 2 min, successfully demonstrated its ability in toxicity screening and monitoring for samples with a few examples for five different stress chemicals. This simple and fast, but not tedious and complicated procedure should be widely and practically used in making cell array chips for the monitoring of environmental toxicity, new-borne chemicals, pharmaceutical drugs and cosmic rays in the space station or spaceships in future.

Bioluminescent Escherichia coli strains for the quantitative detection of phosphate and ammonia in coastal and suburban watersheds

Accumulation of phosphate and ammonia in estuarine systems and subsequent dinoflagellate and algal blooms has been implicated in fish kills and in health risks for fishermen. Analytic chemistry kits are used to measure phosphate and ammonia levels in water samples, but their sensitivity is limited due to specificity for inorganic forms of these moieties. An Escherichia coli bioluminescent reporter system measured the bioavailability of inorganic nutrients through fusion of E. coli promoters (phoA or glnAp2) to the luxCDABE operon of Vibrio fischeri carried either on the chromosome or on a multicopy plasmid vector, resulting in emission of light in response to phosphate or ammonia starvation. Responses were shown to be under the control of expected physiological regulators, phoB and glnFG, respectively. Standard curves were used to determine the phosphate and ammonia levels in water samples from diverse watersheds located in the northeastern United States. Bioluminescence produced in response to nutrient starvation correlated with concentrations of phosphate (1-24 ppm) and ammonia (0.1-1.6 ppm). While the ammonia biosensor measured nutrient concentrations in tested water samples that were comparable to the amounts reported by a commercial kit, the phosphate biosensor reported higher levels of phosphate in Chesapeake water samples than did the kit.

Escherichia coli as a bioreporter in ecotoxicology

Ecotoxicological assessment relies to a large extent on the information gathered with surrogate species and the extrapolation of test results across species and different levels of biological organisation. Bacteria have long been used as a bioreporter for genotoxic testing and general toxicity. Today, it is clear that bacteria have the potential for screening of other toxicological endpoints. Escherichia coli has been studied for years; in-depth knowledge of its biochemistry and genetics makes it the most proficient prokaryote for the development of new toxicological assays. Several assays have been designed with E. coli as a bioreporter, and the recent trend to develop novel, better advanced reporters makes bioreporter development one of the most dynamic in ecotoxicology. Based on in-depth knowledge of E. coli, new assays are being developed or existing ones redesigned, thanks to the availability of new reporter genes and new or improved substrates. The technological evolution towards easier and more sensitive detection of different gene products is another important aspect. Often, this requires the redesign of the bacterium to make it compatible with the novel measuring tests. Recent advances in surface chemistry and nanoelectronics open the perspective for advanced reporter based on novel measuring platforms and with an online potential. In this article, we will discuss the use of E. coli-based bioreporters in ecotoxicological applications as well as some innovative sensors awaited for the future.

A bacterial biosensor for oxidative stress using the constitutively expressed redox-sensitive protein roGFP2

A highly specific, high throughput-amenable bacterial biosensor for chemically induced cellular oxidation was developed using constitutively expressed redox-sensitive green fluorescent protein roGFP2 in E. coli (E. coli-roGFP2). Disulfide formation between two key cysteine residues of roGFP2 was assessed using a double-wavelength ratiometric approach. This study demonstrates that only a few minutes were required to detect oxidation using E. coli-roGFP2, in contrast to conventional bacterial oxidative stress sensors. Cellular oxidation induced by hydrogen peroxide, menadione, sodium selenite, zinc pyrithione, triphenyltin and naphthalene became detectable after 10 seconds and reached the maxima between 80 to 210 seconds, contrary to Cd²⁺, Cu²⁺, Pb²⁺, Zn²⁺ and sodium arsenite, which induced the oxidation maximum immediately. The lowest observable effect concentrations (in ppm) were determined as 1.0 × 10⁻⁷ (arsenite), 1.0 × 10⁻⁴ (naphthalene), 1.0 × 10⁻⁴ (Cu²⁺), 3.8 × 10⁻⁴ (H₂O₂), 1.0 × 10⁻³ (Cd²⁺), 1.0 × 10⁻³ (Zn²⁺), 1.0 × 10⁻² (menadione), 1.0 (triphenyltin), 1.56 (zinc pyrithione), 3.1 (selenite) and 6.3 (Pb²⁺), respectively. Heavy metal-induced oxidation showed unclear response patterns, whereas concentration-dependent sigmoid curves were observed for other compounds. In vivo GSH content and in vitro roGFP2 oxidation assays together with E. coli-roGFP2 results suggest that roGFP2 is sensitive to redox potential change and thiol modification induced by environmental stressors. Based on redox-sensitive technology, E. coli-roGFP2 provides a fast comprehensive detection system for toxicants that induce cellular oxidation.

Whole-cell fluorescent biosensors for bioavailability and biodegradation of polychlorinated biphenyls

Whole-cell microbial biosensors are one of the newest molecular tools used in environmental monitoring. Such biosensors are constructed through fusing a reporter gene such as lux, gfp or lacZ, to a responsive promoter. There have been many reports of the applications of biosensors, particularly their use in assaying pollutant toxicity and bioavailability. This paper reviews the basic concepts behind the construction of whole-cell microbial biosensors for pollutant monitoring, and describes the applications of two such biosensors for detecting the bioavailability and biodegradation of Polychlorinated Biphenyls (PCBs).

Microbial cell arrays

The coming of age of whole-cell biosensors, combined with the continuing advances in array technologies, has prepared the ground for the next step in the evolution of both disciplines - the whole cell array. In the present chapter, we highlight the state-of-the-art in the different disciplines essential for a functional bacterial array. These include the genetic engineering of the biological components, their immobilization in different polymers, technologies for live cell deposition and patterning on different types of solid surfaces, and cellular viability maintenance. Also reviewed are the types of signals emitted by the reporter cell arrays, some of the transduction methodologies for reading these signals, and the mathematical approaches proposed for their analysis. Finally, we review some of the potential applications for bacterial cell arrays, and list the future needs for their maturation: a richer arsenal of high-performance reporter strains, better methodologies for their incorporation into hardware platforms, design of appropriate detection circuits, the continuing development of dedicated algorithms for multiplex signal analysis, and - most importantly - enhanced long term maintenance of viability and activity on the fabricated biochips.

Improving the sensitivity of bacterial bioreporters for heavy metals

Whole-cell bacterial bioreporters represent a convenient testing method for quantifying the bioavailability of contaminants in environmental samples. Despite the fact that several bioreporters have been constructed for measuring heavy metals, their application to environmental samples has remained minimal. The major drawbacks of the available bioreporters include a lack of sensitivity and specificity. Here, we report an improvement in the limit of detection of bacterial bioreporters by interfering with the natural metal homeostasis system of the host bacterium. The limit of detection of a Pseudomonas putida KT2440-based Zn/Cd/Pb-biosensor was improved by a factor of up to 45 by disrupting four main efflux transporters for Zn/Cd/Pb and thereby causing the metals to accumulate in the cell. The specificity of the bioreporter could be modified by changing the sensor element. A Zn-specific bioreporter was achieved by using the promoter of the cadA1 gene from P. putida as a sensor element. The constructed transporter-deficient P. putida reporter strain detected Zn(2+) concentrations about 50 times lower than that possible with other available Zn-bioreporters. The achieved detection limits were significantly below the permitted limit values for Zn and Pb in water and in soil, allowing for reliable detection of heavy metals in the environment.

Prokaryotic real-time gene expression profiling for toxicity assessment

Examining global effects of toxins on gene expression profiles is proving to be a powerful method for toxicity assessment and for investigating mechanisms of toxicity. This study demonstrated the application of prokaryotic real-time gene expression profiling in Escherichia coli for toxicity assessment of environmental pollutants in water samples, by use of a cell-array library of 93 E. coli K12 strains with transcriptional green fluorescent protein (GFP) fusions covering most known stress response genes. The high-temporal-resolution gene expression data, for the first time, revealed complex and time-dependent transcriptional activities of various stress-associated genes in response to mercury and mitomycin (MMC) exposure and allowed for gene clustering analysis based on temporal response patterns. Compound-specific and distinctive gene expression profiles were obtained for MMC and mercury at different concentrations. MMC (genotoxin) induced not only the SOS response, which regulates DNA damage and repair, but also many other stress genes associated with drug resistance/sensitivity and chemical detoxification. A number of genes belonging to the P-type ATPase family and the MerR family were identified to be related to mercury resistance, among which zntA was found to be up-regulated at an increasing level as the mercury concentration increased. A mechanism-based evaluation of toxins based on real-time gene expression profiles promises, to be an efficient and informative method for toxicity assessment in environmental samples.

Risk reduction of adverse effects due to di-(2-ethylhexyl) phthalate (DEHP) by utilizing microbial degradation

Di-(2-ethylhexyl) phthalate (DEHP), one of the major phthalates, was reported to be a suspected endocrine-disrupting chemical (EDC) that might produce developmental or reproductive toxicities. Therefore, much effort was undertaken to reduce the potential risk of adverse effects of DEHP on humans by diminishing environmental exposure to this chemical. A bacterium was isolated from soil contaminated with DEHP at a poly(vinyl chloride) (PVC) manufacturing site. Biodegradation kinetic experiments on DEHP-contaminated soil samples were performed in a slurry phase system. The DEHP concentration was decreased to a concentration of 0.5 g/kg by the addition of 1% culture medium to the soil. The microorganism degraded DEHP through the formation of a mono-2-ethylhexyl phthalate (MEHP), which was subsequently metabolized to phthalic acid (PA), as detected by GC-MS analysis. Micrococcus luteus was able to degrade almost 90% of the initial DEHP within 12 d. In addition, the microbial toxicity study of DEHP and its degradation products MEHP or PA, using recombinant bioluminescent bacteria, showed that PA or the mixture produced protein or DNA damage. Data thus suggest that a new strain of Micrococcus luteus with a strong ability to degrade DEHP into nontoxic metabolites may contribute to decontamination of environmental phthalates and consequently risk reduction of human exposure to DEHP.

Electrochemical cell-based sensors

One of the recently developed monitoring technologies involves the use of whole cell biosensors. Such biosensors can be constructed to detect expression of genes of interest and the effect of the environment on this expression. These biosensors are essential for monitoring environmental stress, such as general toxicity or specific toxicity caused by pollutants. Currently, a large spectrum of microbial biosensors have been developed that enable the monitoring of gene expression by measuring light, fluorescence, color, or electric current. The electrochemical monitoring is of special interest for in situ measurements as it can be performed using simple, compact, and mobile equipment and is easily adaptable for online measurements. Here we survey the potential application of electrochemical biosensors with special focus on monitoring environmental pollution.

Toxicant identification by a luminescent bacterial bioreporter panel: application of pattern classification algorithms

Genetically engineered microorganisms, tailored to respond by a dose-dependent signal to the presence of toxic chemicals, are a potentially useful tool for environmental monitoring. One manifestation of this approach is based on a panel of luminescent bacterial bioreporters, harboring fusions of the luxCDABE operon to various stress-responsive gene promoters. Such sensors can report by a dose-dependent luminescent signal on the stress sensed by the cells and thus on the presence of toxic compound(s), but they lack the ability to identify the chemicals involved. Here, we demonstrate how the use of a panel of such sensors might offer a solution to this drawback. Five selected Escherichia coli reporter strains harboring fusions of selected gene promoters (grpE, nhoA, oraA, lacZ, and mipA) to luxCDABE were exposed to five model toxicants and to a toxicant-free control in a 40-repetition format. Each of the six treatments activated different promoters to different extents, producing its own unique fingerprint. Two machine learning schemes were challenged with the obtained data set: Bayesian decision theory and the nonparametric nearest-neighbor technique. The Bayesian classifiers performed better and were able to identify the sample's contents within 30 min with an error rate estimate that did not exceed 3% at a 95% confidence level and with zero false negatives. Performance in tap water and wastewater samples was similar. Given the coming of age of whole-cell sensing devices, pattern classification algorithms such as the ones described here offer a step toward the incorporation of reporter cells into future biosensor formats, including whole-cell arrays.

Suicide through stress: A bacterial response to sub-lethal injury in the food environment

The response of bacteria to sub-lethal injury is an important aspect of food microbiology as many inimical processes to which bacteria are subjected during processing are non-lethal. For pathogens like Salmonella and Escherichia coli, the difference in injury levels of exponential phase cells compared to their stationary phase counterparts in this regard is well recognised and evident for a variety of inimical processes. The expression of a range of stress resistance genes under the control of the sigma factor RpoS provides some explanation for the greater resistance of stationary phase cells. However in 1997 the suicide response hypothesis was put forward as an explanation for the observed response of Salmonella and E. coli to sub-lethal stresses. This hypothesis arose as an explanation for the observed protection of Salmonella and E. coli strains to heat and freeze-thaw injury by the presence of a high level of competitor organisms, a protection that had been shown to be RpoS independent. The central tenet of this theory was that under sub-lethal stress bacteria produce a burst of intracellular free radicals and it is these that lead to sub-lethal injury and/or death. Exponential phase cells because of their more active metabolism are more susceptible to this effect and suffer greater damage. This paper reviews the origins of this theory, the evidence for a free radical response and explores the potential mechanisms by which competitor cells produce a protective effect.

Application of bioinformatics resources for genosensor design

Two novel databases, GenSensor and ConSensor, have been developed. GenSensor accumulates information on the sensitivities of the prokaryotic genes to external stimuli and may facilitate designing of novel genosensors; ConSensor contains data about the structure and efficiency of the available genosensor plasmid constructs. Using these databases, candidate genes for the design of novel multiple functional genosensors were searched, and the Escherichia coli dps gene was chosen as the candidate. The genetic construct derived from its promoter was developed and tested for its sensitivity to various stress agents: hydrogen peroxide (oxidative stress), phenol (protein and membrane damaging), and mitomycin C (DNA damaging). This genosensor was found to be sensitive to all stress conditions applied confirming its ability to serve as multi-functional genosensor. The GenSensor and ConSensor databases are available at http://wwwmgs.bionet.nsc.ru/mgs/dbases/gensensor/index.html.

Biosensing environmental pollution

Whole-cell biosensors are finding increasing use for the detection of environmental pollution and toxicity. These biosensors are constructed through the fusion of promoters, responsive to the relevant environmental conditions, to easily monitored reporter genes. Depending on the choice of reporter gene, expression can be monitored by the production of colour, light, fluorescence or electrochemical reactions. Recent advances in this area have included the development of biosensors of compact size that enable the on-line and in situ monitoring of a large number of environmental parameters.

Screening of target-specific stress-responsive genes for the development of cell-based biosensors using a DNA microarray

In this study, we describe a straightforward strategy to develop whole cell-based biosensors using fusions of the bacterial bioluminescence genes and the promoters from chemically responsive genes within Escherichia coli, in which chemical target-responsive genes were screened by using the information of gene expression data obtained from DNA microarray analysis. Paraquat was used as a model chemical to trigger gene expression changes of E. coli and to show the DNA microarray-assisted development of whole cell-based biosensors. Gene expression data from the DNA microarray were obtained by time course analysis (10, 30, and 60 min) after exposure to paraquat. After clustering gene expression data obtained by time course analysis, a group of highly expressed genes over the all time courses could be classified. Within this group, three genes expressed highly for overall time points were selected and promoters of these genes were used as fusion partners with reporter genes, lux CDABE, to construct whole cell-based biosensors. The constructed biosensors recognized the presence of model inducer, paraquat, and structural analogue chemicals of paraquat with a high specificity, and the results were reconfirmed by using DNA microarray experiments for those structural analogues. This strategy to develop whole cell-based biosensors assisted by DNA microarray information should be useful in general for constructing chemical-specific or stress-specific biosensors with a high-throughput manner.

Improved luciferase tagging system for Listeria monocytogenes allows real-time monitoring in vivo and in vitro

An improved system for luciferase tagging Listeria monocytogenes was developed by constructing a highly active, constitutive promoter. This construct gave 100-fold-higher activity in broth than any native promoter tested and allowed for imaging of lux-tagged L. monocytogenes in food products, during murine infections, and in tumor targeting studies.

Quantification of ecotoxicological tests based on bioluminescence using Polaroid film

Assays based on the measurement of bacterial luminescence are widely used in ecotoxicology. Bacterial strains responding either to general toxicity or specific pollutants are rapid, cost-effective and easy to use. However, quantification of the signal requires relatively expensive instrumentation. We show here that the detection of luminescence of BioTox, a Vibrio fischeri-based toxicity test, and of a specific recombinant bacterial strain for arsenic determination, is possible using common Polaroid film. The exposed films can be used for visual or computer-assisted quantification of the signal. Qualitative visual comparison to standards can be used in the rapid and relatively accurate estimation of toxicity or pollutant concentration. The computer-assisted method significantly improves the accuracy and quantification of the results. The results obtained by computer-assisted quantification were in good agreement with the values obtained with a luminometer.

Applications of whole-cell bacterial sensors in biotechnology and environmental science

Biosensors have major advantages over chemical or physical analyses with regard to specificity, sensitivity, and portability. Recently, many types of whole-cell bacterial biosensors have been developed using recombinant DNA technology. The bacteria are genetically engineered to respond to the presence of chemicals or physiological stresses by synthesizing a reporter protein, such as luciferase, beta-galactosidase, or green fluorescent protein. In addition to an overview of conventional biosensors, this minireview discusses a novel type of biosensor using a photosynthetic bacterium as the sensor strain and the crtA gene, which is responsible for carotenoid synthesis, as the reporter. Since bacteria possess a wide variety of stress-response mechanisms, including antioxidation, heat-shock responses, nutrient-starvation, and membrane-damage responses, DNA response elements for several stress-response proteins can be fused with various reporter genes to construct a versatile set of bacterial biosensors for a variety of analytes. Portable biosensors for on-site monitoring have been developed using a freeze-dried biosensing strain, and cell array biosensors have been designed for high-throughput analysis. Moreover, in the future, the use of single-cell biosensors will permit detailed analyses of samples. Signals from such sensors could be detected with digital imaging, epifluorescence microscopy, and/or flow cytometry.

Enhanced degradation and toxicity reduction of dihexyl phthalate by Fusarium oxysporum f. sp. pisi cutinase

AIMS

This research aims to investigate the efficiency of two lipolytic enzymes--fungal cutinase and yeast esterase--upon the biodegradation of dihexyl phthalate (DHP).

METHOD AND RESULTS

During the enzymatic degradation of DHP dissolved in methanol, several degradation products were detected and their time-course changes were monitored using GC/MS. The DHP-degradation rate of cutinase was surprisingly high; i.e. almost 70% of the initial DHP (500 mg l(-1)) was decomposed within 4.5 h. Although the same amount of esterase was employed, more than 85% of the DHP remained after 3 days. Almost all the DHP was converted by cutinase into 1,3-isobenzofurandione (IBF), whereas hexyl methyl phthalate and IBF were abundantly produced by esterase. In addition, the toxicities of the DHP-degraded products by esterase were evaluated using various recombinant bioluminescent bacteria, which caused oxidative and protein damage, whereas the hydrolysis products from cutinase never caused any cellular damage in the methanol-containing reaction system.

CONCLUSIONS

Cutinase starts to act as a DHP-degrader much earlier and faster than esterase, with high stability in ester-hydrolytic activity, therefore a plausible approach to the practical application of cutinase for DHP degradation in the DHP-contaminated environments may be possible.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study describes the enhanced degradation and detoxification of DHP using Fusarium oxysporum f. sp. pisi cutinase.

Intracellular redox equilibrium and growth phase affect the performance of luciferase-based biosensors

Light emission from the bacterial luciferase operon has been variously exploited during last two decades. The use of convenient inducible promoters has granted significant degrees of specificity to whole cell-based assays for high-throughput screening and environmental monitoring. Nevertheless, unexplained unspecific responses have been repeatedly reported. Here, we show that the impairment of the intracellular biochemical equilibrium interferes with the luminescence produced by Escherichia coli and Staphylococcus aureus strains carrying the lux operon under constitutive or inducible control. Compounds as trimethoprim and methotrexate, by indirectly inducing NADPH accumulation, enhance light emission. Conversely, molecules driving the cell toward an oxidized state, as dimethyl sulfoxide, inhibit luminescence. These findings fit into the accepted biochemical pathway for bioluminescence, where NADPH and reducing equivalents are necessary for the production of luciferase substrates, although they do not directly take part into the light-emitting reaction. Moreover, we investigated the influence of induction timing upon the bioluminescence response from inducible reporter systems and demonstrated a correlation between the emitted light and the growth phase at which induction is performed. Our results provide explanations for some unspecific responses recorded so far in whole cell-based luminescent biosensors and emphasize the intrinsic limitations of this kind of reporting system.

Novel luciferase reporter system for in vitro and organ-specific monitoring of differential gene expression in Listeria monocytogenes

In this paper we describe construction of a luciferase-based vector, pPL2lux, and use of this vector to study gene expression in Listeria monocytogenes. pPL2lux is a derivative of the listerial integration vector pPL2 and harbors a synthetic luxABCDE operon encoding a fatty acid reductase complex (LuxCDE) involved in synthesis of the fatty aldehyde substrate for the bioluminescence reaction catalyzed by the LuxAB luciferase. We constructed pPL2lux derivatives in which the secA and hlyA promoters were translationally fused to luxABCDE and integrated as a single copy into the chromosome of L. monocytogenes EGD-e. Growth experiments revealed that hlyA was expressed predominantly in the stationary phase in LB medium buffered at pH 7.4, whereas secA expression could be detected in the exponential growth phase. Moreover, the correlation between luciferase activity and transcription levels, as determined by reverse transcriptase PCR, was confirmed using conditions known to lead to repression and activation of hemolysin expression (addition of cellobiose and activated charcoal, respectively). Furthermore, hemolysin expression could be monitored in real time during invasion of an intact monolayer of C2Bbe1 (Caco-2-derived) cells. Finally, hemolysin expression could be detected in the livers, spleens, and kidneys of mice 3 days postinfection. These experiments clearly established the effectiveness of pPL2lux as a quantitative reporter system for real-time, noninvasive evaluation of gene expression in L. monocytogenes.

Accelerated Degradation of Dipentyl Phthalate by Fusarium oxysporum f. sp. pisi Cutinase and Toxicity Evaluation of Its Degradation Products Using Bioluminescent Bacteria

The efficiency of two lipolytic enzymes (fungal cutinase and yeast esterase) in the degradation of dipentyl phthalate (DPeP) was investigated. The DPeP degradation rate of fungal cutinase was surprisingly high, i.e., almost 60% of the initial DPeP (500 mg/L) was decomposed within 2.5 hours, and nearly 40% of the degraded DPeP disappeared within the initial 15 minutes. With the yeast esterase, despite the same concentration, >87% of the DPeP remained even after 3 days of treatment. The final chemical composition after 3 days was significantly dependent on the enzyme used. During degradation with cutinase, most DPeP was converted into 1,3-isobenzofurandione (IBF) by diester hydrolysis. However, in the degradation by esterase, pentyl methyl phthalate, in addition to IBF, was produced in abundance. Toxicity monitoring using various recombinant bioluminescent bacteria showed that the degradation products from yeast esterase contained a toxic hazard, causing oxidative stress and damage to protein synthesis.

2-D PAGE analysis of pesticide-induced stress proteins of E. coli

Logarithmically growing batch cultures of Escherichia coli were exposed to sublethal concentrations of pyrethroid and carbamate pesticides of four different technical grades. This induced 17-20 stress proteins, as observed using two-dimensional polyacrylamide gel electrophoresis. An E. coli culture growing exponentially in Luria Bertani medium (cell density approximately 2.3x10(9) cells/ml) was exposed to predetermined sublethal doses of individual pesticides. The cells were harvested after 30 minutes of induction and the stress response was developed in fresh LB medium for three hours under the same growth conditions. Cell pellets were obtained and stored in sonication buffer. Two-dimensional polyacrylamide gel electrophoresis was performed to resolve the proteins. Visualization of the protein spots by rapid silver staining showed 17-20 stress proteins which were absent in the standard protein profile of E. coli. On average 29% of these stress proteins were unique to the pollutant, while the remaining stress proteins overlapped with those of other pesticides. The iso-electric points (PIs) and molecular weights of the proteins were determined by comparing with protein markers with known PIs and molecular weights. Furthermore, upon comparing the pesticide-induced proteins within the same class and between the two different classes (pyrethroid and carbamate), it was apparent that the general nature of the stress remained the same throughout, which indirectly proved that the gene or set of genes responsible for stress expression are also the same, irrespective of the chemical nature of the substituents of the pesticides.

Biodegradation of dipropyl phthalate and toxicity of its degradation products: a comparison of Fusarium oxysporum f. sp. pisi cutinase and Candida cylindracea esterase

The efficiency of two lypolytic enzymes (fungal cutinase, yeast esterase) in the degradation of dipropyl phthalate (DPrP) was investigated. The DPrP-degradation rate of fungal cutinase was surprisingly high, i.e., almost 70% of the initial DPrP (500 mg/l) was decomposed within 2.5 h and nearly 50% of the degraded DPrP disappeared within the initial 15 min. With the yeast esterase, despite the same concentration, more than 90% of the DPrP remained even after 3 days of treatment. During the enzymatic degradation of DPrP, several DPrP-derived compounds were detected and time-course changes in composition were also monitored. The final chemical composition after 3 days was significantly dependent on the enzyme used. During degradation with fungal cutinase, most DPrP was converted into 1,3-isobenzofurandione (IBF) by diester hydrolysis. However, in the degradation by yeast esterase, propyl methyl phthalate (PrMP) was produced in abundance in addition to IBF. The toxic effects of the final degradation products were investigated using various recombinant bioluminescent bacteria. As a result, the degradation products (including PrMP) from yeast esterase severely caused oxidative stress and damage to protein synthesis in bacterial cells, while in the fungal cutinase processes, DPrP was significantly degraded to non-toxic IBF after the extended period (3 days).

A cell array biosensor for environmental toxicity analysis

In this study, a cell-based array technology that uses recombinant bioluminescent bacteria to detect and classify environmental toxicity has been implemented to develop two biosensor arrays, i.e., a chip and a plate array. Twenty recombinant bioluminescent bacteria, having different promoters fused with the bacterial lux genes, were immobilized within LB-agar. About 2 microl of the cell-agar mixture was deposited into the wells of either a cell chip or a 384-well plate. The bioluminescence (BL) from the cell arrays was measured with the use of highly sensitive cooled CCD camera that measured the bioluminescent signal from the immobilized cells and then quantified the pixel density using image analysis software. The responses from the cell arrays were characterized using three chemicals that cause either superoxide damage (paraquat), DNA damage (mitomycin C) or protein/membrane damage (salicylic acid). The responses were found to be dependent upon the promoter fused upstream of the lux operon within each strain. Therefore, a sample's toxicity can be analyzed and classified through the changes in the BL expression from each well. Moreover, a time of only 2 h was needed for analysis, making either of these arrays a fast, portable and economical high-throughput biosensor system for detecting environmental toxicities.

Biodegradation and detoxification of organophosphate insecticide, malathion by Fusarium oxysporum f. sp. pisi cutinase

Efficiencies of two lypolytic enzymes (fungal cutinase and yeast esterase) in malathion degradation were investigated. Surprisingly, degradation rate of malathion by fungal cutinase was very high, i.e. almost 60% of initial malathion (500 mg l(-1)) was decomposed within 0.5 h, and nearly 50% of the degraded malathion disappeared within initial 15 min. With the yeast esterase, despite the same concentration, more than 65% of malathion remained even after 2-day treatment. During enzymatic degradation of malathion, two malathion-derived compounds were detected, and time-course changes in composition were also monitored. In the degradation by both fungal cutinase and yeast esterase, two additional organic chemicals were produced from malathion: malathion monoacid (MMA) and malathion diacid (MDA) by ester hydrolysis. Final chemical composition after 2 d was significantly dependent on the enzyme used. Fungal cutinase produced MDA as a major degradation compound. However in the malathion degradation by yeast esterase, an isomer of MMA was produced in abundance in addition to MDA. Toxic effects of malathion and its final degradation products were investigated using various recombinant bioluminescent bacteria. As a result, the degradation products (including MMA) by esterase severely caused membrane damage and inhibition of protein synthesis in bacterial cells, while in the fungal cutinase processes, malathion was significantly degraded to non-toxic MDA after the extended period (2 days).

Chemical stress sensitive luminescent human cells: Molecular biology approach using inducible Drosophila melanogaster hsp22 promoter

A whole-cell bioassay has been developed for the total toxicity testing of liquid samples. The method is based on the induction of the bioluminescent activity of genetically manipulated mammalian cells. For that purpose, transfection was used to introduce, in HeLa cells, a DNA sensing element that responds to chemical stress agents (heavy metals, genotoxic agents, and endocrine-disrupting chemicals). Such element was designed to direct the expression of a reporting gene (firefly luciferase) through the activation of Drosophila melanogaster hsp22 promoter. A molecular approach was conducted to optimize hsp22 promoter element in order to decrease the background expression level of the reporting gene and to increase the sensitivity of the bioassay for testing endocrine disruptors. As a result, in the presence of 20-100 microM cadmium chloride, a 6-fold increase in luciferase expression was obtained using a specially designed truncated hsp22 promoter construction. The following chemicals known to be found in the polluted samples were tested: CdCl2, Cd(NO3)2, NaAsO2, alachlore, fentine acetate, thiram, and maneb. The stressing effect of each of them was sensitively detected by the present bioassay in the 0.05-50 microM concentration range.

Quantitative and kinetic study of oxidative stress regulons using green fluorescent protein

Potentially damaging reactive oxygen species (ROS) are involved in a number of pathways ranging from signal transduction to apoptosis. Cells have adapted this alteration in redox status into a complex regulatory mechanism. ROS are specifically able to induce the expression of a multitude of genes. We constructed and characterized "oxidative stress probes" consisting of promoter fusions of several ROS-induced genes and the green fluorescent protein (GFP) reporter gene. Specifically, the sodA, fumC, zwf, acnA, acrAB, and soxS genes from the SoxRS regulon and the katG and ahpC genes from OxyR regulon, which respond to the superoxide anion and hydrogen peroxide, were studied. Our results revealed not only different levels of background transcription, but different induction levels both in terms of timing and strength. These systematic studies were performed under a uniform parallel platform and have provided insight into the complicated gene regulation of the oxidative stress regulons.

Enzymatic degradation of dibutyl phthalate and toxicity of its degradation products

Dibutyl phthalate (DBP) was more efficiently degraded by cutinase compared to yeast esterase; i.e. almost 80% of initial DBP (500 mg l(-1)) was decomposed within 7.5 h, and nearly 50% of the degraded DBP disappeared within the initial 30 min. The toxicity of the final DBP degradation products were investigated using various recombinant bioluminescent bacteria. Butyl methyl phthalate, the major product of degradation by the esterase, was an oxidative toxic hazard that damaged protein synthesis.

Effects of environmental stresses on the activities of the uspA, grpE and rpoS promoters of Escherichia coli O157:H7

Heat shock proteins and RNA polymerase sigma factor play an important role in protecting cells against environmental stresses, including starvation, osmotic and oxidative stresses, and cold shock. In this study, the effect of environmental stresses on activity of the auto-fluorescent Escherichia coli O157:H7 generated by the fusion of gfp(uv) to E. coli uspA, grpE and rpoS promoters were examined. Osmotic shock caused about a 4-fold increase in green fluorescence of E. coli O157:H7 harboring uspA::gfp(uv) or rpoS::gfp(uv) at 37 degrees C and room temperature whereas osmotic shock at 5 degrees C did not induce green fluorescence. When starved, E. coli O157:H7 possessing grpE::gfp(uv) was more sensitive for evaluating stress at low temperature while uspA::gfp(uv) was better suited for detecting the stress response at higher temperature. The uspA, grpE and rpoS promoters were up-regulated to varying degrees by stresses commonly encountered during food processing.

Water toxicity detection by a panel of stress-responsive luminescent bacteria

A panel of Escherichia coli strains harbouring different stress-responsive promoters fused to a lux reporter system was used to assess the potential toxicity of 17 unknown model water samples. Using liquid cultures, nine out of 14 toxic samples were properly identified as toxic, whereas five were false negatives. All three non-toxic controls were identified correctly (no false positives). Two strains containing promoter-lux fusions were also tested when immobilized onto fibre-optic tips. One genotoxic sample and six toxic samples were correctly identified in this manner. The potential advantages and limitations in the use of genetically engineered bacteria as biosensors for water toxicity are discussed in view of these results.

Determination of geno- and cytotoxicity of groundwater and sediments using the recombinant SWITCH test

The recombinant Salmonella typhimurium TA1535 strain carrying the SWITCH plasmid (combined construct of the SOS-Lux plasmid pPLS-1 and the LAC-Fluoro plasmid pGFPuv: pSWITCH) was treated with control substances for genotoxic (2-aminoanthracene) and cytotoxic (aureomycin) potency as well as with 18 environmental samples (groundwater, river water, sediments) provided at the SENSPOL Technical Meeting on Problems Related to Diffuse Pollution Sources (Characterization of Sediment, Dredged Material, and Groundwater) organized by the Federal Institute of Hydrology in Koblenz, Germany, during late October 2003. For metabolic activation the samples were treated with S9 mix (5% S9 fraction in cofactor mix, Aroclor 1254-induced rat liver microsomes, Moltox Inc., Mol, Belgium). Simultaneously determined cytotoxicity and genotoxicity data were derived through the Multilabel Counter 1420 Victor (PerkinElmer, Boston, MA, USA), by sequential measurement of luminescence, absorbance, and fluorescence. The newly designed SWITCH test, as it was applied at the Koblenz meeting, displays a comparable sensitivity for test samples with known cyto- and genotoxic potential. Groundwater samples from the wells at the former gas plant site Kiel Canal and the agricultural area at Niederwerth expressed neither genotoxic nor cytotoxic responses of the bacteria for both metabolic conditions (+/-S9). Spiked groundwater samples from the Niederwerth well BW1 and the Urmitz well U12 located on the river Rhine were identified to be positive in terms of genotoxicity for the direct and the metabolic approach. Samples from the lake Tiefer See in the city of Potsdam showed a reduction of GFPuv expression as an indication for cytotoxicity, while luminescence output of incubated bacteria remained unaffected. This reflects the well-known presence of contaminations (especially cyanides) in the lake sediment as well as in the acetonic extract. The results obtained at the SENSPOL Technical Meeting show the SWITCH test to be of major relevance not only for the analysis of chemicals under laboratory conditions but also for environmental samples polluted by diffuse industrial sources.

In situ measurement of bioluminescence and fluorescence in an integrated microbioreactor

Reporter strains of bacteria that emit light or a fluorescent marker in response to specific conditions in their environment are having a significant impact in many areas of biology, including toxicity assays for environmental pollutants, chemical detection, and gene expression profiling. We have demonstrated methods for in situ measurements of bioluminescence and fluorescence from bacterial cultures grown in 50 microL instrumented microbioreactors. Results from microbioreactors were compared to results obtained from conventional 500 mL batch bioreactors and shake flasks. Experiments were conducted with reporter strains of Escherichia coli in which luxCDABE or gfp was fused to a promoter that was either expressed constitutively, or that responded to oxygen limitation. With these reporter strains, we have demonstrated the ability to obtain information on growth conditions within the microbioreactor. We have also shown that the large aspect ratio of the microbioreactor provides a unique advantage over measurements in larger bioreactors by reducing the inner filter effect in on-line measurements and eliminating the need for error-prone off-line dilutions. In addition, continuous on-line monitoring of genes in real-time, when expanded to include entire reporter libraries, could potentially provide a true dynamic picture of cellular gene expression from which the kinetics of gene expression can be untangled and elucidated.

Stress responsive bacteria: biosensors as environmental monitors

The delicate and dynamic balance of the physiological steady state and its maintenance is well characterized by studies of bacterial stress response. Through the use of genetic analysis, numerous stress regulons, their physiological regulators and their biochemical processes have been delineated. In particular, transcriptionally activated stress regulons are subjects of study and application. These regulons include those that respond to macromolecular damage and toxicity as well as to nutrient starvation. The convenience of reporter gene fusions has allowed the creation of biosensor strains, resulting from the fusion of stress-responsive promoters with a variety of reporter genes. Such cellular biosensors are being used for monitoring dynamic systems and can report the presence of environmental stressors in real time. They provide a greater range of sensitivity, e.g. to sub-lethal concentrations of toxicants, than the simple assessment of cell viability. The underlying physiological context of the reporter strains results in the detection of bioavailable concentrations of both toxicants and nutrients. Culture conditions and host strain genotypes can be customized so as to maximize the sensitivity of the strain for a particular application. Collections of specific strains that are grouped in panels are used to diagnose targets or mode of action for unknown toxicants. Further application in massive by parallel DNA and gene fusion arrays greatly extends the information available for diagnosis of modes of action and may lead to development of novel high-throughput screens. Future studies will include more panels, arrays, as well as single reporter cell detection for a better understanding of the population heterogeneity during stress response. New knowledge of physiology gained from further studies of novel systems, or using innovative methods of analysis, will undoubtedly yield still more useful and informative environmental biosensors.

Enhancement of the multi-channel continuous monitoring system through the use of Xenorhabdus luminescens lux fusions

The enhancement of the multi-channel continuous toxicity monitoring system developed previously was studied. To achieve better and more stable results from the system, the use of thermo-lux fusion strains that express the luxCDABE genes from Xenorhabdus luminescens was evaluated. A total of six recombinant Escherichia coli strains with the promoters from three oxidative-stress responsive genes, i.e. the katG, sodA and pqi-5 genes, fused to either the lux genes from Vibrio fischeri or X. luminescens were characterized and their responses to different chemicals compared. It was found that the basal level bioluminescence (BL) from the thermo-lux fusion strains was always higher while that of the V. fischeri lux strains were always near or below the lower limit of detection of the system. For example, the katG::V. fischeri lux strain, DPD2511, gave no discernible response due to its low level expression while a fusion of the katG promoter with the X. luminescens lux operon was clearly responsive and capable of detecting hydrogen peroxide down to about 1 ppm. The use of the thermo-lux strains found them to be as sensitive as the V. fischeri lux strains while providing a brighter, more stable basal level bioluminescence, making the analysis and monitoring of water-borne toxicity more reliable.

Assessing wastewater toxicity to activated sludge: recent research and developments

Toxicants in municipal sewage treatment plant (STP) influent wastewater may inhibit the biological activity of the activated sludge and cause treatment plant process upsets. Such process upsets may be avoided if influent wastewater is monitored for toxicity and protective actions are taken when toxicity is detected. A comprehensive review of the methods that can be used for assessing wastewater toxicity to biological treatment systems was conducted several years ago and the resultant report was published in 2000 by Water Environment Research Foundation (WERF). The WERF report also specified the criteria for influent wastewater toxicity monitoring methods and suggested research needs to be addressed. A significant amount of effort was made since the publication of the WERF report to develop new assays or devices and to improve existing ones. In this manuscript, recent research and developments in methods for assessing wastewater toxicity to activated sludge were reviewed. The literature indicates that bioluminescence- and respirometry-based methods received much attention in recent research. A comparison of the new/improved methods with the criteria described in the WERF report reveals that none of these methods has been shown to meet all the specified criteria. The present review also indicates that research efforts since 2000 have not fully taken into account the criteria for influent wastewater toxicity monitoring methods and have not addressed the research needs proposed in the WERF report.

Living Bacterial Cell Array for Genotoxin Monitoring

A biosensor composed of a high-density living bacterial cell array was fabricated by inserting bacteria into a microwell array formed on one end of an imaging fiber bundle. The size of each microwell allows only one cell to occupy each well. In this biosensor, E. coli cells carrying a recA::gfp fusion were used as sensing components for genotoxin detection. Each fiber in the array has its own light pathway, enabling thousands of individual cell responses to be monitored simultaneously with both spatial and temporal resolution. The biosensor was capable of performing cell-based functional sensing of a genotoxin with high sensitivity and short incubation times (1 ng/mL mitomycin C after 90 min). Dose-response curves for several genotoxins were obtained. The biosensors demonstrated an active sensing lifetime of more than 6 h and a shelf lifetime of two weeks.

Construction and characterization of novel dual stress-responsive bacterial biosensors

Using the genes for the green fluorescence protein and Xenorhabdus luminescens luciferase operon and the promoters for the recA and katG genes, two stress-responsive Escherichia coli biosensor strains have been constructed that can individually or concurrently respond to oxidative and genotoxic conditions. Strain DUO-1 carries the pRGDK1 plasmid, which has the recA::GFPuv4 and katG::luxCDABE fusion genes oriented divergently with each other, while in DUO-2, i.e., pRGDK2, they are in a tandem orientation, with the recA promoter showing run-though transcription of the katG::luxCDABE fusion. These two strains and their responses were characterized using several known hydroxyl radical-forming chemicals, e.g., hydrogen peroxide and cadmium chloride, along with some genotoxins, e.g., mitomycin C and methyl-N-nitro-N-nitrosoguanidine, and some general toxicants. Both strains showed an induction of green fluorescent protein (GFP) and bioluminescence when they experienced DNA and oxidative damage, respectively, while the tandem orientation of the two fusion genes within DUO-2 allowed it to also sensitively respond to genotoxins via the production of bioluminescence. However, the characteristics of DUO-2's bioluminescent response to each stress were easily distinguishable, making it useful for the detection of both stresses. Furthermore, tests with mixtures of chemicals showed that both DUO-1 and DUO-2 were responsive when chemicals causing oxidative or genotoxic stress were present as a single chemical or within complex chemical mixtures.