Development of a yeast biosensor–biocatalyst for the detection and biodegradation of the organophosphate paraoxon

Novel yeast-based biosensor for environmental monitoring of tebuconazole

Due to increasing demand for high and stable crop production, human populations are highly dependent on pesticide use for growing and storing food. Environmental monitoring of these agrochemicals is therefore of utmost importance, because of their collateral effects on ecosystem and human health. Even though most current-use analytical methods achieve low detection limits, they require procedures that are too complex and costly for routine monitoring. As such, there has been an increased interest in biosensors as alternative or complementary tools to streamline detection and quantification of environmental contaminants. In this work, we developed a biosensor for environmental monitoring of tebuconazole (TEB), a common agrochemical fungicide. For that purpose, we engineered S. cerevisiae cells with a reporter gene downstream of specific promoters that are expressed after exposure to TEB and characterized the sensitivity and specificity of this model system. After optimization, we found that this easy-to-use biosensor consistently detects TEB at concentrations above 5 μg L-1 and does not respond to realistic environmental concentrations of other tested azoles, suggesting it is specific. We propose the use of this system as a complementary tool in environmental monitoring programs, namely, in high throughput scenarios requiring screening of numerous samples. KEY POINTS: • A yeast-based biosensor was developed for environmental monitoring of tebuconazole. •The biosensor offers a rapid and easy method for tebuconazole detection ≥ 5 μg L-1. •The biosensor is specific to tebuconazole at environmentally relevant concentrations.

New approaches in bioprocess-control: Consortium guidance by synthetic cell-cell communication based on fungal pheromones

Bioconversions in industrial processes are currently dominated by single-strain approaches. With the growing complexity of tasks to be carried out, microbial consortia become increasingly advantageous and eventually may outperform single-strain fermentations. Consortium approaches benefit from the combined metabolic capabilities of highly specialized strains and species, and the inherent division of labor reduces the metabolic burden for each strain while increasing product yields and reaction specificities. However, consortium-based designs still suffer from a lack of available tools to control the behavior and performance of the individual subpopulations and of the entire consortium. Here, we propose to implement novel control elements for microbial consortia based on artificial cell-cell communication via fungal mating pheromones. Coupling to the desired output is mediated by pheromone-responsive gene expression, thereby creating pheromone-dependent communication channels between different subpopulations of the consortia. We highlight the benefits of artificial communication to specifically target individual subpopulations of microbial consortia and to control e.g. their metabolic profile or proliferation rate in a predefined and customized manner. Due to the steadily increasing knowledge of sexual cycles of industrially relevant fungi, a growing number of strains and species can be integrated into pheromone-controlled sensor-actor systems, exploiting their unique metabolic properties for microbial consortia approaches.

Suitability of a Saccharomyces cerevisiae-based assay to assess the toxicity of pyrimethanil sprayed soils via surface runoff: comparison with standard aquatic and soil toxicity assays

The present study is aimed at evaluating whether a gene expression assay with the microbial eukaryotic model Saccharomyces cerevisiae could be used as a suitable warning tool for the rapid preliminary screening of potential toxic effects on organisms due to scenarios of soil and water contamination with pyrimethanil. The assay consisted of measuring changes in the expression of the selected pyrimethanil-responsive genes ARG3 and ARG5,6 in a standardized yeast population. Evaluation was held by assessing the toxicity of surface runoff, a major route of pesticide exposure in aquatic systems due to non-point-source pollution, which was simulated with a pyrimethanil formulation at a semifield scale mimicking worst-case scenarios of soil contamination (e.g. accident or improper disposal). Yeast cells 2-h exposure to the runoff samples led to a significant 2-fold increase in the expression of both indicator genes. These results were compared with those from assays with organisms relevant for the aquatic and soil compartments, namely the nematode Caenorhabditis elegans (reproduction), the freshwater cladoceran Daphnia magna (survival and reproduction), the benthic midge Chironomus riparius (growth), and the soil invertebrates Folsomia candida and Enchytraeus crypticus (survival and reproduction). Under the experimental conditions used to simulate accidental discharges into soil, runoff waters were highly toxic to the standard test organisms, except for C. elegans. Overall, results point out the usefulness of the yeast assay to provide a rapid preview of the toxicity level in preliminary screenings of environmental samples in situations of inadvertent high pesticide contamination. Advantages and limitations of this novel method are discussed.

Environmental applications of photoluminescence-based biosensors

For monitoring and treatment of soil and water, environmental scientists and engineers require measurements of the concentration of chemical contaminants. Although laboratory-based methods relying on gas or liquid chromatography can yield very accurate measurements, they are also complex, time consuming, expensive, and require sample pretreatment. Furthermore, they are not readily adapted for in situ measurements.Sensors are devices that can provide continuous, in situ measurements, ideally without the addition of reagents. A biosensor incorporates a biological component coupled to a transducer, which translates the interaction between the analyte and the biocomponent into a signal that can be processed and reported. A wide range of transducers have been employed in biosensors, the most common of which are electrochemical and optical. In this contribution, we focus on photoluminescence-based biosensors of potential use in the applications described above.Following a review of photoluminescence and a discussion of the optoelectronic hardware part of these biosensor systems, we provide explanations and examples of optical biosensors for specific chemical groups: hydrocarbons and alcohols, halogenated organics, nitro-, phospho-, sulfo-, and other substituted organics, and metals and other inorganics. We also describe approaches that have been taken to describe chemical mixtures as a whole (biological oxygen demand and toxicity) since most environmental samples contain mixtures of unknown (and changing) composition. Finally, we end with some thoughts on future research directions that are necessary to achieve the full potential of environmental biosensors.

Novel bacterial bioassay for a high-throughput screening of 4-hydroxyphenylpyruvate dioxygenase inhibitors

Plant 4-hydroxyphenylpyruvate dioxygenase (HPPD) is the molecular target of a range of synthetic β-triketone herbicides that are currently used commercially. Their mode of action is based on an irreversible inhibition of HPPD. Therefore, this inhibitory capacity was used to develop a whole-cell colorimetric bioassay with a recombinant Escherichia coli expressing a plant HPPD for the herbicide analysis of β-triketones. The principle of the bioassay is based on the ability of the recombinant E. coli clone to produce a soluble melanin-like pigment, from tyrosine catabolism through p-hydroxyphenylpyruvate and homogentisate. The addition of sulcotrione, a HPPD inhibitor, decreased the pigment production. With the aim to optimize the assay, the E. coli recombinant clone was immobilized in sol-gel or agarose matrix in a 96-well microplate format. The limit of detection for mesotrione, tembotrione, sulcotrione, and leptospermone was 0.069, 0.051, 0.038, and 20 μM, respectively, allowing to validate the whole-cell colorimetric bioassay as a simple and cost-effective alternative tool for laboratory use. The bioassay results from sulcotrione-spiked soil samples were confirmed with high-performance liquid chromatography.

Generation of a mutagenized organophosphorus hydrolase for the biodegradation of the organophosphate pesticides malathion and demeton-S

AIMS

The bacterial organophosphorus hydrolase (OPH) enzyme hydrolyses and detoxifies a broad range of toxic organophosphate pesticides and warfare nerve agents by cleaving the various phosphorus-ester bonds (P-O, P-F, P-CN, P-S); however, OPH hydrolyses these bonds with varying efficiencies. The aim of this study was to generate a variant OPH enzyme with improved hydrolytic efficiency against the poorly hydrolysed P-S class of organophosphates.

METHODS AND RESULTS

The gene encoding OPH was sequentially mutated at specific codons by saturation mutagenesis and screened for improved activity against the P-S substrates demeton-S methyl and malathion. Escherichia coli lysates harbouring the variants displayed up to 177- and 1800-fold improvement in specific activity against demeton-S methyl and malathion, respectively, compared to the wild-type lysates. The specificity constants of the purified variant proteins were improved up to 25-fold for demeton-S methyl and malathion compared to the wild-type. Activity was associated with organophosphate detoxification as the hydrolysed substrate lost the ability to inhibit acetylcholinesterase. The improved hydrolytic efficiency against demeton-S translated to the improved ability to hydrolyse the warfare agent VX.

CONCLUSIONS

OPH variant enzymes were generated that displayed significantly improved ability to hydrolyse and detoxify organophosphates harbouring the P-S bond.

SIGNIFICANCE AND IMPACT OF THE STUDY

The long-term goal is to generate an environmentally-friendly enzyme-mediated bioremediation approach for the removal of toxic organophosphate compounds in the environment.