Construction of a modified mini-Tn5 luxCDABE transposon for the development of bacterial biosensors for ecotoxicity testing

Microbial whole-cell biosensors: Current applications, challenges, and future perspectives

Microbial Whole-Cell Biosensors (MWCBs) have seen rapid development with the arrival of 21st century biological and technological capabilities. They consist of microbial species which produce, or limit the production of, a reporter protein in the presence of a target analyte. The quantifiable signal from the reporter protein can be used to determine the bioavailable levels of the target analyte in a variety of sample types at a significantly lower cost than most widely used and well-established analytical instrumentation. Furthermore, the versatile and robust nature of MWCBs shows great potential for their use in otherwise unavailable settings and environments. While MWCBs have been developed for use in biomedical, environmental, and agricultural monitoring, they still face various challenges before they can transition from the laboratory into industrialized settings like their enzyme-based counterparts. In this comprehensive and critical review, we describe the underlying working principles of MWCBs, highlight developments for their use in a variety of fields, detail challenges and current efforts to address them, and discuss exciting implementations of MWCBs helping redefine what is thought to be possible with this expeditiously evolving technology.

Implication of the Type III Effector RipS1 in the Cool-Virulence of Ralstonia solanacearum Strain UW551

Members of the Ralstonia solanacearum species complex cause a variety of wilting diseases across a wide range of hosts by colonizing and blocking xylem vessels. Of great concern are race 3 biovar 2 strains of R. solanacearum capable of causing brown rot of potato at cool temperatures, which are select agents in the United States. To gain a better understanding of cool-virulence mechanisms, we generated libraries of transposon mutants in the cool-virulent R. solanacearum strain UW551 and screened 10,000 mutants using our seedling assay for significantly reduced virulence at 20°C. We found several mutants that exhibited reduced virulence at 28 and 20°C and also mutants that were only affected at the cooler temperature. One mutant of the latter chosen for further study had the transposon inserted in an intergenic region between a type III secretion system effector gene ripS1 and a major facilitator superfamily (MFS) protein gene. Gene expression analysis showed that expression of ripS1 was altered by the transposon insertion, but not the MFS protein gene. An independent mutant with this insertion upstream of ripS1 was generated and used to confirm virulence and gene expression phenotypes. The effector, RipS1, has unknown function and is part of a family of effectors belonging to the largest known type III effectors. The functional connection between RipS1 and cool-virulence of R. solanacearum UW551 suggests that RipS1 (and/or its upstream promoter element) may serve as a potential target for development of cool-virulence-specific diagnostic tools to differentiate the highly regulated cool-virulent strains from non-cool-virulent strains of R. solanacearum. Our results provide important information for continued work toward a better understanding of cool-virulence of R. solanacearum and development of proper control strategies to combat this important plant pathogen.

A high-throughput virulence screening method for the Ralstonia solanacearum species complex

Ralstonia solanacearum species complex strains are the causative agents for wilting diseases of many plants, including the economically important brown rot of potato. We developed a high-throughput virulence screen that is implemented in 96-well microtiter plates using seedlings grown in soft water agar to save space, effort, and resources. Nicotiana glutinosa was determined to be the most effective host for this assay, and we confirmed bacterial growth and systemic spread in inoculated seedlings. In our assay, N. glutinosa seeds were sown quickly and easily on top of individual water agar wells of a 96-well plate by pipetting out desired number of seeds in an aqueous suspension. They were inoculated on the same day by first touching a bacterial colony with an autoclaved toothpick and then stabbing the toothpick into the center of the water agar well. Such inoculation method resulted in inocula above a threshold of 2 × 10⁴ CFU per well achieving consistent virulence results and enabling reduction of inoculum preparation efforts to facilitate high-throughput screening. Our assay is suitable for forward genetic screening of a large number of strains, isolates or mutants for disease symptoms under both cool (20 °C) and warm (28 °C) temperature conditions before detailed studies can be narrowed down to a manageable number of desired candidates. Our virulence screen method provides a valuable tool for future work in understanding genetics of virulence of Rssc, especially cool virulence of the highly regulated race 3 biovar 2 group of R. solanacearum, leading toward development of effective control strategies.

Propagation capacity of bacterial contaminants in platelet concentrates using a luciferase reporter system

INTRODUCTION

Currently the use of molecular tools and techniques of Genetic Engineering in the study of microbial behavior in blood components has replaced the employment of classical methods of microbiology. This work focuses on the use of a novel lux reporter system for monitoring the contaminating propagation capacity of bacteria present in platelet concentrates under standard storage conditions in the blood bank.

METHODS

A miniTn5 promotor probe carrying the lux operon from Photorhabdus luminiscens (pUTminiTn5luxCDABEKm2) was used to construct four bacterial bioluminescent mutants: Escherichia coli, Salmonella typhi, Proteus mirabilis and Pseudomonas aeruginosa. Luminescent mutants were used for contamination tests with 20 CFU in platelet concentrates bags and were stored under standard storage conditions in the blood bank (100 rpm at 22 °C). The measurements of luminous activity and optical density were used to monitor bacterial proliferation during 7 days (168 h).

RESULTS

During the exponential growth phase (log) of bacterial strains, a lineal correlation between luminous activity vs biomass was observed (R(2) = 0.985, 0.976, 0.981) for E. coli::Tn5luxCDABEKm2, P. mirabilis::Tn5luxCDABEKm2 and P. auriginosa::Tn5luxCDABEKm2, respectively. The above indicates that metabolic activity (production of ATP) is directly related to biomass in this phase of microbial growth. While conducting experiments, the inability to propagate S. typhi::Tn5luxCDABEKm2 was detected. We can speculate that platelet concentrates contain specific components that prevent the propagation of S. typhi.

CONCLUSION

The use of luxCDABE system for the quantification of luminous activity is a rapid and sensitive alternative to study the propagation and auto-sterilization of bacterial contaminants in platelet concentrates.

Evaluation of the ecotoxicity of pollutants with bioluminescent microorganisms

This chapter deals with the use of bioluminescent microorganisms in environmental monitoring, particularly in the assessment of the ecotoxicity of pollutants. Toxicity bioassays based on bioluminescent microorganisms are an interesting complement to classical toxicity assays, providing easiness of use, rapid response, mass production, and cost effectiveness. A description of the characteristics and main environmental applications in ecotoxicity testing of naturally bioluminescent microorganisms, covering bacteria and eukaryotes such as fungi and dinoglagellates, is reported in this chapter. The main features and applications of a wide variety of recombinant bioluminescent microorganisms, both prokaryotic and eukaryotic, are also summarized and critically considered. Quantitative structure-activity relationship models and hormesis are two important concepts in ecotoxicology; bioluminescent microorganisms have played a pivotal role in their development. As pollutants usually occur in complex mixtures in the environment, the use of both natural and recombinant bioluminescent microorganisms to assess mixture toxicity has been discussed. The main information has been summarized in tables, allowing quick consultation of the variety of luminescent organisms, bioluminescence gene systems, commercially available bioluminescent tests, environmental applications, and relevant references.

Genetically modified whole-cell bioreporters for environmental assessment

Living whole-cell bioreporters serve as environmental biosentinels that survey their ecosystems for harmful pollutants and chemical toxicants, and in the process act as human and other higher animal proxies to pre-alert for unfavorable, damaging, or toxic conditions. Endowed with bioluminescent, fluorescent, or colorimetric signaling elements, bioreporters can provide a fast, easily measured link to chemical contaminant presence, bioavailability, and toxicity relative to a living system. Though well tested in the confines of the laboratory, real-world applications of bioreporters are limited. In this review, we will consider bioreporter technologies that have evolved from the laboratory towards true environmental applications, and discuss their merits as well as crucial advancements that still require adoption for more widespread utilization. Although the vast majority of environmental monitoring strategies rely upon bioreporters constructed from bacteria, we will also examine environmental biosensing through the use of less conventional eukaryotic-based bioreporters, whose chemical signaling capacity facilitates a more human-relevant link to toxicity and health-related consequences.

Impact of sublethal levels of environmental pollutants found in sewage sludge on a novel Caenorhabditis elegans model biosensor

A transgenic strain of the model nematode Caenorhabditis elegans in which bioluminescence reports on relative, whole-organism ATP levels was used to test an environmentally-relevant mixture of pollutants extracted from processed sewage sludge. Changes in bioluminescence, following exposure to sewage sludge extract, were used to assess relative ATP levels and overall metabolic health. Reproductive function and longevity were also monitored. A short (up to 8 h) sublethal exposure of L4 larval stage worms to sewage sludge extract had a concentration-dependent, detrimental effect on energy status, with bioluminescence decreasing to 50-60% of the solvent control (1% DMSO). Following longer exposure (22-24 h), the energy status of the nematodes showed recovery as assessed by bioluminescence. Continuous exposure to sewage sludge extract from the L4 stage resulted in a shorter median lifespan relative to that of solvent or medium control animals, but only in the presence of 400-600 µM 5-fluoro-2'-deoxyuridine (FUdR), which was incorporated to inhibit reproduction. This indicated that FUdR increased lifespan, and that the effect was counteracted by SSE. Exposure to sewage sludge extract from the L1 stage led to slower growth and a delayed onset of egg laying. When L1 exposed nematodes reached the reproductive stage, no effect on egg laying rate or egg number in the uterus was observed. DMSO itself (1%) had a significant inhibitory effect on growth and development of C. elegans exposed from the L1 stage and on reproduction when exposed from the L4 stage. Results demonstrate subtle adverse effects on C. elegans of a complex mixture of environmental pollutants that are present, individually, in very low concentrations and indicate that our biosensor of energy status is a novel, sensitive, rapid, quantitative, whole-organism test system which is suitable for high throughput risk assessment of complex pollutant mixtures.

Real-time monitoring of metabolic shift and transcriptional induction of yciG::luxCDABE E. coli reporter strain to a glucose pulse of different concentrations

Ineffective mixing entailing heterogeneity issue within industrial bioreactors has been reported to affect microbial physiology and consequently bioprocess performances. Alteration of these performances results from microorganism ability to modulate their physiology at metabolic and/or transcriptional levels in order to survive in a given environment. Until now, dynamics of both metabolic and transcriptional microbial response to external stimuli have been investigated using mainly ex situ measurements with sampling and/or quenching constraints. This work showed an in situ bioluminescence approach for real-time monitoring of characteristic stress responses of Escherichia coli containing yciG::luxCDABE reporter to glucose pulses in well-controlled steady-state chemostat cultures. Reproducibility of in situ bioluminescence profiles was assessed. A dramatic transient increase in the bioluminescence intensity (sharp peak) was observed for a complete depletion of sugars and for a sudden decrease in the dilution rate. This response was connected to a sudden change of the metabolic activity. On the contrary a bell curve of bioluminescence intensity, dose-dependent, was related to an induction of transcriptional activity. Real-time monitoring of the bioluminescence signal with time-span less than a second gave access to the characteristic times of the metabolic shift and transcriptional induction of the stress response.

lux-Marking and application of carbofuran degrader Burkholderia cepacia PCL3

A luxAB-mutant of the carbofuran degrading bacterium Burkholderia cepacia PCL3 was successfully constructed with the capability to emit a luminescence signal of 1.6×10(-3)RLUcfu(-1). The mutant has a growth pattern and carbofuran degradation ability similar to PCL3 wild-type. The luminescent emission by PCL3:luxAB1 directly correlated with the metabolic activity of the cells. The optimal pH, temperature and n-decanal concentration for luminescence emission are 7.0, 35°C and 0.01%, respectively. PCL3:luxAB1 was used to assess the toxicity of carbofuran and carbofuran phenol in basal salt medium (BSM) in which the different sensitivity of the cells is dependent on the biomass concentration. With the luciferase system, the degradative fraction of the augmented PCL3:luxAB1 and the difference between the active augmented PCL3:luxAB1 and indigenous microorganisms at the contaminated site could be indicated.

Predicting bioremediation of hydrocarbons: laboratory to field scale

There are strong drivers to increasingly adopt bioremediation as an effective technique for risk reduction of hydrocarbon impacted soils. Researchers often rely solely on chemical data to assess bioremediation efficiently, without making use of the numerous biological techniques for assessing microbial performance. Where used, laboratory experiments must be effectively extrapolated to the field scale. The aim of this research was to test laboratory derived data and move to the field scale. In this research, the remediation of over thirty hydrocarbon sites was studied in the laboratory using a range of analytical techniques. At elevated concentrations, the rate of degradation was best described by respiration and the total hydrocarbon concentration in soil. The number of bacterial degraders and heterotrophs as well as quantification of the bioavailable fraction allowed an estimation of how bioremediation would progress. The response of microbial biosensors proved a useful predictor of bioremediation in the absence of other microbial data. Field-scale trials on average took three times as long to reach the same endpoint as the laboratory trial. It is essential that practitioners justify the nature and frequency of sampling when managing remediation projects and estimations can be made using laboratory derived data. The value of bioremediation will be realised when those that practice the technology can offer transparent lines of evidence to explain their decisions.

Monitoring of environmental pollutants by bioluminescent bacteria

This review deals with the applications of bioluminescent bacteria to the environmental analyses, published during the years 2000-2007. The ecotoxicological assessment, by bioassays, of the environmental risks and the luminescent approaches are reported. The review includes a brief introduction to the characteristics and applications of bioassays, a description of the characteristics and applications of natural bioluminescent bacteria (BLB), and a collection of the main applications to organic and inorganic pollutants. The light-emitting genetically modified bacteria applications, as well as the bioluminescent immobilized systems and biosensors are outlined. Considerations about commercially available BLB and BLB catalogues are also reported. Most of the environmental applications, here mentioned, of luminescent organisms are on wastewater, seawater, surface and ground water, tap water, soil and sediments, air. Comparison to other bioindicators and bioassay has been also made. Various tables have been inserted, to make easier to take a rapid glance at all possible references concerning the topic of specific interest.

Application of luminescent biosensors for monitoring the degradation and toxicity of BTEX compounds in soils

AIMS

To assess the changes in acute toxicity and biodegradation of benzene, toluene, ethylbenzene and xylene (collectively referred to as BTEX) compounds in soil over time and compare the performances of biological and chemical techniques.

METHODS AND RESULTS

Biological methods (lux-based bacterial biosensors, basal respiration and dehydrogenase activity) were related to changes in the concentration of the target compounds. There was an initial increase in toxicity determined by the constitutively expressed biosensor, followed by a continual reduction as degradation proceeded. The biosensor with the BTEX-specific promoter was most induced when BTEX concentrations were highest. The treatment with nutrient amendment had a significant increase in microbial activity, while the sterile control produced the lowest level of degradation.

SIGNIFICANCE AND IMPACT OF THE STUDY

Luminescent biosensors were able to monitor changes in contaminant toxicity and bioavailability in aqueous extracts from BTEX-impacted soils as degradation proceeded. The integration of biological tests with chemical analysis enables a fuller understanding of the biodegradation processes occurring at their relative rates.

CONCLUSIONS

The biological methods were successfully used in assessing the performance of different treatments for enhancing natural attenuation of BTEX from contaminated soils. While, chemical analysis showed biodegradation of parent BTEX compounds in biologically active soils, the biosensor assays reported on changes in bioavailability and potentially toxic intermediate fractions as they estimated the integrative effect of contaminants.

The potential of site-specific recombinases as novel reporters in whole-cell biosensors of pollution

DNA recombinases show some promise as reporters of pollutants providing that appropriate promoters are used and that the apparent dependence of expression on cell density can be solved. Further work is in progress using different recombinases and other promoters to optimize recombinase expression as well as to test these genetic constructs in contaminated environmental samples such as soil and water. It may be that a graded response reflecting pollutant concentration may not be possible. However, they show great promise for providing definitive detection systems for the presence of a pollutant and may be applicable to address the problem of bioavailability of pollutants in complex environments such as soil.

Novel cyanobacterial biosensor for detection of herbicides

The aim of this work was to generate a cyanobacterial biosensor that could be used to detect herbicides and other environmental pollutants. A representative freshwater cyanobacterium, Synechocystis sp. strain PCC6803, was chromosomally marked with the luciferase gene luc (from the firefly Photinus pyralis) to create a novel bioluminescent cyanobacterial strain. Successful expression of the luc gene during growth of Synechocystis sp. strain PCC6803 cultures was characterized by measuring optical density and bioluminescence. Bioluminescence was optimized with regard to uptake of the luciferase substrate, luciferin, and the physiology of the cyanobacterium. Bioassays demonstrated that a novel luminescent cyanobacterial biosensor has been developed which responded to a range of compounds including different herbicide types and other toxins. This biosensor is expected to provide new opportunities for the rapid screening of environmental samples or for the investigation of potential environmental damage.

Vectors to express foreign genes and techniques to monitor gene expression in Pseudomonads

Improved tools for Pseudomonas research include small, broad-host-range vectors that allow regulated expression from the lac operon and T7 promoters whose biology is well understood and adaptable to many bacteria. To facilitate studies on gene regulation, tracking and monitoring of bacteria in diverse environments, and the construction of biosensors, various reporter genes with versatile assay formats have been developed that can be delivered on plasmid, transposon and integration-proficient vectors.