Microscale application of the SOS-LUX-TEST as biosensor for genotoxic agents

Development and validation of a higher throughput screening approach to genotoxicity testing using the GADD45a-GFP GreenScreen HC assay

There is a pressing need to develop rapid yet accurate screening assays for the identification of genotoxic liability and for early hazard assessment in drug discovery. The GADD45a-GFP human cell-based genotoxicity assay (GreenScreen HC) has been reformatted to test 12 compounds per 96-well microplate in a higher throughput, automated screening mode and the protocol applied to the analysis of 1266 diverse, pharmacologically active compounds. Testing from a fixed starting concentration of 100 AmicroM and over 3 serial dilutions, the hit rates for genotoxicity (7.3%) and cytotoxicity (33%) endpoints of the assay have been determined in a much wider chemical space than previously reported. The degree of interference from color, autofluorescence, and low solubility has also been assessed. The assay results have been compared to an in silico approach to genotoxicity assessment using Derek for Windows software. Where carcinogenicity data were available, GreenScreen HC demonstrated a higher specificity than in silico methods while identifying genotoxic species that were not highlighted for genotoxic liability in structure-activity relationship software. Higher throughput screening from a fixed, low concentration reduces sensitivity to less potent genotoxins, but the maintenance of the previously reported high specificity is essential in early hazard assessment where misclassification can lead to the needless rejection of potentially useful compounds in drug development.

A fluorescence-based screening assay for DNA damage induced by genotoxic industrial chemicals

A rapid screening assay to detect chemically-induced DNA damage resulting from exposure of surrogate DNA to genotoxic compounds is reported. This assay is based on changes in the melting and annealing behavior observed for damaged DNA. Exposure of calf thymus DNA to genotoxic industrial chemicals reduced the extent to which the DNA annealed as measured using a double strand DNA selective fluorescent indicator dye. Formaldehyde, acrolein, crotonaldehyde and bromoethane showed the most prominent effects, chloroacetone and allylamine exhibited lesser effects, and acryrlonitrile showed no statistically significant assay response. The assay response for formaldehyde and crotonaldehyde were measured over the concentration range of 10-100 mM and 50-300 mM, respectively. This assay showed little response for the cytotoxic compounds phenol, cyclohexane and toluene but was sensitive to the effects of DNA damaging compounds such as mitomycin C and glutaraldehyde.

Bioluminescent bacterial biosensors for the assessment of metal toxicity and bioavailability in soils

A major factor governing the toxicity of heavy metals in soils is their bioavailability. Traditionally, sequential extraction procedures using different extractants followed by chemical analysis have been used for determining the biologically available fraction of metals in soils. Yet, the transfer of results obtained on non-biological systems to biological ones is certainly questionable. Therefore, bioluminescence-based bacterial biosensors have been developed using genetically engineered microorganisms, constructed by fusing transcriptionally active components of metal resistance mechanisms to lux genes from naturally bioluminescent bacteria like Vibrio fischeri for the assessment of metal toxicity and bioavailability in polluted soils. As compared to chemical methods, bacterial biosensors present certain advantages, such as selectivity, sensitivity, simplicity, and low cost. Despite certain inherent limitations, bacterial bioluminescent systems have proven their usefulness in soils under laboratory and field conditions. Finally, green fluorescent protein-based bacterial biosensors are also applicable for determining with high sensitivity the bioavailability of heavy metals in soil samples.

The SOS-LUX-TOXICITY-Test on the International Space Station

For the safety of astronauts and to ensure the stability and integrity of the genome of microorganisms and plants used in bioregenerative life support systems, it is important to improve our knowledge of the combined action of (space) radiation and microgravity. The SOS-LUX-TOXICITY test, as part of the TRIPLE-LUX project (accepted for flight at Biolab in Columbus on the International Space Station, (ISS)), will provide an estimation of the health risk resulting from exposure of astronauts to the radiation environment of space in microgravity. The project will: (i) increase our knowledge of biological/health threatening action of space radiation and enzymatic DNA repair; (ii) uncover cellular mechanisms of synergistic interaction of microgravity and space radiation; (iii) provide specified biosensors for spacecraft milieu examination; and (iv) provide experimental data on stability and integrity of bacterial DNA in spacecrafts. In the bacterial biosensor "SOS-LUX-Test" developed at DLR (patent), bacteria are transformed with the pBR322-derived plasmid pPLS-1 or the similar, advanced plasmid SWITCH, both carrying the promoterless lux operon of Photobacterium leiognathi as the reporter element controlled by a DNA damage-dependent SOS promoter as sensor element. A short description of the space experiment is given, and the current status of adaptation of the SOS-LUX-Test to the ISS, i.e. first results of sterilization, biocompatibility and functional tests performed with the already available hardware and bread board model of the automated space hardware under development, is described here.

Further development of the beta-lactamase MutaGen assay and evaluation by comparison with Ames fluctuation tests and the umu test

A rapid, high-throughput bacterial mutagenicity test system has been developed (MutaGen test) that detects reversions of inactivating base-pair substitutions and frameshifts in a TEM-1 class A beta-lactamase (ampicillinase) gene. To quickly and sensitively detect mutagens, the system utilises a series of plasmids that contain the mutated ampicillinase gene and the mucAB operon. Inactivating mutations in the ampicillinase gene include frameshifts integrated into repetitive GC-sequences and G-runs known to be mutagenic hot-spots, and base-pair substitutions inserted in or around the beta-lactamase active site. Frameshift mutations completely inactivated the enzyme only when located downstream of the active-site serine (Ser68). Previous (reporter gene based) assays with this system have detected reversion to ampicillin resistance by luminescence driven by induction of the tet-promotor controlled lacZ gene. In the present study, we describe the construction and evaluation of 19 additional potential tester strains. We also developed conditions for detecting reversions by pH shift using bromocresol purple and by directly detecting the enzymatic activity of beta-lactamase using nitrocefin. A 384-well format version of the pH shift MutaGen test was used to assay more than 20 chemicals. The responses in the assay were compared with responses for the same chemicals in the umu test and Ames fluctuation assays. The results indicate that the MutaGen test has high specificity for detecting specific mutations and, in some instances, better sensitivity than the other tests. Since the test is easy to conduct, sterile working conditions are not necessary, and the mutagenicity results are available either within one working day or overnight, the assay shows promise for the rapid screening of potentially genotoxic substances.

Immobilization as a technical possibility for long-term storage of bacterial biosensors

For applications in field experiments, the recombinant strain Salmonella typhimurium TA1535 was immobilized to permit its immediate utilization after long storage periods. Salmonella typhimurium TA1535 cells contain the plasmid that has an inducible SOS promoter fused to a promoterless luxCDABFE operon from Photobacterium leiognathi. The induction of bioluminescence occurs in the presence of the DNA-damaging agent mitomycin C which stimulates the bacterial SOS response. Early stationary phase cells were immobilized at a cell concentration of 10(10) CFU/ml in microtiter plates and stored up to 6 weeks at 4 degrees C in a sealed container. Even after 4 weeks of storage, the bioluminescence kinetics and yield in response to different concentrations of mitomycin C were not significantly different from those of freshly prepared samples.

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.

The SOS-LUX-LAC-FLUORO-Toxicity-test on the International Space Station (ISS)

In the 21st century, an increasing number of astronauts will visit the International Space Station (ISS) for prolonged times. Therefore it is of utmost importance to provide necessary basic knowledge concerning risks to their health and their ability to work on the station and during extravehicular activities (EVA) in free space. It is the aim of one experiment of the German project TRIPLE-LUX (to be flown on the ISS) to provide an estimation of health risk resulting from exposure of the astronauts to the radiation in space inside the station as well as during extravehicular activities on one hand, and of exposure of astronauts to unavoidable or as yet unknown ISS-environmental genotoxic substances on the other. The project will (i) provide increased knowledge of the biological action of space radiation and enzymatic repair of DNA damage, (ii) uncover cellular mechanisms of synergistic interaction of microgravity and space radiation and (iii) examine the space craft milieu with highly specific biosensors. For these investigations, the bacterial biosensor SOS-LUX-LAC-FLUORO-Toxicity-test will be used, combining the SOS-LUX-Test invented at DLR Germany (Patent) with the commercially available LAC-FLUORO-Test. The SOS-LUX-Test comprises genetically modified bacteria transformed with the pBR322-derived plasmid pPLS-1. This plasmid carries the promoterless lux operon of Photobacterium leiognathi as a reporter element under control of the DNA-damage dependent SOS promoter of ColD as sensor element. This system reacts to radiation and other agents that induce DNA damages with a dose dependent measurable emission of bioluminescence of the transformed bacteria. The analogous LAC-FLUORO-Test has been developed for the detection of cellular responses to cytotoxins. It is based on the constitutive expression of green fluorescent protein (GFP) mediated by the bacterial protein expression vector pGFPuv (Clontech, Palo Alto, USA). In response to cytotoxic agents, this system reacts with a dose-dependent reduction of GFP-fluorescence. Currently, a fully automated miniaturized hardware system for the bacterial set up, which includes measurements of luminescence and fluorescence or absorption and the image analysis based evaluation is under development. During the first mission of the SOS-LUX-LAC-FLUORO-Toxicity-Test on the ISS, a standardized, DNA-damaging radiation source still to be determined will be used as a genotoxic inducer. A panel of recombinant Salmonella typhimurium strains carrying either the SOS-LUX plasmid or the fluorescence-mediating lac-GFPuv plasmid will be used to determine in parallel on one microplate the genotoxic and the cytotoxic action of the applied radiation in combination with microgravity. Either in addition to or in place of the fluorometric measurements of the cytotoxic agents, photometric measurements will simultaneously monitor cell growth, giving additional data on survival of the cells. The obtained data will be available on line during the TRIPLE-LUX mission time. Though it is the main goal during the TRIPLE-LUX mission to measure the radiation effect in microgravity, the SOS-LUX-LAC-FLUORO-Toxicity-test in principle is also applicable as a biomonitor for the detection and measurement of genotoxic substances in air or in the (recycled) water system on the ISS or on earth in general.

Construction of high-density bacterial colony arrays and patterns by the ink-jet method

We have developed a method for fabricating bacterial colony arrays and complex patterns using commercially available ink-jet printers. Bacterial colony arrays with a density of 100 colonies/cm(2) were obtained by directly ejecting Escherichia coli (E. coli) onto agar-coated substrates at a rapid arraying speed of 880 spots per second. Adjusting the concentration of bacterial suspensions allowed single colonies of viable bacteria to be obtained. In addition, complex patterns of viable bacteria as well as bacteria density gradients were constructed using desktop printers controlled by a simple software program.

HUMEX, a study on the survivability and adaptation of humans to long-duration exploratory missions, part I: lunar missions

The European Space Agency has recently initiated a study of the human responses, limits and needs with regard to the stress environments of interplanetary and planetary missions. Emphasis has been laid on human health and performance care as well as advanced life support developments including bioregenerative life support systems and environmental monitoring. The overall study goals were as follows: (i) to define reference scenarios for a European participation in human exploration and to estimate their influence on the life sciences and life support requirements; (ii) for selected mission scenarios, to critically assess the limiting factors for human health, wellbeing, and performance and to recommend relevant countermeasures; (iii) for selected mission scenarios, to critically assess the potential of advanced life support developments and to propose a European strategy including terrestrial applications; (iv) to critically assess the feasibility of existing facilities and technologies on ground and in space as testbeds in preparation for human exploratory missions and to develop a test plan for ground and space campaigns; (v) to develop a roadmap for a future European strategy towards human exploratory missions, including preparatory activities and terrestrial applications and benefits. This paper covers the part of the HUMEX study dealing with lunar missions. A lunar base at the south pole where long-time sunlight and potential water ice deposits could be assumed was selected as the Moon reference scenario. The impact on human health, performance and well being has been investigated from the view point of the effects of microgravity (during space travel), reduced gravity (on the Moon) and abrupt gravity changes (during launch and landing), of the effects of cosmic radiation including solar particle events, of psychological issues as well as general health care. Countermeasures as well as necessary research using ground-based test beds and/or the International Space Station have been defined. Likewise advanced life support systems with a high degree of autonomy and regenerative capacity and synergy effects were considered where bioregenerative life support systems and biodiagnostic systems become essential. Finally, a European strategy leading to a potential European participation in future human exploratory missions has been recommended.