Bacterial and biochemical tests for assessing chemical toxicity in the aquatic environment: A review

A Saccharomyces cerevisiae-based bioassay for assessing pesticide toxicity

This study evaluates the toxic effect of three pesticides (Azoxystrobin, Cymoxanil, and Diuron) on the yeast Saccharomyces cerevisiae for the development of a new bioassay based on inhibition of S. cerevisiae metabolic activity at the level of adenosine-5-triphosphate (ATP) synthesis, as compared with two different toxicity tests based on inhibition of Daphnia magna mobility (NF EN ISO 6341) and inhibition of Vibrio fisheri activity (NF EN ISO 11348). The S. cerevisiae bioassay is cheaper and 96 times faster than the D. magna toxicity bioassay, but has lower sensitivity. It is as fast as the V. fisheri bioassay and more sensitive. Thus, this new toxicity test can be proposed for rapid detection of pesticide residues in environmental samples as a complement to the more expensive and time-consuming D. magna toxicity test.

Real-time Monitoring of Non-specific Toxicity Using a Saccharomyces cerevisiae Reporter System

Baker's yeast, Saccharomyces cerevisiae, is the simplest and most well-known representative of eukaryotic cells and thus a convenient model organism for evaluating toxic effects in human cells and tissues. Yeast cell sensors are easy to maintain with short generation times, which makes the analytical method of assessing antifungal toxicity cheap and less-time consuming. In this work, the toxicity of test compounds was assessed in bioassays based on bioluminescence inhibition and on traditional growth inhibition on agar plates. The model organism in both tests was a modified S. cerevisiae sensor strain that produces light when provided with D-luciferin in an insect luciferase reporter gene activity assay. The bioluminescence assay showed toxic effects for yeast cell sensor of 5,6-benzo-flavone, rapamycin, nystatin and cycloheximide at concentrations of nM to μM. In addition, arsenic compounds, cadmium chloride, copper sulfate and lead acetate were shown to be potent non-specific inhibitors of the reporter organism described here. The results from a yeast agar diffusion assay correlated with the bioluminescence assay results.

A rapid radiochemical bacterial bioassay to evaluate copper toxicity in freshwaters

A rapid, highly sensitive bacterial bioassay to determine copper toxicity in freshwaters was developed based on the inhibition of cellular assimilation of radiolabeled glucose. The test used a copper-sensitive bacterium isolated from a freshwater stream. Employing sensitive radiochemical techniques enabled environmentally relevant concentrations of the test bacterium (10(5) cells mL(-1)) and a short incubation period (4 hours) to be used, which minimized the potential for changes in copper speciation during the test. The 4-hour median effective concentration (EC(50)) for inorganic copper at pH 7.5 in synthetic freshwater was 0.6 microg L(-1) (95% confidence limits 0.4 to 1.0 microg L(-1)). This compared well with chronic growth inhibition of this bacterium in minimal medium (48-hour EC(50) of 0.9 microg L(-1) [95% confidence limits 0.7 to 1.0 microg L(-1)]). MINEQL + software (Environmental Research Software) was used to calculate copper (II) ion concentrations in synthetic freshwater at pH 7.5, giving an EC(50) value of pCu(2+) 8.8. However, using nitrilotriacetic acid metal-ion buffers (Cu-NTA), 50% inhibition occurred at a pCu(2+) of 9.7, suggesting this bacterium was markedly more inhibited by copper in these Cu(2+)-buffered solutions. This may indicate that the Cu-NTA species was contributing to toxicity. The radiochemical bioassay was evaluated further using freshwater samples from both copper-impacted and pristine environments. Measured EC(50) values ranged from 3.4 to 34.0 microg L(-1)inorganic copper and were strongly correlated with dissolved organic carbon (DOC) concentrations (r = 0.88, p < 0.05).

Patterns of metals and arsenic poisoning in Vibrio fischeri bacteria

The Microtox bioassay was used to establish dose-response curves for some toxic elements in aqueous solutions, namely, Zn(II), Pb(II), Cu(II), Hg(II), Ag(I), Co(II), Cd(II), Cr(VI), As(V) and As(III). Experiments were carried out at either pH 6.0 or pH 7.0 to indicate that pH may influence the measured toxicity of some elements due to pH-related changes of their chemical speciation. EC20 values, which represent a measurable threshold of toxicity, were determined for each element and were found to rank as Pb(II)>Ag(I)>Hg(II) approximately Cu(II)>Zn(II)>As(V)>Cd(II) approximately Co(II)>As(III)>Cr(VI). These values were compared to the limit concentrations allowed in industrial wastewater according to the official regulations in Catalonia (Spain). It appears that the Microtox test is sensitive enough for detecting some of the tested elements with respect to official regulations of Catalonia (Spain) dealing with pollution control, with the exception of cadmium, mercury, arsenate, arsenite and chromate.

Toxicity of the 13 priority pollutant metals to Vibrio fisheri in the Microtox chronic toxicity test

The Microtox Acute Toxicity Test has been successfully used to measure the toxicity of metals and other pollutants at high concentrations (ppm) in selected environmental samples. However, metals and other toxicants are often found in much lower concentrations (ppb) in many municipal wastewaters and receiving waters. In order to assess the toxicity of these pollutants in these samples, a more sensitive toxicity assay is needed. The Microtox chronic toxicity test has been developed to measure the sublethal effect of toxicants over multiple generations of the test species, Vibrio fisheri. In this study, the toxicity of the 13 priority pollutant metals [i.e. As, Se, Cd, Cr (III and VI), Cu, Pb, Sb, Ag, Tl, Zn, Be, Hg and Ni] to V. fisheri was evaluated using the Microtox chronic toxicity test. In this test, the inhibitory concentration (IC), lowest observable effect concentration (LOEC), and no observable effect concentration (NOEC) were obtained after 22-h of incubation at 27+/-1 degrees C, by comparing the light output of the control to that of the test sample. Among the 13 priority pollutant metals, beryllium (Be) was found to be the most toxic in the test (LOEC=0.742-1.49 microg/l) while thallium (Tl) was the least toxic (LOEC=3840-15300 microg/l). The LOECs for copper (as Cu) and lead (Pb) in reagent (ASTM Type I) water were 6.78-13.6 microg/l and 626-1251 microg/l, respectively. The toxicity of copper sulfate (as Cu) in reagent water was shown and significantly reduced with the addition of natural organic matter (fulvic acid) or EDTA to the sample. The LOEC values for the 13 priority pollutant metals in this test were comparable to or lower than those reported for commonly used aquatic toxicity tests, such as the Ceriodaphnia dubia assay.

The cytotoxic and genotoxic potential of surface water and wastewater effluents as determined by bioluminescence, umu-assays and selected biomarkers

Two bacterial tests employing Photobacterieum phosphoreum (Microtox bioluminescence test) and Salmonella typhimurium TA 1535 pSK1002 (umu-assay) were evaluated to estimate the cytotoxic and genotoxic potential of water samples from the selected rivers in Germany as well as the primary and secondary effluents of some sewage treatment plants. Rainbow trout (Onchorynchus mykiss) were exposed to different concentrations (20-40%) of secondary effluent in the model online aquatic monitoring plant WaBoLu-Aquatox. The toxic potential of water samples from the exposure tanks was determined in two prokaryotic test systems and the biomarkers acethylcholinesterase (AChE) activity in muscle tissue and DNA unwinding assay in liver tissue of fish. Samples from the tested rivers showed no inhibition of the bioluminescence of P. phosphoreum or growth of umu-bacteria. Only primary effluent samples from the treatment plants at the Saale River inhibited the light emission or the growth of test bacteria by more than 20%. The induction ratio of umu-bacteria was in most of the river samples less than the threshold for genotoxicity (IR < 1.5). Only some samples from the Saale River, especially at sites downstream of secondary effluents caused genotoxic responses in the umu-assay. Samples of primary effluents contained the greatest genotoxic potential up to GEUI = 6 which was not detectable in samples of secondary effluents. A concentration range 20-40% secondary effluent inhibited AChE activity in muscle tissue and significantly increased DNA fragmentation in liver tissue of rainbow trout. In contrast, no cytotoxic or genotoxic responses in the umu-assay were caused by water samples. Both bacterial methods can be successfully used to analyse the cytotoxic and genotoxic response of industrial and domestic wastewater and to estimate the effectiveness of sewage treatment units. However, because of their low sensitivity and high susceptibility, they are not reliable as a single test for the detection of cytotoxicity and genotoxicity in surface water. The application of prokaryotic tests systems with biomarkers such as AChE activity and DNA fragmentation in different tissues of test organisms seems to be a useful combination for the assessment of cytotoxic and genotoxic potential in surface water and secondary effluent.

Toxicity assessment of 16 inorganic environmental pollutants by six bioassays

The relative toxicity of 16 environmental pollutants, such as inorganic elements (Ba, Cd, Co, Cr, Cu, Fe, Ge, Hg, Mn, Nb, Pb, Sb, Sn, Ti, V, and Zn), is evaluated on the L-929 established cell line of murine fibroblasts, with five bioassays [RNA synthesis rate assay (RNA), MTT reduction assay (MTT), neutral red incorporation assay (NRI), Coomassie blue assay, and cellular growth rate assay], and on the ciliated protozoa Tetrahymena pyriformis GL [doubling time of T. pyriformis GL population assay (DTP)]. For each inorganic substance, the six bioassays allowed the toxicological index IC50 ("inhibitory concentration 50%") to be calculated. The IC50 values are useful to rank the tested elements and to compare the features of the six bioassays. The most sensitive assays were the RNA, MTT, NRI, and DTP assays. Moreover, the in vitro IC50 values correlated with the in vivo LD50 values; these results were close to those obtained with established lines of human, murine, or fish cells. The sensitivity and the complementarity of these bioassays would be in favor of their incorporation in a "battery" of tests used for toxicological screening studies of xenobiotics.

Heavy metal toxicity testing in environmental samples

The toxicity of heavy metals in the environment depends on a number of physicochemical and biological factors. The complexity of these relationships has encouraged the use of bioassays for direct measurement of the [table: see text] impact of toxic metals on selected test species. Fish and daphnid bioassays are well accepted by the scientific and regulatory communities, but their length (48 h or more) and the considerable time and effort needed to culture the test organisms make their application to sample screening problematical. Microbial and biochemical assays based on the inhibition of bioluminescence, enzyme activity, enzyme biosynthesis, growth, respiration, and heat production are typically faster and less expensive than the traditional and fish bioassays. Some of these tests approach or equal the sensitivity of daphnids to heavy metals. Since the soil acts as a sink for airborne and waste-applied metals, the uptake of metals by plants and the associated toxic impacts are important. Growth inhibition, enzyme induction, and production of stress proteins have been considered as toxicity end points. Enzymatic tests have been developed that are specific for heavy metal toxicity. Such tests can facilitate toxicity reduction evaluations. Detection of individual metals in the environment may eventually be possible using biosensors consisting of genetically engineered microorganisms. Direct solid-phase tests for soil, sediment, or sludge toxicity, using bacterial bioluminescence or enzyme activity as end points, have been developed. Such tests may complement traditional solid-phase toxicity tests using nematodes or earthworms as indicator organisms. Based on the work reviewed, we draw the following conclusions: 1. The Microtox assay is sensitive to mercury but would fail to detect the toxicity of certain metals, such as cadmium. Among all the microbial assays reviewed, the bioassay based on growth inhibition of the alga Selenastrum capricornutum appears to give the lowest EC50s, similar to those seen for daphnid bioassays. 2. Biosensors, using genetically engineered microorganisms, offer an elegant means of detecting the presence of specific heavy metals in environmental samples. However, at the present time, they are not designed for assessing heavy metal toxicity. 3. The use of bioassays specific for heavy metal toxicity can be useful for directly assessing the bioavailability of these toxicants in environmental samples, thus avoiding the need for fractionation.+4

Whole-cell biosensors for environmental monitoring

Concern over the pollution risk to drinking water from industry and agriculture is growing, and the need for continuous on-line monitoring recognised. There is increasing use of living organisms as the sensitive agent to detect the presence of pollutants, and whole-cell biosensors are seen to have particular advantages in such environmental monitoring. The development of a mediated amperometric biosensor, incorporating the cyanobacterium Synechococcus as the biocatalyst, for on-line herbicide monitoring is described. The biosensor is able to detect a wide range of herbicides with sites of action on the photosynthetic electron transport chain, at concentrations down to 20 micrograms litre-1 and possesses a working life of up to 7 days. The use of alginate immobilisation of the biocatalyst to overcome the problems associated with obtaining a realistic shelf life for the biosensor is discussed.

Variables Affecting Two Electron Transport System Assays

Several methodological variables were critical in two commonly used electron transport activity assays. The dehydrogenase assay based on triphenyl formazan production exhibited a nonlinear relationship between formazan production (dehydrogenase activity) and sediment dilution, and linear formazan production occurred for 1 h in sediment slurries. Activity decreased with increased time of sediment storage at 4°C. Extraction efficiencies of formazan from sediment varied with alcohol type; methanol was unsatisfactory. Phosphate buffer (0.06 M) produced higher activity than did either U.S. Environmental Protection Agency reconstituted hard water or Tris buffer sediment diluents. Intracellular formazan crystals were dissolved within minutes when in contact with immersion oil. Greater crystal production (respiration) detected by a tetrazolium salt assay occurred at increased substrate concentrations. Test diluents containing macrophyte exudates produced greater activity than did phosphate buffer, U.S. Environmental Protection Agency water, or ultrapure water diluents. Both assays showed decreases in sediment or bacterial activity through time.