Additivity in microbial toxicity of nonuniform mixtures of organic chemicals

Transcriptional regulation of organohalide pollutant utilisation in bacteria

Organohalides are organic molecules formed biotically and abiotically, both naturally and through industrial production. They are usually toxic and represent a health risk for living organisms, including humans. Bacteria capable of degrading organohalides for growth express dehalogenase genes encoding enzymes that cleave carbon-halogen bonds. Such bacteria are of potential high interest for bioremediation of contaminated sites. Dehalogenase genes are often part of gene clusters that may include regulators, accessory genes and genes for transporters and other enzymes of organohalide degradation pathways. Organohalides and their degradation products affect the activity of regulatory factors, and extensive genome-wide modulation of gene expression helps dehalogenating bacteria to cope with stresses associated with dehalogenation, such as intracellular increase of halides, dehalogenase-dependent acid production, organohalide toxicity and misrouting and bottlenecks in metabolic fluxes. This review focuses on transcriptional regulation of gene clusters for dehalogenation in bacteria, as studied in laboratory experiments and in situ. The diversity in gene content, organization and regulation of such gene clusters is highlighted for representative organohalide-degrading bacteria. Selected examples illustrate a key, overlooked role of regulatory processes, often strain-specific, for efficient dehalogenation and productive growth in presence of organohalides.

Quantitative Structure-Toxicity Relationship analysis of combined toxic effects of lignocellulose-derived inhibitors on bioethanol production

This study performed a Quantitative Structure-Toxicity Relationship (QSTR) model to evaluate the combined toxicity of lignocellulose-derived inhibitors on bioethanol production. Compared with all the control groups, the combined systems exhibited lower conductivity values, higher oxidation-reduction potential values, as well as maximum inhibition rates. These results indicated that the presence of combined inhibitors had a negative effect on the bioethanol fermentation process. Meanwhile, QSTR model was excellent for evaluating the combined toxic effects at lower ferulic acid concentration (([1:4] × IC50)) and (([1:1] × IC50)), due to higher R² values (0.994 and 0.762), lower P values (0.000 and 0.023) and relative error values (less than 30%). The obtained results also showed that the combined toxic effects of ferulic acid and representative lignocellulose-derived inhibitors were relevant to different molecular descriptors. Meanwhile, the interactions of combined inhibitors were weaker when ferulic acid was at low concentration ([1:4] × IC50).

Combined Toxicity of Nitro-Substituted Benzenes and Zinc to Photobacterium Phosphoreum: Evaluation and QSAR Analysis

The single toxicity (IC₅₀) of zinc (Zn) and 11 nitro-substituted benzenes to Photobacterium phosphoreum were determined, respectively. On basis of single toxicity, the joint toxicity of binary mixtures of Zn and 11 nitro-substituted benzenes at different Zn concentrations of 0.2 IC₅₀, 0.5 IC₅₀, and 0.8 IC₅₀ were measured. The joint toxicity was evaluated by toxic unit (TU) and additive index (AI) methods. The results indicated that the joint toxicity was not only depending on the Zn concentrations but also on the substituted groups of nitro-substituted benzenes. The quantitative structure-activity relation (QSAR) equations were developed and the results showed that the toxicity of nitro-substituted benzenes has different joint effect at the different Zn concentrations. At the Zn concentration of 0.2 IC₅₀, the binary joint effects were mainly antagonism and the joint toxicity was negatively related to descriptors called VE2_B(p) and TIC3. At the Zn concentration of 0.5 IC₅₀ and 0.8 IC₅₀, the binary joint effects were mainly antagonism and simple addition, and the joint toxicity was related to the same descriptor Eig06_ AEA(dm). It indicated that the joint toxic actions were similar when combined at the medium and high concentrations of Zn.

Selection of organosilicone surfactants for tank-mixed pesticides considering the balance between synergistic effects on pests and environmental risks

In this study, the bioactivities of binary mixtures of organosilicone surfactants and indoxacarb against two Lepidopteran pests were investigated along with their environmental risks. All of the tested organosilicone surfactants had obvious synergistic effects on the contact toxicity of indoxacarb against Spodoptera exigua and Agrotis ipsilon. However, all of the organosilicone surfactants exhibited certain antagonism for indoxacarb against S. exigua in terms of stomach & contact toxicity; both Silwet-408 and Silwet-806 exhibited additivity against A. ipsilon, whereas Silwet-618 and Silwet-DRS-60 exhibited synergism and slight antagonism, respectively. All of the tested chemicals were highly toxic to Daphnia magna, among which Silwet-DRS-60 had the lowest acute toxicity (EC₅₀ of 94.91 μg/L). However, these chemicals were less toxic to Brachydanio rerio. Silwet-DRS-60 had a low toxicity to B. rerio, while Silwet-408, Silwet-806 and Silwet-618 were moderately toxic to B. rerio. For the joint toxicity evaluation of organosilicone surfactants and indoxacarb to D. magna and B. rerio, the additive index method, concentration addition method and toxicity unit method were robust in judging synergism or antagonism, whereas other methods were more conservative; the V-value method and equilibrium curve method exhibited high robustness and viability in evaluating the combined effects of binary mixtures. Overall, we should carefully select organosilicone surfactants for premixed or tank-mixed pesticides in agriculture to obtain a balance between synergistic effects on pests and environmental risks.

Alcohol ethoxylates significantly synergize pesticides than alkylphenol ethoxylates considering bioactivity against three pests and joint toxicity to Daphnia magna

Seeking alternatives for alkylphenol ethoxylates (APEOs) have been a heavily researched topic in the surfactant industry and agricultural systems. In this study, the combined effects of different ethoxylates and pesticides on the bioactivity against three pests and toxicological risks to Daphnia magna were investigated. Results showed that alcohol ethoxylates (AEOs) had higher synergistic effects on the bioactivity of pesticides against Spodoptera exigua, Agrotis ipsilon and Aphis citricola than did APEOs. In terms of the joint toxicity of the ethoxylates and pesticides to D. magna, additive index method, toxicity unit method, V value method and isobologram method were used in the tests. All of these methods indicated that the joint effects of APEOs + acetamiprid and APEOs + indoxacarb upon D. magna turned from synergism to antagonism with the increasing EO (ethylene oxide) numbers. Those of AEOs exhibited similar trends. Overall, AEOs may be potential alternatives for APEOs in agriculture as they synergize pesticides against three pests significantly more than do APEOs. However, further research should investigate the compounds' environmental risks to aquatic organisms because the AEOs were highly toxic to D. magna.

Evaluation of joint toxicity of nitroaromatic compounds and copper to Photobacterium phosphoreum and QSAR analysis

The individual toxicities of Cu and 11 nitroaromatic compounds to Photobacterium phosphoreum were determined. The toxicity was expressed as the concentrations causing a 50% inhibition of bioluminescence after 15 min exposure (IC(50)). To evaluate the joint effect between the metal ion and the 11 nitroaromatic compounds, the joint toxicity of Cu and 11 nitroaromatic compounds were measured at different Cu concentrations (0.2IC(50), 0.5IC(50) and 0.8IC(50)), respectively. The result shows that the binary joint effect between Cu and nitroaromatic compounds is mainly simple addition at the low Cu concentration (0.2IC(50)). However, an antagonism effect, 55% and 64%, was observed between Cu and 11 nitroaromatic compounds for Cu at medium and high concentrations (0.5IC(50) and 0.8IC(50)). Quantitative structure-activity relationship (QSAR) analysis was performed to study the joint toxicity for the 11 nitroaromatic compounds. The result shows that the toxicity of nitroaromatic compounds is related to descriptors of Connolly solvent-excluded volume (CSEV) and dipolarity/polarizability (S) at low Cu concentration. On the other hand, the toxicity is related to Connolly accessible area (CAA) at medium and high Cu concentrations. The result indicates that different QSAR models on complex mixtures need to be developed to assess the ecological risk in real environments. Using single toxic data to evaluate the toxic effect of mixtures may result in wrong conclusions.

Ecological risk assessment of urban and industrial systems: a review

Numerous ecological risk assessment methodologies have been developed over the last twenty years around the world for evaluating urban and industrial systems and installations, by both the organisations responsible for implementing regulations and the scientific community. Although these methodologies share the general principle underlying their use, they differ widely with respect to the approaches chosen and the resources employed to apply them. Also, they may even have different objectives: prior assessment as part of an impact study before building a new installation, or retrospective assessment, for example, in view to explaining the reasons for an impact recorded or for forecasting additional expected impacts. This article provides a synthesis of the different approaches used around the world for carrying out each of the major steps common to all ecological risk assessment methodologies. The advantages and limitations of these different options are discussed in order to provide elements for formulating any new methodology adapted to a given scenario. To conclude, perspectives for improving the tools required for these methodologies are proposed, and the research works to which priority should be given are identified.

Evaluation of the combined toxicity of 15 pesticides by uniform design

BACKGROUND

Environmental pesticides, including insecticides and herbicides, are frequently encountered as mixtures and threaten non-target organisms in water. Evaluation of the combined toxicity of diverse pesticides with various concentration combinations is important, especially using limited experimental effort. Uniform design (UD) is one optimal experimental technique that can rationally arrange the concentrations of mixture components so that, with a minimum number of experimental runs, the combined toxicity of multiple pesticide mixtures can be evaluated.

RESULTS

The concentration compositions of 18 pesticide mixture points designed by UD covered almost all possible concentration ranges of the mixture components on account of the two merits of 'space filling' and 'multiple levels'. The combined toxicities of 18 mixture rays extended by using the fixed-ratio ray design (FRRD) from 18 UD mixture points were evaluated by concentration addition (CA) and independent action (IA) models. It was found that the concentration-response curves (CRCs) predicted by CA were, on the whole, located between the 95% confidence intervals of the experimental CRCs, which implied that the combined toxicity of the pesticide mixture rays could be evaluated by CA. The CRCs predicted by IA were very similar to those from CA.

CONCLUSION

The model developed from the UD mixture rays can effectively simulate mixtures with arbitrary concentration compositions of 15 pesticides. The CA model can accurately evaluate and predict the combined toxicity of the pesticides, which provides a useful tool for risk assessment of a mixture of multiple pesticides in the aquatic environment.

Evaluation of combined toxicity of phenols and lead to Photobacterium phosphoreum and quantitative structure-activity relationships

The combined toxicity of lead (Pb) and nine phenols were measured. The result indicated that the combined toxicity is not only dependent on the Pb concentrations but also on the positions of substituted groups of phenols. Quantitative structure-activity relationship equations were built from the combined toxicity and physico-chemical descriptors of phenols in the different Pb concentrations. The combined toxicity was related to water solubility and the third order molecular connectivity index ((3)X) in low Pb concentration, to solute excess molar refractivity (E) and ionization constant (pKa) in medium Pb concentration and to dipolarity/polarizability (S) in high Pb concentration.

Combined photobacterium toxicity of herbicide mixtures containing one insecticide

To test whether the dose-addition (DA) model can predict the combined toxicity of the mixtures of herbicides that coexisted with insecticide(s), we selected five herbicides (simetryn, prometon, bromacil, velpar, and diquat) and one organophosphorus insecticide (dichlorvos) as the test components. The inhibition toxicities of the six pesticides as well as those of their mixtures to Vibrio qinghaiensis sp.-Q67 were determined by using the microplate toxicity test procedure. The dose-response curves (DRCs) between the observed inhibition toxicities and the doses of the pesticides or the mixtures were modeled by using the nonlinear least square fitting. It was shown that all dose-response relationships were effectively described by the Weibull function. To fully explore the combined toxicities of mixtures including various concentration compositions, we designed three equivalent-effect concentration ratio (EECR) mixtures and six uniform design concentration ratio (UDCR) mixtures. The combined toxicity of a mixture is identified by inspecting whether the DRC predicted by the dose addition (DA) or independent action (IA) locates in the 95% confidence interval of the DRC of the mixture. Furthermore, the possible reason for the three mixtures to depart from the DA action was the very high concentration ratio of diquat in the mixtures.

A model to analyze effects of complex mixtures on survival

In ecotoxicology there is a growing interest in effects of mixtures. The aim of this research was to develop a biology-based model that describes effects of mixtures on survival in time. The model works from the individual compounds in the mixture. Such an approach requires parameters for each individual compound in the mixture. For narcotic compounds we underpinned theoretical relations between the toxic parameters and the logK(ow) with experimental data by analyzing almost 300 datasets from the open literature, allowing a vast reduction in effort in the assessment of effects of mixtures. To illustrate the use of the model we simulated the effect of a mixture of 14 PAHs on the survival of Pimephales promelas. The simulation showed that due to the combined effect of the compounds in the mixture effects can be seen at very low concentrations.

Prediction for the mixture toxicity of six organophosphorus pesticides to the luminescent bacterium Q67

Organophosphorus (OP) pesticides are ubiquitous in the surface water as mixtures. To examine the mixture toxicity in the multi-component space, the uniform design (UD) which can explore the concentration changes with few experimental efforts was employed to design the mixtures. On the other hand, the fixed concentration ratio ray was applied into six UD mixtures and two equivalent-effect concentration mixtures to build the whole concentration-response curves to overcome the demerit of the classical "point-to-point" method. The experimental toxicities of six pesticides and their mixtures to the luminescent bacterium Q67 were determined. The mixture toxicities were predicted by two models, concentration addition (CA) and independent action (IA). The results showed that all the mixture toxicities observed had no significant differences from the ones predicted by CA. However, the mixture toxicities were also well predicted by IA especially at the low-concentration section.

Modeling the effects of binary mixtures on survival in time

In general, effects of mixtures are difficult to describe, and most of the models in use are descriptive in nature and lack a strong mechanistic basis. The aim of this experiment was to develop a process-based model for the interpretation of mixture toxicity measurements, with effects of binary mixtures on survival as a starting point. The survival of Folsomia candida was monitored daily for 21 d during the exposure to six binary mixtures of cadmium, copper, lead, and zinc in a loamy sand soil. The measurements were used to develop a model to describe survival in time. The model consists of two parts: A one-compartment model that describes uptake and elimination of the compounds, and a hazard model describing survival. The model was very successful in describing the data and at finding possible interactions. The mixture of copper and lead showed a slight antagonistic effect, the other mixtures showed no interaction. The model is straightforward in its biological assumptions and does not require a mode-of-action a priori choice of the mixture that might influence the modeled interaction of the components in the mixture. The model requires measurements at intermediate time points, but runs with relatively few parameters and is robust in finding interactions. When mixture effects are considered at only one time point, care should be taken with the assignment of interactions because these may be different for different points during the time course of the experiments.

Prediction of noninteractive mixture toxicity of organic compounds based on a fuzzy set method

Current methods for the prediction of mixture toxicity have shown to be valid for mixtures that conform to some assumptions that were ideally formulated for mixtures comprising constituents exhibiting either completely similar or dissimilar mechanisms of action. Approaches are needed that predict the toxicity of mixtures representative of real environmental occurrences i.e., those comprising constituents of mixed similar and dissimilar compounds and therefore are more complex. In this paper such a methodology is proposed which uses molecular descriptors and fuzzy set theory to characterize the degree of similarity and dissimilarity of mixture constituents, integrates the concentration addition and independent action models, and therefore is called INFCIM (INtegrated Fuzzy Concentration addition--Independent action Model). INFCIM is tested in two case studies using toxicity data of four mixtures, and its performance is compared against those of both concentration addition and independent action models. Mixture 1 consists of 18 s-triazines acting on green freshwater algae scenedemus vacuolatus. Mixture 2 comprises 16 acting constituents tested on scenedemus vacuolatus. Both mixtures inhibit reproduction in the biological assays. There are 10 quinolone compounds in mixture 3 and 16 phenol derivative compounds in mixture 4 all causing long-term inhibition of bioluminescence in the marine bacterium Vibrio fischeri. It was shown that INFCIM performed comparably or better than the best performing existing model in the original studies for all the mixtures tested.

Predictability of the mixture toxicity of 12 similarly acting congeneric inhibitors of photosystem II in marine periphyton and epipsammon communities

Testing of single chemicals with single species is common ecotoxicological practice in contrast to contaminated environments where highly diverse biological communities are exposed to highly diverse mixtures of chemical compounds. We, therefore, investigated whether mixture toxicity approaches that have been used successfully for single species, might also be applied on a community level of biological complexity. Twelve inhibitors of photosystem II, selected by QSAR and chemometrical approaches as the structurally most similar from a congeneric group of phenylurea herbicides, were tested singly and as mixtures on two types of marine microalgal communities, periphyton and epipsammon. Inhibition of photosynthesis was measured in short-term tests using incorporation of radiolabelled carbon (14C) to estimate photosynthetic rates. Two basic concepts, concentration addition (CA) and independent action (IA), were used to predict the toxicities of the mixtures. Congeneric and similar-acting substances such as the phenylureas are expected to comply with CA rather than IA. The aim of the present study was to evaluate whether these concepts can be used to predict mixture toxicity also to periphyton and epipsammon photosynthesis, i.e. at the level of natural communities. We found that deviations between observed and predicted mixture toxicity were relatively small but that CA predictions were the more accurate ones. The predictions proved to be robust, when based on single substance information even from different seasons, years, and sites. We conclude that the concept of CA for predicting mixture toxicity applies also at the community level of algal testing; at least when a physiological short-term effect indicator is used that matches the mechanism of action of the substances.

Optimisation of a microbial bioassay for contaminated soil monitoring: bacterial inoculum standardisation and comparison with Microtox assay

This work represents the first step to set up a toxicity testing procedure and to evaluate the sensitivity of the test microorganism to several classes of environmental pollutants. First, three different techniques were employed to standardise the microbial inoculum, then two different toxicity assessment protocols have been compared: Microtox and a dehydrogenase (DHase) activity inhibition test. The main goal was the optimisation of a microbial bioassay based on the dehydrogenase activity (DHase) inhibition in Pseudomonas fluorescens bacterial strain ATCC 13525. Triphenyl tetrazolium chloride (TTC) was used as electron acceptor and its reduction produces Triphenyl formazane (TPF). The P. fluorescens DHase inhibition bioassay was investigated for being a reliable and rapid method for assessing toxicity. The optimisation of the operating conditions resulted in a repeatable bioassay. Then, P. fluorescens and Vibrio fischeri sensitivity were firstly compared by testing Zn++, one of the reference compounds for Microtox test. In addition, other compounds (Ni++, Cd++, Cu++, phenol) were also tested with both bioassays. A high statistical significance of data was obtained with the logistic curve. The present work has demonstrated that P. fluorescens is as sensitive as Microtox culture (V. fischeri), for some of the metal ions. With reference to organic compounds, the lower sensitivity of P. fluorescens to phenol makes its use difficult in organic polluted samples.

Predictability of combined effects of eight chloroacetanilide herbicides on algal reproduction

Chloroacetanilides are pre-emergence herbicides for the control of annual grasses and broadleaf weeds. As a result of their extensive use, residues are often found in surface waters. Observed simultaneous occurrence of different chloroacetanilide herbicides gives reason for concern about potential combination effects on aquatic non-target organisms. This study aimed to clarify whether joint effects of various chloroacetanilide herbicides may be predictable from knowledge of concentration-response relationships of single substances. Whether the chloroacetanilides all share the same mode of action is unclear. Therefore we investigated the predictive value of two alternative concepts for the prediction of combined effects: concentration addition, which assumes a similar mode of action, and independent action, which is based on the idea of a dissimilar mode of action of the mixture components. Eight chloroacetanilides (acetochlor, alachlor, butachlor, dimethachlor, metazachlor, metolachlor, pretilachlor and propachlor) were experimentally tested for their individual as well as for their combined effects in mixtures on the reproduction of the green alga Scenedesmus vacuolatus. Individual chloroacetanilides impaired algal reproduction, with EC50 values ranging from 3 to 232 microg litre(-1). The differences in EC50 values were strongly correlated with the lipophilicities of the compounds. Effects of chloroacetanilide mixtures were considerably higher than those of the individual components: a complete inhibition of algal reproduction was observed when every mixture component was present in a concentration that would cause only 5% effect if applied singly. However, the combined effects proved to be predictable by using the concept of concentration addition. The alternative concept, independent action, distinctly underestimated the mixture toxicity. These findings (1) indicate a similar mechanism of action of chloroacetanilides in algae and (2) reinforce the view that concentration addition is a reasonable assumption for the predictive hazard assessment of groups of similarly acting herbicides.

Synergistic effects of peroxynitrite on arecoline-induced cytotoxicity in human buccal mucosal fibroblasts

Epidemiological studies have demonstrated a clear association between betel nut chewing and an increased risk for oral mucosal lesions. Arecoline, the most abundant betel alkaloid, is considered the most important etiologic factor in betel nuts. In addition, most betel nut chewers are also smokers. In order to elucidate the potential toxicological implications of interactions of arecoline and peroxynitrite (a reaction product of cigarette smoking), cell viability, and cellular levels of glutathione (GSH) were investigated, using cultured human buccal mucosal fibroblasts. At a concentration higher than 0.8 mM, arecoline was cytotoxic to buccal mucosal fibroblasts in a concentration- and time-dependent manner. Arecoline also depleted intracellular GSH in a dose-dependent manner (P<0.05). The addition of extracellular peroxynitrite acted as a synergistic effect on the arecoline-induced cytotoxicity (P<0.05). Furthermore, at a concentration of 0.8 mM, arecoline depleted intracellular GSH by about 42%, while 2 mM peroxynitrite enhanced the arecoline-depleted GSH level further to 86% as compared with the control. During GSH depletion, arecoline may render the human buccal mucosal fibroblasts more vulnerable to other reactive agents within cigarette smoking. Taken together, we suggest that people who combine the habits of betel nut chewing with cigarette smoking could be more susceptible to oral mucosal damage than betel quid chewing alone.

Synergistic DNA damaging effects of 4-nitroquinoline-1-oxide and non-effective concentrations of methyl methanesulfonate in human fibroblasts

DNA damage and DNA repair in human fibroblasts induced by the combination mixture of the genotoxic agents methyl methanesulfonate (MMS) and 4-nitroquinoline-1-oxide (4-NQO) were studied using the comet assay and the unscheduled DNA synthesis (UDS), respectively. Cells were simultaneously treated for 1h with the no observed effect concentration (noec) of MMS and increasing concentrations of 4-NQO or vice versa. Different results were obtained with the two types of mixtures. When the noec of 4-NQO was combined with increasing concentrations of MMS, no combination effects were observed. However, in experiments with increasing concentrations of 4-NQO and the noec of MMS, an increase in DNA damage and repair (and an enhancement of cytotoxicity) was demonstrated. Quantitative analysis of the effects by the isobologram method confirmed synergistic responses in both tests. We are proposing interactive actions between 4-NQO and MMS, whereby 4-NQO facilitates the attack of MMS on the DNA bases.

The single substance and mixture toxicity of quinolones to the bioluminescent bacterium Vibrio fischeri

Quinolones are one of the most important group of synthetic antibiotics used in aquaculture. We studied the single substance and mixture toxicity of ten quinolones using a long term bioluminescence inhibition assay with the marine bacterium Vibrio fischeri as the test organism. All tested quinolones are highly toxic to the test organism with EC50 values ranging from 14 µg/l for ofloxacin to 1020 µg/l for pipemidic acid. Adapting the approach outlined in EEC directive 93/21/EEC to these results, all but one of the ten quinolones belong to the group classified as being 'very toxic to aquatic organisms' (EC50 below 1 mg/l). On the basis of the concentration-response relationships of the single compounds, the mixture toxicity of the ten compounds was estimated by the concepts of concentration addition and independent action. Complete concentration-response relationships were experimentally recorded for the quinolone mixture in three different mixture ratios, based on the relative toxicity of the components (EC50, EC1 and NOEC). The results show that the mixture toxicity of the quinolones is best predictable by concentration addition, whereas independent action underestimates the toxicity of the mixture. As the quinolones have an identical specific mechanism of action (the inhibition of bacterial gyrases), these results are in agreement with the pharmacological assumptions that form the basis of the concept of concentration addition. It is therefore concluded, that concentration addition can be useful for hazard assessment procedures of mixtures of similarly acting compounds. One important implication of this concept is that even mixture components that are present only at their individual no observed effect concentrations (NOECs) contribute to the overall toxicity of the mixture. Under these conditions more than 99% effect of the quinolone mixture are observed. This result emphasises the unsuitability of NOECs as an approximation of a 'safe' concentration.