Prediction of mixture toxicity from the hormesis of a single chemical: A case study of combinations of antibiotics and quorum-sensing inhibitors with gram-negative bacteria

Single and mixture toxicity evaluation of avobenzone and homosalate to male zebrafish and H295R cells

Avobenzone and homosalate are widely used in sunscreens to provide ultraviolet (UV) protection, either as single compounds or in combination. Some UV filters exhibit estrogenic or anti-androgenic activities, however, studies regarding their interactions and toxicity in mixtures are limited. In this study, the effect of the toxicity of a binary mixture comprising avobenzone (0.72 μg L-1) and homosalate (1.02 and 103 μg L-1) on steroid hormone biosynthesis were investigated using male zebrafish and human adrenocortical carcinoma (H295R) cells. In fish exposed to homosalate, a significant decrease in the gonadosomatic index, testosterone level, and transcription of several genes (e.g, hsd3b2, cyp17a1, and hsd17b1) and a significant increase in the hepatosomatic index, liver steatosis, 17β-estradiol level, and transcription of vtg gene were observed. These results suggest that estrogenic and anti-androgenic effects of homosalate were mediated by the steroidogenic pathway. The presence of 0.72 μg L-1 of avobenzone augmented the anti-androgenic responses in male fish. The testosterone level in the H295R cells were significantly decreased after they were exposed to homosalate alone or in combination with avobenzone, which is consistent with observations in male zebrafish. Further studies need to be conducted to understand the endocrine disrupting properties of long-term exposure to substances typically used in sunscreens.

Comprehensive assessment of toxicity and environmental risk associated with sulfamethoxazole biodegradation in sulfur-mediated biological wastewater treatment

Incomplete mineralization of sulfamethoxazole (SMX) in wastewater treatment systems poses a threat to ecological health. The toxicity and environmental risk associated with SMX biodegradation in the sulfur-mediated biological process were examined for the first time through a long-term (180 days) bioreactor study and a series of bioassays. The results indicated that the sulfur-mediated biological system was highly resistant and tolerant to SMX toxicity, as evidenced by the enrichment of sulfate-reducing bacteria (SRB), the improved microbial metabolic activity, and the excellent performance on pollutants removal under long-term SMX exposure. SMX can be effectively biodegraded by the cleavage and rearrangement of the isoxazole ring, hydrogenation and hydroxylation reactions in sulfur-mediated biological wastewater system. These biodegradation pathways effectively reduced the acute toxicity, antibacterial activity, and ecotoxicities of SMX and its biotransformation products (TPs) in the effluent of the sulfur-mediated biological system. The TPs produced via hydrogenation (TP1), hydroxylation, and isoxazole ring cleavage (TP3, TP4, TP5, TP8, and TP9) exhibited lower toxicity than SMX. Under SMX stress, although the abundance of sulfonamide resistance genes increased, the total abundance of ARGs decreased due to the extrusion of some intracellular SMX by the efflux pump genes and the inactivation of some SMX through the biodegradation process. Efflux pump and inactivation, as the main resistance mechanisms of antibiotics in the sulfur-mediated biological system, play a crucial role in microbial self-defense. The findings of this study demonstrate the great potential of the sulfur-mediated biological system in SMX removal, detoxication, and ARGs environmental risk reduction.

Predictive in silico models for aquatic toxicity of cosmetic and personal care additive mixtures

As emerging environmental contaminants, cosmetic and personal care additives (CPCAs) may have less oversight than other consumer products. Their continuous release and pseudopersistence could cause long-term harm to the aquatic environment. Since CPCAs generally exist in the form of mixtures in the environment, prediction and analysis of their mixture toxicity are crucial for ecological risk assessment. In this study, the acute toxicity of five typical CPCA mixtures to Daphnia magna was tested. The combined toxicity of binary mixtures was examined with the traditional concentration addition (CA) and independent action (IA) model. Overall, the synergistic effect of the five CPCAs may be caused mainly by methylparaben. In addition, reliable approaches for quantitative structure-activity relationship (QSAR) model development were explored. Specifically, 18 QSAR models were developed by three dataset partitioning techniques (Kennard-Stone's algorithm division, Euclidean distance based division, and sorted activity based division), two descriptor filtering methods (genetic algorithm and stepwise multiple linear regression) and three regression methods (multiple linear regression, partial least squares and support vector machine). Sixteen equations were applied for the calculation of the mixture descriptors to screen the functional expression of the mixture descriptors with the largest contribution to the mixture toxicity. A new comprehensive parameter that integrates internal and external validation was proposed for QSAR models evaluation. The mixture toxicity is mainly related the 3D distribution of atomic masses and the spatial distribution of the molecule electronic properties. Rigorously validated and externally predictive QSAR models were developed for predicting the toxicity of binary CPCAs mixtures with any ratio, in the applicability domain. The best possible work frame for construction and validation of QSAR models to provide reliable predictions on the mixture toxicity was proposed.

The dissimilarity of antibiotic and quorum sensing inhibitor on activated sludge nitrification system: Microbial communities and antibiotic resistance genes

Effects of antibiotics (azithromycin, AZM, 1-40 mg/L) and quorum sensing inhibitor (QSI, 2(5H)-furanone, 1-40 mg/L) combined pollution with environmental concentration of copper on bacterial/archaeal community and antibiotic resistance genes (ARGs) in activated sludge system were explored. QSI inhibited nitrification more obviously than AZM. AZM and QSI were synergistic inhibitions on bacterial diversity, and AZM inhibited bacterial compositions more than QSI. While, QSI had more impacts on archaeal diversity/compositions. Less interactions among bacteria and archaea communities with Aquimonas as keystone genus. Functional differences in bacteria/archaea communities were little, and AZM had more effects on metabolism. AZM mainly affected nitrifying bacteria (Candidatus Nitrospira nitrificans and Nitrosomonas). Specific denitrifying bacteria were enriched by AZM (Brevundimonas, 1.76-31.69%) and QSI (Comamonas, 0.61-9.61%), respectively. AZM enriched ARGs more easily than QSI and they were antagonistic to proliferation of ARGs. Bacteria were main hosts of ARGs (macrolide-lincosamide-streptogramin B, other/efflux, etc.) and archaea (Methanosphaerula, Methanolobus) carried multiple ARGs.

Prediction on the combined toxicities of stimulation-only and inhibition-only contaminants using improved inverse distance weighted interpolation

The evaluation of ecological risks of contaminant mixtures to organisms is very challenging due to the non-linear response of organisms to each component, especially under the co-existence of both stimulators and inhibitors. Whether the stimulatory effect can reduce or even offset the inhibitory effect would be critical to the risk assessment and the treatment measures of mixed pollutants. Here, the combined toxicity of sodium fluoride (NaF), a stimulator with stimulation rate >100%, and six compounds that cannot induce hormesis (four ionic liquids (ILs) and two pesticides) were studied. The time-dependent toxicity of each toxicant on Vibrio qinghaiensis sp.-Q67 was investigated at 0.25, 2, 4, 6, 8, 10 and 12 h. Results showed that four ILs and two pesticides failed to induce hormesis, while NaF induced hormesis from 2 to 6 h and induced stimulation only after 6 h and reached its maximum (650%) at 12 h. All mixture rays with NaF induced hormesis at different times. In the four NaF-IL mixture systems, the absolute value of maximum stimulation demonstrated an upwards and then a downwards trend with the increasing of mixture ratio of IL. In two NaF-pesticide systems, the maximum stimulation effect declined with the increasing of the mixture ratio of pesticide. The toxicities of the mixture were successfully predicted by the improved inverse distance weighted interpolation, which are not able to be predicted by the commonly used concentration addition or independent action models. This paper shed lights on evaluating the hormesis of mixtures and the ecological risk of fluoride.

Hormetic dose responses induced by antibiotics in bacteria: A phantom menace to be thoroughly evaluated to address the environmental risk and tackle the antibiotic resistance phenomenon

The environmental contamination of antibiotics caused by their over or inappropriate use is a major issue for environmental and human health since it can adversely impact the ecosystems and promote the antimicrobial resistance. Indeed, considering that in the environmental matrices these drugs are present at low levels, the possibility that bacteria exhibit a hormetic response to increase their resilience when exposed to antibiotic subinhibitory concentrations might represent a serious threat. Information reported in this review showed that exposure to different types of antibiotics, either administered individually or in mixtures, is capable of exerting hormetic effects on bacteria at environmentally relevant concentrations. These responses have been reported regardless of the type of bacterium or antibiotic, thus suggesting that hormesis would be a generalized adaptive mechanism implemented by bacteria to strengthen their resistance to antibiotics. Hormetic effects included growth, bioluminescence and motility of bacteria, their ability to produce biofilm, but also the frequency of mutation and plasmid conjugative transfer. The evaluation of quantitative features of antibiotic-induced hormesis showed that these responses have both maximum stimulation and dose width characteristics similar to those already reported in the literature for other stressors. Notably, mixtures comprising individual antibiotic inducing stimulatory responses might have distinct combined effects based on antagonistic, synergistic or additive interactions between components. Regarding the molecular mechanisms of action underlying the aforementioned effects, we put forward the hypothesis that the adoption of adaptive/defensive responses would be driven by the ability of antibiotic low doses to modulate the transcriptional activity of bacteria. Overall, our findings suggest that hormesis plays a pivotal role in affecting the bacterial behavior in order to acquire a survival advantage. Therefore, a proactive and effective risk assessment should necessarily take due account of the hormesis concept to adequately evaluate the risks to ecosystems and human health posed by antibiotic environmental contamination.

Hormesis Effect of Berberine against Klebsiella pneumoniae Is Mediated by Up-Regulation of the Efflux Pump KmrA

Berberine (BBR) is an effective drug for human intestinal inflammation by preventing intestinal adhesion of bacterial pathogens, while its antibacterial activity is ineffective. Although the antimicrobial mechanisms of BBR are intensively studied at high concentrations, the response of pathogens to its low concentrations remains poorly understood. Here we demonstrated that low concentrations of BBR (3 and 6 μg/mL) conferred by hormesis accelerated cell growth of an important Gram-negative pathogen, Klebsiella pneumoniae, in vitro, while higher concentrations (25 and 50 μg/mL) resulted in the opposite. Transcriptome analysis of K. pneumoniae revealed the up-regulated expression of the KmrA efflux pump and further confirmed it was hypersensitive to BBR stress. Strikingly, when cultivated in tetracycline, the growth-promoting effect of BBR became more significant, while this effect was reversed in the presence of the efflux pump inhibitor cyanide-m-chlorophenylhydrazone. The hormesis was also found in Enterobacter cloacae and Acinetobacter baumannii. More importantly, the presence of BBR at low concentrations resulted in higher minimal inhibitory concentrations of efflux-related antibiotics such as rifampicin and azithromycin. Overall, our data demonstrated the hormesis of BBR and revealed the potential risk of its applications against Gram-negative pathogens at low concentrations.

Chemical interactions and mixtures in public health risk assessment: An analysis of ATSDR's interaction profile database

The Agency for Toxic Substances and Disease Registry (ATSDR) develops interaction profiles using binary weight of evidence (BINWOE) methodology to determine interaction directions of common environmental mixtures. We collected direction of interactions, BINWOE score determination, and BINWOE score confidence rating from 13 interaction profiles along with toxicodynamic and toxicokinetic influences on interaction direction. By doing so, we quantified the 1) direction of interaction and indeterminate evaluations; 2) characterized confidence in the BINWOE determinations; and 3) quantified toxicokinetic/toxicodynamic, and other influences on projected BINWOE interaction directions. Thirty-nine percent (130/336) of the attempts to make a BINWOE were indeterminate due to no interaction data or inadequate or conflicting evidence. Out of remaining BINWOEs, 25% were additive, 9% were greater-than-additive, and 27% were less-than-additive interactions. Fifty-five percent of BINWOEs were explained by toxicokinetic interactions, 12% and 5% were explained by toxicodynamic and other explanations, respectively. High quality mixture toxicology in vivo studies along with mixture in vitro and in silico studies will lead to greater confidence in interaction directions and influences. Limitations for interpretation of the data were also included.

Cd induced biphasic response in soil alkaline phosphatase and changed soil bacterial community composition: The role of background Cd contamination and time as additional factors

Hormesis is an intriguing phenomenon characterized by low-dose stimulation and high-dose inhibition. The hormetic phenomena have been frequently reported in the past decades, but the researches on the biphasic responses of soil enzymes are still limited. The main objective of this study is to explore dose response of alkaline phosphatase (ALP) to Cd (0, 0.003, 0.03, 0.3, 3.0 and 30 mg/kg) in the presence of different levels of background Cd contamination (bulk soil with no added Cd, BS; low background Cd, LB; medium background Cd, MB; and high background Cd, HB). ALP activity at 0.003-0.3 mg Cd/kg was 13-39% higher than that of the control (0 mg Cd/kg) for HB after 7 d. Similarly, the enzyme activities at 0.003-0.03 mg Cd/kg were 2-25% and 14-17% higher than those of the controls for MB and HB after 60 d. After 90 d, ALP activities at 0.3-3.0 mg Cd/kg increased by 11-17% for LB. The dose-response curves had the shape of an inverted U, showing biphasic responses at days 7 (HB), 60 (MB and HB) and 90 (LB). After 60 days of exposure, total operational taxonomic units (OTU) numbers and unique species exposed to Cd stress displayed hormetic-response curve for MB. The relative abundances of Agrobacterium, Salinimicrobiums, Bacilllus, and Oceanobacillus displayed significantly positive correlations with ALP activity. This suggested that bacterial communities potentially contribute to ALP's hormesis. This study further provides new insights into the ecological mechanisms of pollutant-induced hormesis, and substantially contributes to the ecological risk assessment of Cd pollution.

Resistance risk induced by quorum sensing inhibitors and their combined use with antibiotics: Mechanism and its relationship with toxicity

The abuse of antibiotics has brought out serious bacterial resistance, which threatens the ecological environment and human health. Quorum sensing inhibitors (QSIs), as a new kind of potential antibiotic substitutes that are theoretically difficult to trigger bacterial resistance, are recommended to individually use or jointly use with traditional antibiotics. However, there are few studies on the resistance risk in the use of QSIs. In this study, the influence of QSIs alone or in combination with sulfonamides (SAs) on conjugative transfer and mutation of Escherichia coli (E. coli) was investigated to explore whether QSIs have the potential to induce bacterial resistance. The results show that QSIs may facilitate plasmid RP4 conjugative transfer by binding with SdiA protein to regulate pilus expression, and interact with LsrR protein to increase SOS gene expression, inducing gene mutation. The QSIs-SAs mixtures could promote plasmid RP4 conjugative transfer and mutation in E. coli, and the main joint effects are synergism and antagonism. Furthermore, there is a good correlation among conjugative transfer, mutation, and growth inhibition of QSIs-SAs to E. coli. It could be speculated that bacteria may delay cell division to provide sufficient energy and time for regulating conjugative transfer and mutation under the stress of QSIs and their combined exposure with antibiotics, which is essentially a balance between bacterial resistance and toxicity. This study provides a reference for the resistance risk assessment of QSIs and benefits the clinical application of QSIs.

Sodium Ascorbate as a Quorum-Sensing Inhibitor Leads to Decreased Virulence in Vibrio campbellii

Vibrio campbellii is one of the major bacterial pathogens for animals reared in aquaculture, affecting both vertebrates and invertebrates, and causes significant economic losses. It is now evident that the expressions of virulence factors in this pathogen are regulated by the density of the bacterial population. This type of regulation, termed quorum sensing (QS), is mediated by extracellular signal molecules called autoinducers. In this study, the impact of sodium ascorbate (NaAs) on the virulence of V. campbellii was investigated under both in vitro and in vivo conditions, to develop a natural anti-infective strategy to contain V. campbellii infection in aquacultured animals. Results showed that NaAs significantly decreased swimming motility, biofilm production, and the production of virulence enzymes, such as lipase, caseinase, phospholipase, and hemolysin in V. campbellii. Consistent with this, pretreatment of V. campbellii with NaAs before inoculation into the rearing water resulted in significantly increased survival of gnotobiotic brine shrimp larvae, when compared to larvae challenged with untreated V. campbellii. Furthermore, NaAs could interfere with QS-regulated bioluminescence in V. campbellii, suggesting the QS-inhibitory activity largely determines the protective effect of NaAs toward the brine shrimp. In essence, due to the potent anti-virulence effects observed in in vitro studies and the clinical brine shrimp-V. campbellii infection model, NaAs constitute a promising novel strategy for the control of V. campbellii infections in aquaculture.

Time-Dependent Hormetic Response of Soil Alkaline Phosphatase Induced by Cd and the Association with Bacterial Community Composition

Hormetic dose-response that involved Cd in soils is increasingly paid attentions for risk assessment of Cd toxicity, but insufficient studies were conducted to define the temporary modification of soil enzyme and the potential microbial responses. The present study chooses soil alkaline phosphatase (ALP) as endpoint to uncover the time-dependent hormetic responses to low doses of Cd and its association with bacterial community composition. The results showed that addition of 0.01-3.0 mg kg^-1 Cd significantly increased ALP's activities with maximum stimulatory magnitude of 11.4-27.2%, indicating a typical hormesis. The response started at 12 h after Cd addition and maintained about 24 h. This demonstrated that the hormetic response is time-dependent and transient. Changes of soil bacterial community composition showed that, at 6 h, relative abundances (RAs) of Proteobacteria and Firmicutes at phylum and Pontibacter, Bacillaceae-Bacillus, Bacillaceae1-Bacillus, and Paenisporosarcina at genus significantly correlated with ALP's activities at 12-36 h (P < 0.05). This suggests that soil bacteria likely showed an earlier response to Cd and potentially contributes to the subsequent soil enzyme's hormesis. In addition, it was found that Gram-negative bacteria other than Gram-positive bacteria are prone to exhibiting a hormetic response under Cd stress. Our findings provide much insight into ecotoxicological risk assessment for soil Cd pollution.

Magnitude of the mixture hormetic response of soil alkaline phosphatase can be predicted based on single conditions of Cd and Pb

In soil ecosystems, it is very challenging to predict mixture hormesis effects. In the present study, soil alkaline phosphatase (ALP) was selected to investigate and predict its potential hormetic responses under Cd and Pb stresses. Typical reverse U-shaped dose-response relationships between ALP activities and the single and combined Cd and Pb were observed, showing a hormetic response of soil itself. The maximum stimulatory magnitudes ranged in 8.0 - 8.6% under 0.004 - 0.2 mg/kg Cd and 80 - 400 mg/kg Pb, respectively. An enhanced stimulation of 15.7% occurred under the binary mixtures of 0.6 mg/kg Cd and 200 mg/kg Pb. In addition, a dosage-independent binary linear regression model was proposed based on an assumption of a linear relationship between the single and combined hormetic responses under Cd and Pb. Our model can well predict ALP's responses in the presence of the two metals' mixtures (p < 0.1). Our findings provided new understandings to hormesis in soil.

Exploration of Computational Approaches to Predict the Toxicity of Chemical Mixtures

Industrial advances have led to generation of multi-component chemicals, materials and pharmaceuticals which are directly or indirectly affecting the environment. Although toxicity data are available for individual chemicals, generally there is no toxicity data of chemical mixtures. Most importantly, the nature of toxicity of these studied mixtures is completely different to the single components, which makes the toxicity evaluation of mixtures more critical and challenging. Interactions of individual chemicals in a mixture can result in multifaceted and considerable deviations in the apparent properties of its ingredients. It results in synergistic or antagonistic effects as opposed to the ideal case of additive behavior i.e., concentration addition (CA) and independent action (IA). The CA and IA are leading models for the assessment of joint activity supported by pharmacology literature. Animal models for toxicity testing are time- and money-consuming as well as unethical. Thus, computational approaches are already proven efficient alternatives for assessing the toxicity of chemicals by regulatory authorities followed by industries. In silico methods are capable of predicting toxicity, prioritizing chemicals, identifying risk and assessing, followed by managing, the risk. In many cases, the mechanism behind the toxicity from species to species can be understood by in silico methods. Until today most of the computational approaches have been employed for single chemical's toxicity. Thus, only a handful of works in the literature and methods are available for a mixture's toxicity prediction employing computational or in silico approaches. Therefore, the present review explains the importance of evaluation of a mixture's toxicity, the role of computational approaches to assess the toxicity, followed by types of in silico methods. Additionally, successful application of in silico tools in a mixture's toxicity predictions is explained in detail. Finally, future avenues towards the role and application of computational approaches in a mixture's toxicity are discussed.

Prediction of the Toxicity of Binary Mixtures by QSAR Approach Using the Hypothetical Descriptors

Organic compounds are often exposed to the environment, and have an adverse effect on the environment and human health in the form of mixtures, rather than as single chemicals. In this paper, we try to establish reliable and developed classical quantitative structure⁻activity relationship (QSAR) models to evaluate the toxicity of 99 binary mixtures. The derived QSAR models were built by forward stepwise multiple linear regression (MLR) and nonlinear radial basis function neural networks (RBFNNs) using the hypothetical descriptors, respectively. The statistical parameters of the MLR model provided were N (number of compounds in training set) = 79, R² (the correlation coefficient between the predicted and observed activities)= 0.869, LOOq² (leave-one-out correlation coefficient) = 0.864, F (Fisher's test) = 165.494, and RMS (root mean square) = 0.599 for the training set, and N_ext (number of compounds in external test set) = 20, R² = 0.853, qext2 (leave-one-out correlation coefficient for test set)= 0.825, F = 30.861, and RMS = 0.691 for the external test set. The RBFNN model gave the statistical results, namely N = 79, R² = 0.925, LOOq² = 0.924, F = 950.686, RMS = 0.447 for the training set, and N_ext = 20, R² = 0.896, qext2 = 0.890, F = 155.424, RMS = 0.547 for the external test set. Both of the MLR and RBFNN models were evaluated by some statistical parameters and methods. The results confirm that the built models are acceptable, and can be used to predict the toxicity of the binary mixtures.

The joint toxicity effect of five antibiotics and dibutyl phthalate to luminescent bacteria (Vibrio fischeri)

Antibiotics and phthalate esters are two kinds of emerging pollutants and are ubiquitous in the aquatic ecosystem. To date, few studies analyzed the combined toxicity of the mixtures of antibiotics and phthalate esters, and their joint toxicity effect mode remains unknown. Here, we investigated the single and joint toxicity of dibutyl phthalate (DBP) and five antibiotics, namely, oxytetracycline hydrochloride (OTC), chlortetracycline hydrochloride (CTC), sulfamethazine (SMZ), sulfamerazine (SMR), and sulfadiazine (SD), to luminescent bacteria of Vibrio fischeri. The median effect concentration (EC₅₀) values of the test chemicals were ranked as CTC (6.67 mg/L) > OTC (25.12 mg/L) > SD (67.61 mg/L) > SMR (141.51 mg/L) > DBP (148.38 mg/L) > SMZ (245.07 mg/L). The joint toxicities of the binary mixtures of antibiotics and DBP were evaluated by the concentration addition (CA) and independent action (IA) models. The joint toxicity effects of CTC-DBP, OTC-DBP, SMZ-DBP, SMR-DBP, and SD-DBP all appeared to be synergism. Our study revealed that sulfonamides combined with DBP could be as toxic as or even more toxic than tetracycline. Thus, the joint toxicity effect should be considered when assessing the ecological risks of binary or multicomponent pollutants.

A QSAR-based mechanistic study on the combined toxicity of antibiotics and quorum sensing inhibitors against Escherichia coli

Quorum sensing inhibitors (QSIs) have attracted increasing attention due to their potential roles as the antibiotic alternatives. The combination of QSIs and antibiotics in clinical use and their subsequent release into the environment may result in joint effects on the ecology and environment, which has not received enough concerns yet. In this study, eight potential QSIs and three types of commonly used antibiotics, i.e., sulfonamides (SAs), β-lactams and tetracyclines (TCs), were investigated for their combined toxicity on Escherichia coli (E. coli). The QSAR models for the combined toxicity were constructed using the interaction energies between the chemicals and their target proteins as calculated by molecular docking. It was revealed that the SAs and QSIs presented either additive or antagonistic joint effects in the mixture toxicity test, while β-lactams and TCs showed only antagonistic effects with the QSIs. The analysis on the coefficients in the QSAR models suggested that the QSIs in the mixtures were more involved in the interaction with the proteins than the antibiotics. This study will help better understand the risks of joint exposure to the antibiotics and QSIs, and provide a new perspective for the study of the combined toxicity mechanism.

In vitro and in silico investigations of the binary-mixture toxicity of phthalate esters and cadmium (II) to Vibrio qinghaiensis sp.-Q67

Phthalate esters (PAEs) are widely used as plasticizers and have become one of the emerging contaminants with an increasing public concern. The residues of PAEs frequently co-exist with heavy metals such as cadmium (Cd) in waters; however, their joint ecotoxicity remains largely unknown. We herein investigated the single and joint toxicity of commonly used PAEs and Cd using freshwater luminescent bacteria Vibrio qinghaiensis sp.-Q67. The median effective concentration (EC₅₀) of benzyl butyl phthalate (BBP), dibutyl phthalate (DBP), diethyl phthalate (DEP), dimethyl phthalate (DMP), diisooctyl phthalate (DIOP) and di-n-octyl phthalate (DOP) were determined to be in the range from 134.4mg/L to as high as 1000mg/L, indicating very weak toxicity to Vibrio qinghaiensis sp.-Q67. The toxicity of single PAEs showed a significant linear relationship with Log K_ow, indicating the dependence of the elevated toxicity on the increasing hydrophilicity. The toxicity of binary mixture of PAEs was further evaluated in silico using the independent action (IA) model and concentration addition (CA) model. DMP-DEP, DEP-DBP or DMP-DBP exhibited antagonistic effects with the toxic unit value higher than 1.2. The CA and IA models poorly predicted the joint toxicity of DMP-DEP, DEP-DBP or DMP-DBP. The joint toxicity of the binary mixtures of DMP, DEP or DBP with Cd was simple additive as predicted by the CA and IA models. Our results indicated the potentially higher risk of PAEs in the presence of Cd, emphasizing the importance of determining the impact of their joint effects on aquatic organisms. The integrated in vitro and in silico methods employed in this study will be beneficial to study the joint toxicity and better assess the aquatic ecological risk of PAEs.

Hormetic effects of metal ions upon V. fischeri and the application of a new parameter for the quantitative assessment of hormesis

Hormesis is an intriguing phenomenon that is characterized by low dose stimulation and high dose inhibition. Several traditional parameters, such as the concentration of the zero equivalent point (ZEP) and the maximal stimulatory effect (Y_max), have been used to characterize the zone of hormesis or the extent of the stimulatory effect. However, the characteristics of hormesis for chemicals cannot be quantified completely by one parameter, which is important to accurately compare the hormetic effects of chemicals and to describe the combined effects of chemical mixtures at low doses. In the present study, a novel parameter, termed the relative standard area of hormetic zone (Hor_Area^R), was developed and proposed to quantify the hormetic effects (24h exposure) of nine metal ions (Cr³⁺, Cu²⁺, Mg²⁺, Cd²⁺, Fe³⁺, Ni²⁺, Zn²⁺, Co²⁺, Cs⁺) towards Vibrio fischeri, both individually and as binary mixtures. The results indicate that Hor_Area^R can be used not only to accurately assess the hormetic effects and its relationship with structural characteristics but also to conveniently describe the combined effects of interactive mixtures at low dose. Thus, the Hor_Area^R parameter can quantitatively assess the hormetic effects and can offer a useful approach to perform environmental risk assessments of chemicals at low doses.

A new parameter for the stimulation effect and its application in the prediction of the hormetic effect in chemical mixtures

To study the prediction methods is important for chronic mixture toxicity at low concentration, particularly mixtures containing chemicals with hormetic effects because pollutants in the real environment exist at low-doses in the form of mixtures.