| Carbon nanotubes（CNTs）is one of the most extensively used nanomaterials.After being discharged,CNTs has the possibility of interacting with co-existing contaminants,endangering the ecological environment.At present,the case study of the joint toxicity of CNTs and organic contaminants（OCs）and the understanding of relative mechanisms are very insufficient.It is necessary to study the joint toxicity of CNTs and OCs to bacteria and the influence of contaminant properties and bacterial species.Pentachlorophenol（PCP）,listed as priority pollutant and human carcinogen,is a typical traditional OCs.Ciprofloxacin（CIP）,as a representative of fluoroquinolones,has been identified as a common new OCs in the environment.PCP and CIP are hydrophobic and hydrophilic substances,respectively,and they may have differences in the interaction and joint toxicity with CNTs.Gram-negative Escherichia coli and positive Bacillus subtilis possess different physicochemical properties and physiological characteristics,and their responses to a same exposure system may be different.Based on microscopic observation,determination of biochemical indexes and bioaccumulation,and the analyses of transcriptomics and metabonomics,the involved interactions and toxic mechanisms of oxidized multi-walled carbon nanotubes（OCNTs）with PCP or CIP to E.coli and B.subtilis were investigated.The main results of the work are as follows:（1）Both OCNTs and PCP significantly inhibited the growth and reproduction of E.coli through membrane disruption,oxidative stress,and genetic damage.The half inhibitory concentration(IC50)values for OCNTs and PCP after 3-h exposure were 14.4±1.8 and 11.5±1.6 mg/L,respectively.Their co-exposure exhibited antagonistic toxicity.PCP increased the electrostatic repulsion between bacteria and OCNTs and decreased cell-surface hydrophobicity（CSH）.The bioaccumulation of OCNTs and subsequent OCNTs-induced intracellular reactive oxygen species accumulation were therefore reduced.OCNTs attenuated the PCP-induced disturbances to gene expressions in biosynthetic,protein metabolic,and small molecule metabolic processes,as well as for organelles.（2）The 3-h IC50 of CIP to E.coli growth was 244±14 mg/L,and the co-exposure of CIP and OCNTs revealed an antagonistic toxicity.Mitigations in cell membrane disruption and oxidative stress were involved in the antagonistic action.CIP decreased the bioaccumulation of OCNTs via reducing CSH,which attenuated the OCNTs toxicity.OCNTs alleviated the disturbance of CIP to gene expressions,especially related to nitrogen compound metabolism,oxidoreductase activity,and iron-sulfur protein maturation,through relieving the CIP-induced inhibition of DNA gyrase activity.Further,OCNTs reduced the impact of CIP on bacterial metabolome via the regulation of biosynthesis of unsaturated fatty acids and metabolisms of some amino acids and glutathione.（3）The 3-h IC50 values of OCNTs,PCP,and CIP to B.subtilis were 12.5±2.6,3.5±0.5,and 0.46±0.03 mg/L,respectively.In the co-exposure of OCNTs with PCP or CIP,there were additive toxicity or synergistic toxicity,respectively.The pre-exposure of PCP or CIP before its co-exposure with OCNTs had significantly increased the joint toxicity.All of the three pollutants caused cell membrane damage,aggravation of oxidative stress,and changes in the contents of fatty acids,amino acid,glycerol,osamine,and small molecular acid.PCP reduced the bioaccumulation of OCNTs,while CIP had no significant influence on the OCNTs bioaccumulation.OCNTs increased the bioaccumulation of PCP and CIP,and may enhance the toxicity of CIP through the‘Trojan horse effect’.The co-existence of OCNTs increased the inhibition effect of CIP on the activity of topoisomeraseⅣ,and interfered with the expression of genes related to ABC transporters and lysine biosynthesis. |
