Metabolomic and proteomic investigations of impacts of titanium dioxide nanoparticles on <i>Escherichia coli</i>

In a previous study, it was demonstrated that the toxic impact of titanium dioxide nanoparticles on Escherichia coli starts at 10 ppm and is closely related to the presence of little aggregates. It was also assumed that only a part of the bacterial population is able to adapt to this stress and attempts to survive. Proteomic analyses, supported by results from metabolomics, reveal that exposure of E. coli to nano-TiO2 induces two main effects on bacterial metabolism: firstly, the up-regulation of proteins and the increase of metabolites related to energy and growth metabolism; secondly, the down-regulation of other proteins resulting in an increase of metabolites, particularly amino acids. Some proteins, e.g. chaperonin 1 or isocitrate dehydrogenase, and some metabolites, e.g. phenylalanine or valine, might be used as biomarkers of nanoparticles stress. Astonishingly, the ATP content gradually rises in relation with the nano-TiO2 concentration in the medium, indicating a dramatic release of ATP by the damaged cells. These apparently contradictory results accredit the thesis of a heterogeneity of the bacterial population. This heterogeneity is also confirmed by SEM images which show that while some bacteria are fully covered by nano-TiO2, the major part of the bacterial population remains free from nanoparticles, resulting in a difference of proteome and metabolome. The use of combined–omics has allowed to better understand the heterogeneous bacterial response to nano-TiO2 stress due to heterogeneous contacts between the protagonists under environmental conditions.既往研究证实，二氧化钛纳米颗粒（titanium dioxide nanoparticles）对大肠杆菌（Escherichia coli）的毒性效应始于10 ppm浓度，且与微小聚集体的存在密切相关。另有研究推测，仅部分细菌种群可适应该胁迫并尝试存活。依托代谢组学（metabolomics）结果佐证的蛋白质组分析（proteomic analyses）显示，大肠杆菌暴露于纳米二氧化钛（nano-TiO2）后，其代谢过程会出现两种主要变化：一是与能量及生长代谢相关的蛋白质表达上调、对应代谢物水平升高；二是部分蛋白质表达下调，进而引发包括氨基酸在内的多种代谢物含量上升。部分蛋白质（如伴侣蛋白1（chaperonin 1）、异柠檬酸脱氢酶（isocitrate dehydrogenase））与部分代谢物（如苯丙氨酸（phenylalanine）、缬氨酸（valine））或可作为纳米颗粒胁迫的生物标志物（biomarkers）。令人诧异的是，培养基内纳米二氧化钛浓度越高，细菌的三磷酸腺苷（ATP）含量反而逐步升高，这表明受损细胞会大量释放ATP。这些看似矛盾的结果印证了细菌种群存在异质性的论点。扫描电子显微镜（SEM）图像也证实了这一异质性：部分细菌完全被纳米二氧化钛包裹，而绝大多数细菌并未接触纳米颗粒，进而导致不同细菌的蛋白质组与代谢组产生差异。鉴于环境中作用因子间的接触存在异质性，采用组学联合分析（combined–omics）能够更清晰地解析细菌对纳米二氧化钛胁迫的异质性应答机制。