State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Jiangsu 210023, China.
College of Resources and Environmental Sciences, Nanjing Agricultural University, Jiangsu 210095, China.
Sci Total Environ. 2020 Jul 1;724:138291. doi: 10.1016/j.scitotenv.2020.138291. Epub 2020 Mar 28.
Redox transformations of organic contaminants by bacterial extracellular polymeric substances (EPS) and the associated electron transfer mechanisms are rarely reported. Here we show that a nitroaromatic compound (1,3-dinitrobenzene) can be readily reduced to 3-hydroxylaminonitrobenzene and 3-nitroaniline in aqueous suspension of common bacteria (E. coli or B. subtilis) or in aqueous dissolved EPS extracted from the bacteria. The loss ratio of 1,3-dinitrobenzene by E. coli was unaffected after knocking out the nfsA gene encoding nitroreductase, but was suppressed by removing EPS attached to cells. In contrast, the loss ratio was enhanced by adding aqueous dissolved EPS to E. coli or B. subtilis suspension. The residual 1,3-dinitrobenzene and products formed after reduction were only presented outside the bacterial cells. Thus, bacterial reduction of 1,3-dinitrobenzene was mediated by nonenzymatic extracellular reduction. This was further corroborated by the observation that the stoichiometric demand of electrons in 1,3-dinitrobenzene reduction was nearly equal to the quantity of electrons donated by bacterial cells in the electrochemical cell experiment. Inhibition on the reduction of 1,3-dinitrobenzene by chemical probes combined with fluorescence detection demonstrated that reducing sugars in EPS might act as electron donors, while cytochromes and some low-molecular weight molecules (flavins and quinones) were involved as electron transfer mediators. Linear relationships were observed between the reduction kinetics and the one-electron reduction potentials for a series of substituted dinitrobenzenes in the presence of bacterial cells or dissolved EPS. Their close linear regression slope values suggest that the extracellular matrix and the exfoliated EPS utilized the same reducing agents (likely hydroquinones and reduced flavins) as terminal electron donors to reduce NACs. These results reveal a previously unrecognized mechanism for nonenzymatic extracellular reduction of NACs by common bacteria. CAPSULE: The extracellular matrix of E. coli or B. subtilis supplies both electron donors and electron transfer mediators to efficiently reduce nitroaromatic compounds.
细菌细胞外聚合物（EPS）对有机污染物的氧化还原转化及其相关的电子传递机制鲜有报道。本研究表明，在常见细菌（大肠杆菌或枯草芽孢杆菌）的水悬浮液中或从细菌中提取的水溶解 EPS 中，硝基芳烃化合物（1,3-二硝基苯）可被迅速还原为 3-羟氨基硝基苯和 3-硝基苯胺。敲除编码硝基还原酶的 nfsA 基因后，大肠杆菌对 1,3-二硝基苯的损失率没有影响，但去除与细胞结合的 EPS 会抑制损失率。相反，向大肠杆菌或枯草芽孢杆菌悬浮液中添加水溶解的 EPS 会增强损失率。还原后残留的 1,3-二硝基苯和形成的产物仅存在于细菌细胞外。因此，细菌对 1,3-二硝基苯的还原是由非酶促细胞外还原介导的。这一结论在电化学池实验中得到进一步证实，在该实验中，1,3-二硝基苯还原的电子计量需求几乎与细菌细胞提供的电子数量相等。通过化学探针抑制 1,3-二硝基苯的还原并结合荧光检测，结果表明 EPS 中的还原糖可能作为电子供体，而细胞色素和一些低分子量分子（黄素和醌）则作为电子传递介体。在存在细菌细胞或溶解的 EPS 时，一系列取代的二硝基苯的还原动力学与单电子还原电位之间存在线性关系。它们接近的线性回归斜率值表明，细胞外基质和剥落的 EPS 利用相同的还原剂（可能是氢醌和还原的黄素）作为末端电子供体来还原 NACs。这些结果揭示了常见细菌对 NACs 进行非酶促细胞外还原的一种先前未被认识的机制。
