Adaptive Responses of Shewanella decolorationis to Toxic Organic Extracellular Electron Acceptor Azo Dyes in Anaerobic Respiration

Bacterial anaerobic respiration using an extracellular electron acceptor plays a predominant role in global biogeochemical cycles. However, the mechanisms of bacterial adaptation to the toxic organic pollutant as the extracellular electron acceptor during anaerobic respiration are not clear, which limits our ability to optimize the strategies for the bioremediation of a contaminated environment. Here, we report the physiological characteristics and the global gene expression of an ecologically successful bacterium, Shewanella decolorationis S12, when using a typical toxic organic pollutant, amaranth, as the extracellular electron acceptor. Our results revealed that filamentous shift (the cells stretched to fiber-like shapes as long as 18 mu m) occurred under amaranth stress. Persistent stress led to a higher filamentous cell rate and decolorization ability in subcultural cells compared to parental strains. In addition, the expression of genes involved in cell division, the chemotaxis system, energy conservation, damage repair, and material transport in filamentous cells was significantly stimulated. The detailed roles of some genes with significantly elevated expressions in filamentous cells, such as the outer membrane porin genes ompA and ompW, the cytochrome c genes arpC and arpD, the global regulatory factor gene rpoS, and the methyl-accepting chemotaxis proteins genes SHD_2793 and SHD_0015, were identified by site-directed mutagenesis. Finally, a conceptual model was proposed to help deepen our insights into both the bacterial survival strategy when toxic organics were present and the mechanisms by which these toxic organics were biodegraded as the extracellular electron acceptors.IMPORTANCE Keeping toxic organic pollutants (TOPs) in tolerable levels is a huge challenge for bacteria in extremely unfavorable environments since TOPs could serve as energy substitutes but also as survival stresses when they are beyond some thresholds. This study focused on the underlying adaptive mechanisms of ecologically successful bacterium Shewanella decolorationis S12 when exposed to amaranth, a typical toxic organic pollutant, as the extracellular electron acceptor. Our results suggest that filamentous shift is a flexible and valid way to solve the dilemma between the energy resource and toxic stress. Filamentous cells regulate gene expression to enhance their degradation and detoxification capabilities, resulting in a strong viability. These novel adaptive responses to TOPs are believed to be an evolutionary achievement to succeed in harsh habitats and thus have great potential to be applied to environment engineering or synthetic biology if we could picture every unknown node in this pathway.

利用胞外电子受体的细菌厌氧呼吸在全球生物地球化学循环中起着主导作用。然而，细菌在厌氧呼吸过程中适应作为胞外电子受体的有毒有机污染物的机制尚不清楚，这限制了我们优化污染环境生物修复策略的能力。在此，我们报道了一种在生态上成功的细菌——希瓦氏菌S12在使用一种典型的有毒有机污染物苋菜红作为胞外电子受体时的生理特性和全局基因表达情况。我们的结果显示，在苋菜红胁迫下发生了丝状转变（细胞伸长为长达18微米的纤维状）。与亲本菌株相比，持续的胁迫导致继代培养细胞中丝状细胞比例更高以及脱色能力更强。此外，丝状细胞中参与细胞分裂、趋化系统、能量守恒、损伤修复和物质运输的基因表达受到显著刺激。通过定点突变确定了一些在丝状细胞中表达显著升高的基因的详细作用，例如外膜孔蛋白基因ompA和ompW、细胞色素c基因arpC和arpD、全局调控因子基因rpoS以及甲基受体趋化蛋白基因SHD_2793和SHD_0015。最后，提出了一个概念模型，以帮助我们深入了解存在有毒有机物时细菌的生存策略以及这些有毒有机物作为胞外电子受体被生物降解的机制。 重要性：在极其不利的环境中，将有毒有机污染物（TOPs）保持在可耐受水平对细菌来说是一个巨大的挑战，因为当TOPs超过一定阈值时，它们既可以作为能量替代物，也可以作为生存压力。本研究重点关注了生态上成功的细菌希瓦氏菌S12在暴露于一种典型的有毒有机污染物苋菜红作为胞外电子受体时的潜在适应机制。我们的结果表明，丝状转变是解决能量来源和毒性压力之间困境的一种灵活有效的方法。丝状细胞调节基因表达以增强其降解和解毒能力，从而具有很强的生存能力。这些对TOPs的新型适应反应被认为是在恶劣栖息地中取得成功的一种进化成果，因此如果我们能够描绘出这条途径中的每一个未知节点，它们在环境工程或合成生物学中具有巨大的应用潜力。