硫还原Geobacter是一种模型微生物,用于阐明几种生物地球化学循环中的胞外电子转移机制,生物电化学应用,和微生物金属腐蚀。先前有多条证据表明,导电菌毛(e-pili)是硫还原G.中远距离细胞外电子传输的重要管道。然而,最近有报道,硫还原G.不表达e-pili,并且由多种血红素c型细胞色素组成的细丝负责远程电子传递。通过检查细胞直接研究了这种可能性,而不是长丝制剂,原子力显微镜。来自野生型细胞的大约90%的细丝的直径(3nm)和电导率与以前从硫还原G.收获的或在大肠杆菌中异基因表达的e-菌毛的报道一致。剩余的10%的丝具有与包含c型细胞色素OmcS的丝一致的形态。表达修饰的菌毛基因的菌株旨在产生导电性差的菌毛,表达了90%直径为3nm的细丝,但电导率大大降低,进一步表明野生型菌株中3-nm直径的导电丝是e-pili。一种菌株,其中五个最丰富的外表面c型细胞色素的基因,包括OmcS,删除仅产生3nm直径的细丝,其电导率与野生型相同。这些结果表明,e-pili是由硫还原G.表达的最丰富的导电细丝,与以前的功能研究一致,这些研究表明需要e-pili进行远程细胞外电子转移。重要性电活性微生物具有显著的环境影响,以及在生物能源和生物修复中的应用。组成,函数,甚至导电菌毛(e-pili)的存在一直是电微生物学研究中最具争议的领域之一,部分原因是e-pili提供了一种远程电子传输机制,该机制不涉及许多生物电子传输中常见的金属辅因子。这项研究表明,e-pili是来自硫化还原Geobacter的丰富细丝,作为直接种间电子转移中的远程细胞外电子转移的模型,异化金属还原,微生物电极交换,和由Fe(0)直接吸收电子引起的腐蚀。本研究中描述的方法提供了一种简单的策略,用于评估导电细丝在整个微生物世界中的分布,该方法避免了可能与生理无关的细丝的人为生产和/或富集。
Geobacter sulfurreducens is a model microbe for elucidating the mechanisms for extracellular electron transfer in several biogeochemical cycles, bioelectrochemical applications, and microbial metal corrosion. Multiple lines of evidence previously suggested that electrically conductive pili (e-pili) are an essential conduit for long-range extracellular electron transport in G. sulfurreducens. However, it has recently been reported that G. sulfurreducens does not express e-pili and that filaments comprised of multi-heme c-type cytochromes are responsible for long-range electron transport. This possibility was directly investigated by examining cells, rather than filament preparations, with atomic force microscopy. Approximately 90% of the filaments emanating from wild-type cells had a diameter (3 nm) and conductance consistent with previous reports of e-pili harvested from G. sulfurreducens or heterologously expressed in Escherichia coli from the G. sulfurreducens pilin gene. The remaining 10% of filaments had a morphology consistent with filaments comprised of the c-type cytochrome OmcS. A strain expressing a modified pilin gene designed to yield poorly conductive pili expressed 90% filaments with a 3-nm diameter, but greatly reduced conductance, further indicating that the 3-nm diameter conductive filaments in the wild-type strain were e-pili. A strain in which genes for five of the most abundant outer-surface c-type cytochromes, including OmcS, were deleted yielded only 3-nm-diameter filaments with the same conductance as in the wild type. These results demonstrate that e-pili are the most abundant conductive filaments expressed by G. sulfurreducens, consistent with previous functional studies demonstrating the need for e-pili for long-range extracellular electron transfer. IMPORTANCE Electroactive microbes have significant environmental impacts, as well as applications in bioenergy and bioremediation. The composition, function, and even existence of electrically conductive pili (e-pili) has been one of the most contentious areas of investigation in
electromicrobiology, in part because e-pili offer a mechanism for long-range electron transport that does not involve the metal cofactors common in much of biological electron transport. This study demonstrates that e-pili are abundant filaments emanating from Geobacter sulfurreducens, which serves as a model for long-range extracellular electron transfer in direct interspecies electron transfer, dissimilatory metal reduction, microbe-electrode exchange, and corrosion caused by direct electron uptake from Fe(0). The methods described in this study provide a simple strategy for evaluating the distribution of conductive filaments throughout the microbial world with an approach that avoids artifactual production and/or enrichment of filaments that may not be physiologically relevant.