Tan Yang, Adhikari Ramesh Y, Malvankar Nikhil S, Ward Joy E, Woodard Trevor L, Nevin Kelly P, Lovley Derek R
Department of Microbiology, University of Massachusetts-Amherst, Amherst, Massachusetts, USA.
Department of Physics, University of Massachusetts-Amherst, Amherst, Massachusetts, USA.
mBio. 2017 Jan 17;8(1):e02203-16. doi: 10.1128/mBio.02203-16.
The electrically conductive pili (e-pili) of Geobacter sulfurreducens serve as a model for a novel strategy for long-range extracellular electron transfer. e-pili are also a new class of bioelectronic materials. However, the only other Geobacter pili previously studied, which were from G. uraniireducens, were poorly conductive. In order to obtain more information on the range of pili conductivities in Geobacter species, the pili of G. metallireducens were investigated. Heterologously expressing the PilA gene of G. metallireducens in G. sulfurreducens yielded a G. sulfurreducens strain, designated strain MP, that produced abundant pili. Strain MP exhibited phenotypes consistent with the presence of e-pili, such as high rates of Fe(III) oxide reduction and high current densities on graphite anodes. Individual pili prepared at physiologically relevant pH 7 had conductivities of 277 ± 18.9 S/cm (mean ± standard deviation), which is 5,000-fold higher than the conductivity of G. sulfurreducens pili at pH 7 and nearly 1 million-fold higher than the conductivity of G. uraniireducens pili at the same pH. A potential explanation for the higher conductivity of the G. metallireducens pili is their greater density of aromatic amino acids, which are known to be important components in electron transport along the length of the pilus. The G. metallireducens pili represent the most highly conductive pili found to date and suggest strategies for designing synthetic pili with even higher conductivities.
e-pili are a remarkable electrically conductive material that can be sustainably produced without harsh chemical processes from renewable feedstocks and that contain no toxic components in the final product. Thus, e-pili offer an unprecedented potential for developing novel materials, electronic devices, and sensors for diverse applications with a new "green" technology. Increasing e-pili conductivity will even further expand their potential applications. A proven strategy is to design synthetic e-pili that contain tryptophan, an aromatic amino acid not found in previously studied e-pili. The studies reported here demonstrate that a productive alternative approach is to search more broadly in the microbial world. Surprisingly, even though G. metallireducens and G. sulfurreducens are closely related, the conductivities of their e-pili differ by more than 3 orders of magnitude. The ability to produce e-pili with high conductivity without generating a genetically modified product enhances the attractiveness of this novel electronic material.
嗜硫还原地杆菌的导电菌毛（e - 菌毛）是一种新型长距离细胞外电子转移策略的模型。e - 菌毛也是一类新型生物电子材料。然而，之前研究的唯一其他地杆菌菌毛来自铀还原地杆菌，其导电性很差。为了获取更多关于地杆菌属菌毛电导率范围的信息，对金属还原地杆菌的菌毛进行了研究。在嗜硫还原地杆菌中异源表达金属还原地杆菌的PilA基因，得到了一株嗜硫还原地杆菌菌株，命名为MP菌株，该菌株产生大量菌毛。MP菌株表现出与存在e - 菌毛一致的表型，如高铁（III）氧化物还原速率和石墨阳极上的高电流密度。在生理相关pH 7条件下制备的单个菌毛的电导率为277±18.9 S/cm（平均值±标准差），这比pH 7条件下嗜硫还原地杆菌菌毛的电导率高5000倍，比相同pH条件下铀还原地杆菌菌毛的电导率高近100万倍。金属还原地杆菌菌毛电导率较高的一个潜在解释是其芳香族氨基酸密度更大，已知芳香族氨基酸是沿菌毛长度进行电子传输的重要组成部分。金属还原地杆菌菌毛是迄今为止发现的导电性最高的菌毛，并为设计具有更高电导率的合成菌毛提供了策略。
e - 菌毛是一种卓越的导电材料，可以从可再生原料中通过无苛刻化学过程可持续生产，且最终产品不含有毒成分。因此，e - 菌毛为利用新的“绿色”技术开发用于各种应用的新型材料、电子设备和传感器提供了前所未有的潜力。提高e - 菌毛的电导率将进一步扩大其潜在应用。一个已证实的策略是设计含有色氨酸的合成e - 菌毛，色氨酸是之前研究的e - 菌毛中未发现的芳香族氨基酸。此处报道的研究表明，一种有效的替代方法是在微生物世界中更广泛地寻找。令人惊讶的是，尽管金属还原地杆菌和嗜硫还原地杆菌密切相关，但它们的e - 菌毛电导率相差超过3个数量级。在不产生转基因产物的情况下生产高导电性e - 菌毛的能力增强了这种新型电子材料的吸引力。
