在清除放射性铀污染的队伍中，有望增加一批新成员。这些只有千分之一毫米长的清洁工挥舞着细长的“毛发”，能把溶解在水里的铀清除掉。美国研究人员新近发现，一类称为地杆菌的细菌有潜力用于铀污染的生物治理。
此前已有研究表明，一些地杆菌能够通过还原周围环境里的金属（也就是向金属添加电子）来获取能量。溶解在水里的铀经过这样的还原之后，会变得难以溶解，从而缩小污染范围，并且容易被清除掉。
美国密歇根州立大学的研究人员杰玛·雷格拉猜测，这些细菌外面的细长丝状物——菌毛可能是问题的关键。这些由蛋白质组成的菌毛能够导电，曾被用于研制“纳米电线”。
雷格拉与同事以硫还原地杆菌为对象，培育出因缺乏某种基因而不能产生菌毛的菌株，与能正常产生菌毛的菌株进行比较。结果显示，菌毛能大大增强细菌清除铀污染的能力。
研究发现，如果没有菌毛，铀的还原反应是在细菌内部进行的，会伤害到细菌自身。而有菌毛时，大部分反应围绕着菌毛完成，不仅扩大了反应过程中可用于电子传输的空间，还拉远了铀与细菌的距离，提高安全性。
研究人员用一种荧光染料测量了地杆菌细胞的呼吸酶在接触铀之后的活性。结果显示，有菌毛的细菌呼吸酶活性更高，因而生存能力更强。有菌毛的菌株在接触铀之后还能恢复过来，并且比没有菌毛的菌株生长更快。
雷格拉说，由于菌毛的成分是蛋白质，可以比较容易地往上面添加不同的官能团（决定有机化合物化学性质的原子或原子团），来调节菌毛功能。
相关论9月文5日发表在美国新一期《国家科学院院刊》上。雷格拉认为，这种方法理论上也适用于其他一些金属元素的放射性同位素，包括锝、钚和钴等。因此，该成果不仅可用于治理以往核试验造成的铀污染，还有可能帮助应对日本核电站事故。(生物谷 Bioon.com)
doi:10.1073/pnas.1108616108
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Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism
Cologgi, Dena L.; Lampa-Pastirk, Sanela; Speers, Allison M.; Kelly, Shelly D.; Reguera, Gemma
The in situ stimulation of Fe(III) oxide reduction by Geobacter bacteria leads to the concomitant precipitation of hexavalent uranium [U(VI)] from groundwater. Despite its promise for the bioremediation of uranium contaminants, the biological mechanism behind this reaction remains elusive. Because Fe(III) oxide reduction requires the expression of Geobacter's conductive pili, we evaluated their contribution to uranium reduction in Geobacter sulfurreducens grown under pili-inducing or noninducing conditions. A pilin-deficient mutant and a genetically complemented strain with reduced outer membrane c-cytochrome content were used as controls. Pili expression significantly enhanced the rate and extent of uranium immobilization per cell and prevented periplasmic mineralization. As a result, pili expression also preserved the vital respiratory activities of the cell envelope and the cell's viability. Uranium preferentially precipitated along the pili and, to a lesser extent, on outer membrane redox-active foci. In contrast, the pilus-defective strains had different degrees of periplasmic mineralization matching well with their outer membrane c-cytochrome content. X-ray absorption spectroscopy analyses demonstrated the extracellular reduction of U(VI) by the pili to mononuclear tetravalent uranium U(IV) complexed by carbon-containing ligands, consistent with a biological reduction. In contrast, the U(IV) in the pilin-deficient mutant cells also required an additional phosphorous ligand, in agreement with the predominantly periplasmic mineralization of uranium observed in this strain. These findings demonstrate a previously unrecognized role for Geobacter conductive pili in the extracellular reduction of uranium, and highlight its essential function as a catalytic and protective cellular mechanism that is of interest for the bioremediation of uranium-contaminated groundwater.
