報告書番号

V04006

タイトル（和文）

電気による微生物の制御（その８） -－クロム還元菌の取得ならびに電気化学的集積培養効果－-

タイトル（英文）

Electrochemical control of bacteria (8). Acquisition of a chromium redusing bacterium and its accumulation using electrochemical cultivation.

概要 （図表や脚注は「報告書全文」に掲載しております）

当所では、有害な六価クロムに耐性を有し、これを還元、無害化する微生物の取得に成功した。このクロム還元菌の簡便な集積培養の手段として電気化学的手法を試みた。Cr(VI)イオンを含む培地にクロム還元菌を懸濁させ、嫌気的に4週間の振とう培養を実施した。その結果、Cr(VI)濃度が0.5mM以下では良好な菌の増殖が確認されたが、培地中のCr(VI)イオンの減少に伴い、雑菌の繁殖が生じる結果となった。一方、Cr(VI)イオンが1mMの場合には、クロム還元菌自体の増殖が阻害され、本菌を集積培養するためには低濃度のCr(VI)イオンを維持する必要があると判断された。そこで、Cr(VI)イオンが電気化学的に生成される1.0Vの電位を培養液に印加し、嫌気的に2週間の電気培養を試みた。その結果、形状のそろった単一の微生物の増殖が確認され、電気化学的にクロム還元菌を集積できることが確認できた。

概要 （英文）

We have obtained chromium reducing bacteria from soil. These bacteria not only thrive at concentrations of toxic hexavalent chromium, Cr(VI), ions that are toxic to most bacteria, but under anaerobic conditions, also reduce Cr(VI) to insoluble Cr(III). However, obtaining a pure culture of these bacteria using conventional cultivation condition is difficult because as the concentration of Cr(VI) ions decreases, dominant bacteria from the original inoculation begin to thrive. Here we present results obtained using an electrochemical cell to replenish the Cr(VI) ions thus maintaining an environment not conducive to the growth of the dominant bacteria. Cr(VI) reducing bacteria were collected from an environmental soil sample and cultivated for 1 month under anaerobic condition in a medium with the following contents (g/L); NH4Cl, 0.132; K2HPO4, 0.041; MgCl2 6H2O, 0.49; CaCl2 2H2O, 0.009; KCl, 0.052; K2Cr2O7, 0.015, sodium lactate, 1.0 and Yeast extract, 0.2. After 1 month of cultivation, Cr(VI) in the medium was completely reduced to Cr(III) and several species of bacteria were evident in the culture. An electrochemical cultivation bath was constructed, consisting of a Pt electrode (anode) and a carbon electrode (cathode) separated by an ion exchange membrane. For electrochemical cultivations, 10 mL of the bacteria mixture was inoculated into 90 mL of fresh medium in the anode side in each of three cultivation baths. One hundred mL of Cr(VI) free medium was poured in each cathode. All three baths were set in an anaerobic box and the potentials described below were applied. After 10 days cultivation at 30 ℃, the cell growth for each bath was counted. In addition, shapes of the cultured bacteria were examined using atomic force microscopy (AFM). From a cyclic voltammogram for Cr in solution, reduction and oxidation of Cr was observed at 0.1 V and 1.0 V (vs. Ag/AgCl), respectively. Based on this result, the potentials of the electrochemical cultivation cells were set at 0.2V, 0.6V and 1.0V (vs. Ag/AgCl). After 10 days incubation, cell density in the 0.2 V and 0.6 V baths was about 4 x 10^7 cells/mL, that was almost the same as initial value, while, about 33% higher than the initial. Cell growth in the 1.0 V cell was over 8 x 10^7 cells/mL, about 166% higher than the initial. The potential inducing growth enhancement (1.0V) is well matched to that of Cr(VI) regeneration. Under microscopic and AFM examination, bacteria grown by electrochemical cultivation were nearly uniform in appearance, in contrast to the mixture apparent in the initial solution. Our results show clearly that growth of Cr(VI) reducing bacteria is enhanced by continuously supplying Cr(VI) ions through electrochemical cultivation.

報告者

キーワード