報告書番号

U99012

タイトル（和文）

電気による微生物の制御（その４） -‐性質の異なる鉄酸化細菌に対する電気培養適応性の評価‐-

タイトル（英文）

Electrochemical control of microorganisms 4.-Evaluation of applicability of electrochemical cultivation for various bacterial strains with iron oxidizing capability-

概要 （図表や脚注は「報告書全文」に掲載しております）

当所では電気エネルギーによる有用物質生産系の構築を目指し、独立栄養微生物を電気化学的手法で高密度培養する電気培養を実施してきた。これまで１種類の菌株でのみ検討されてきたが、今回新たに3種類の鉄酸化細菌について、電気培養による増殖の可能性を評価した。鉄酸化活性を有する３種の菌株について、従来法に則した電気培養を実施した結果、菌体の死滅が確認された。この理由として、電気培養の際に電極で生じる反応が、直接、あるいは間接的に微生物に対して何らかのダメージ与えていることが考えられた。そこでより温和な電解条件として、電解電位を酸化側に移行し、還元副反応を起きにくくするとともに、必要に応じて電解をおこなう間欠式の電解を採用した。その結果、3株中2株で電気培養による増殖促進効果が確認された。電気培養が可能であった株について、投入電荷量に対する増殖菌数の割合を調べた結果、ほぼ同じ値を示した。

概要 （英文）

Electrochemical cultivation of microorganism realizes high density cell cultivation within a short term by continuous supply of living energy using electrochemical method. Using this method, we are planning to create the production system for useful organic compounds made by microorganism using electric power. High density cultivations have been achieved so far by the electrochemical cultivation using Thiobacillus ferrooxidans, which is one of the iron oxidizing bacteria. However, the other bacteria had not been checked their ability of electrochemical growing. It is important to seek electrochemically growable bacteria from the point that a creation of useful organic materials from microorganism. In this report, the applicability of electrochemical cultivation was checked using different strains of iron oxidizing bacteria. Three kinds of bacteria were used in this study (Leptospirillum ferrooxidans, T-23 and CF-27). Each strain has iron oxidizing activity. Every electrochemical cultivation was carried out using electro-bath. The electrochemical reduction of Fe(III) to Fe(II) was performed under potential controlled condition. Through the experiment, time cource of cell density, reduction current and Fe(II) concentration were measured.At the first trial, each three strain was cultivated electrochemically under the same condition that of T.ferrooxidans. However, no growth was observed. This indicated the existence of some inhibiting factor among the electrolysis. We proposed two cases of possibility; one was a direct interaction between the surface of cathode and bacterium, and another was an affection via some inhibitors generated by sub reactions of electrodes. In our system, it is thought hydrogen peroxide generated from cathode, and oxygen generated from anode and path through the membrane were possible to inhibit to the cell growth. Then we arranged a moderate condition of electrolysis for these strains, namely, reduction potential was shifted 100mV to the anodic direction, and intermittent electrolysis was carried out at the first term of their cultivation. Under these treatments, two strains out of three (L.ferrooxidans and CF-27) could be grown electrochemically and these cell densities reached 40 to 100-fold of concentrations compared to those under batch cultivation. On the other hand, however, strain T-23 couldn't be grown electrochemically. It suggested the iron oxidizing ability of T-23 did not related to its energy acquisition and the strain would gain its living energy from organic materials included in medium. In comparison of energy efficiency among three electrochemically growable strains contained with T.ferrooxidans, a number of cells per unit charge for every strain showed almost the same value. The result suggested that since these three strain utilize same electron donor and acceptor, namely, Fe(II) and oxygen, they would gain same chemical energy for their growth.

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