Custom-made medium approach for effective enrichment and isolation of chemolithotrophic iron-oxidizing bacteria

内島 智貴*; 加藤 真悟*; 谷本 和也*; 白石 史人*; 濱村 奈津子*; 徳永 紘平   ; 牧田 寛子*; Kondo, Momoko*; 大熊 盛也*; 光延 聖*

Uchijima, Tomoki*; Kato, Shingo*; Tanimoto, Kazuya*; Shiraishi, Fumito*; Hamamura, Natsuko*; Tokunaga, Kohei; Makita, Hiroko*; Kondo, Momoko*; Okuma, Moriya*; Mitsunobu, Satoshi*

鉄をエネルギー源として生きる鉄酸化細菌は、地下水や温泉、湿地など自然界のさまざまな場所に広く分布し、鉄の酸化反応を通じて地球規模の鉄循環に寄与している。さらに、彼らが作り出す鉄酸化物は、ヒ素や銅などの有害元素を強く吸着・固定する能力を持つことから、環境浄化にも応用されている。しかし、このように重要な役割を担っているにもかかわらず、多くの鉄酸化細菌は実験室で純粋培養できず、非常に培養が難しい微生物として知られてきた。実際、陸上環境の独立栄養性鉄酸化細菌の純粋培養報告は世界で8例程度にとどまっており、その生態解明や工学応用は進んでいない。本研究では、これまで純粋培養が非常に難しかった鉄酸化細菌の培養効率を大幅に向上する方法「オーダーメイド培地法」を確立した。本成果は、培養困難だった鉄酸化細菌の生理・生態の解明に向けた重要な一歩となる。また、鉄酸化細菌は地下水や河川水などに含まれる有害物質の浄化に利用される有用微生物であり、本成果は微生物利用型の環境修復(バイオレメディエーション)技術への応用が期待される成果である。

Chemolithotrophic neutrophilic iron (Fe)-oxidizing bacteria, which mainly belong to the family Gallionellaceae, universally prevail in terrestrial environments changing Fe cycling. However, they are typically recognized as difficult-to-culture microbes. Despite efforts, there are few Fe(II)-oxidizing lithotroph isolates; hence, their physiological and ecological knowledge remains limited. This limitation is largely owing to difficulties in their cultivation, and we hypothesize that the difficulty exists because substrate and mineral concentrations in the cultivation medium are not tuned to a specific environmental condition under which these organisms live. To address this hypothesis, this study proposes a novel custom-made medium approach for chemolithotrophic Fe(II)-oxidizing bacteria; a method that manipulates medium components through diligent analysis of field environment. A new custom-made medium simulating energy substrates and nutrients under the field condition was prepared by modifying both chemical composition and physical setup in the glass-tube medium. In particular, the modification of the physical setup in the tube had a significant effect on adjusting dissolved Fe(II) and O concentrations to the field environment. Using the medium, Gallionellaceae members were successfully enriched and a new Gallionellaceae species was isolated from a natural hot spring site. Compared with conventional medium, the custom-made medium has significantly higher ability in enriching Gallionellaceae members.