Six strains of strictly thermophilic, obligately chemolithotrophic, hydrogen-oxidizing bacteria were isolated from hot springs located in Izu and Kyushu, Japan. The bacterial strains which we tested were gram negative, nonmotile, nonsporeforming, long, straight, and rod shaped. The cell size was 0.3 to 0.5 by 2.0 to 3.0 μm. The deoxyribonucleic acid guanine-plus-cytosine contents of the six strains were between 43.5 and 43.9 mol%. The optimal temperature for autotrophic growth on H-O-CO was between 70 and 75°C. None of the strains grew at 37 or 80°C. The neutral pH range was suitable for growth. No strain showed heterotrophic growth at the expense of 48 organic compounds or on complex media, in contrast to all other known aerobic hydrogen-oxidizing bacteria which are facultative autotrophs. The major cellular fatty acids were a straight-chain saturated C acid and a straight-chain unsaturated C acid with one double bond. C and C fatty acids and a C cyclopropane acid were minor components. Cytochromes and were found in all of the strains. The polyacrylamide gel electrophoretic patterns of the total soluble proteins of all of the strains were essentially the same. The name gen. nov., sp. nov., is proposed for the six new isolates, and type strain TK-6 has been deposited with the culture collection of the Institute of Applied Microbiology, University of Tokyo, as strain IAM 12695.


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