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 pm. 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 H2-02-CO2 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 1 8 : O acid and a straight-chain unsaturated CzOT1 acid with one double bond. C16:o and c 1 8 : 1 fatty acids and a C21:0 cyclopropane acid were minor components. Cytochromes b and c 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 Hydrogenobacter thermophilus 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.Five species of aerobic , thermophilic , hydrogen-oxidizing bacteria have been described previously. These are Hydrogenomonas thermophilus (21), Pseudomonas thermophila K-2 (13), Pseudomonas hydrogenothermophila (14), Flavobacterium autothermophilum (14), and Bacillus schlegelii (2, 24). These organisms are all aerobic, facultative chemolithotrophs. Methanobacteriirm thermoautotrophicum (29) is thermophilic, obligately chemolithoautotrophic, and hydrogen oxidizing but is strictly anaerobic and methane producing. The aerobic, hydrogen-oxidizing bacteria (both mesophiles and thermophiles) are all facultative lithoautotrophs that are capable of growing heterotrophically on various kinds of organic compounds. Therefore, the hydrogen-oxidizing bacteria are assumed to form a physiological group containing no obligate chemolithoautotrophs, in contrast to other chemolithoautotrophic bacterial groups.In this paper we describe the phenotypic and chemotaxonomic characteristics of six newly isolated strains of aerobic, thermophilic, hydrogen-oxidizing bacteria which are obligate chemolithoautotrophs. MATERIALS AND METHODSIsolation of bacteria. The isolation procedure which we used has been described previously (18). Water and soil samples were collected from hot springs in Izu and Kyushu, Japan.Bacterial strains isolated. Strains TK-6r (T = type strain), TK-38, TK-120, TK-F, TK-G, and TK-H were isolated and investigated in this study.Culture medium and cultivation of bacteria. The basal medium had the same composition and pH (7.2) as the isolation medium described previously (18), except t...