The properties of an extremely thermophilic bacterium isolated from water at a Japanese hot spring and previously named are described. The cells are gram-negative, nonsporulating, aerobic rods containing yellow pigment. The optimum temperature for growth is between 65 and 72 C, the maximum being 85 C and the minimum being 47 C. The guanine plus cytosine content of the deoxyribonucleic acid of the thermophile is 69 mol %. This microorganism is sensitive to various antibiotics including those which are known to be rather ineffective against gram-negative bacteria. Spheroplast-like bodies are formed upon treating intact cells with egg-white lysozyme at 60 C. The spheres are osmotically more stable than mesophile protoplasts, and their rupture under hypotonic conditions is not complete unless 0.5% Brij 58 is added to the suspension. Bulk protein extracted from this thermophile is much more stable to heat than mesophile proteins, and only about 10% of the total protein is denatured by heating at 110 C for 5 min. Nevertheless, the amino acid composition of the bulk protein is similar to that of mesophile proteins. As the properties of this organism are similar to those of (Brock and Freeze) and inasmuch as is a poorly defined genus, this thermophilic microorganism is transferred to the genus as (Yoshida and Oshima) comb. nov. The type strain is HB8 (=ATCC 27634).


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