S U M M A R YTwo strains of heterotrophic, non-motile, Gram-negative extreme thermophiles have been isolated from hot tap water. These strains (NH and DI) have been characterized and compared with strains of the genus Thermus. Few of the single organic compounds tested supported growth in the presence of ammonium salts, and, like Thermus strains, growth on undefined media was restricted to dilute tryptone-yeast extract-mineral salt solutions. Nutrient agar and similar common laboratory media did not support growth. The growth rate was similar to that of Thermus strains, as was the unusual pattern of antibiotic resistance. Mean base composition of NH was 61.4 guanine plus cytosine (G+C), and of DI was 62.0 to 62-27& G + C, which are several per cent lower than the G + C contents of other strains so far described.

375 ~ ~~The DNA of I 3 yellow-pigmented strains of the genus Thermus, isolated from hot springs in Iceland, had a mean base composition of 60.5 to 65.1 mol % GC. In their growth characteristics and sensitivity to antibiotics, these strains resembled Thermus aquaticus strain YTI. Glutamate, acetate and sucrose, with ammonium ions as a nitrogen source, supported growth. The enzymes of the tricarboxylic acid cycle were present and the glyoxylate cycle was constitutive. The stability of malate dehydrogenase in crude extracts was similar to that of strain Y T I . I N T R O D U C T I O NThe description of a new type of extremely thermophilic, aerobic, Gram-negative rod isolated from a hot spring in Yellowstone National Park by Brock & Freeze (1969) and called Thermus aquaticus has given new impetus to the study of thermophiles, and provided new material for the isolation of thermostable enzymes and for the study of thermostable macromolecules and membranes. Bacteria of this type have been isolated world-wide from neutral or moderately alkaline water between about 50 and 90 "C (Brock ' Egorova & Loginova, 1975;Pask-Hughes & Williams, 1975). A strain with similar physiological properties to Thermus isolates, but with different pigmentation and morphology and a distinctly alkaline pH optimum, has been described as a representative of a new genus, Thermomicrobium (Jackson, Ramaley & Meinschein, I 973).All isolated strains of Thermus grow poorly with single organic compounds as sole carbon source in the presence of mineral salts and ammonium sulphate as a nitrogen source. This paper describes the isolation and properties of a number of yellow-pigmented Thermus strains from hot springs in south-western Iceland, which were studied as part of an investigation of the range of properties of Thermus (Pask-Hughes, 1976) suitable for classification purposes, and to select strains suitable for investigating the problems of thermostability of macromolecules and of thermophily.

Envelopes of Thermus strains had a multilayered Gram-negative appearance. Peptidoglycan contained significant amounts of glycine, and ornithine was the dibasic amino acid. Lipopolysaccharide (LPS) did not contain heptose and 2-keto-3-deoxyoctonate was not positively identified. The polysaccharide from LPS contained glucose, glucosamine, galactose, galactosamine and, in two strains, mannose. The fatty acids identified in lipopolysaccharide were C14, C16 and CIS, both saturated and unsaturated.