For a detailed pheno- and genotypic characterization of the genus , the following seven representative strains were analyzed: Three extremely thermophilic strains (optimal temperature for growth, 65 to 75°C), DSM 625, “” DSM 674, and “” DSM 579; and four moderately thermophilic strains (optimal temperature for growth 55 to 60°C), DSM 1279 and three new isolates (strains H1, H2, and H3) from sewage. All of these strains exhibited isobranched pentadecanoic and heptadecanoic acids as principal components of their fatty acids, possessed unsaturated menaquinones with eight isoprene units, and had deoxyribonucleic acid guanine-plus-cytosine contents of 59 to 65.5 mol%, and their cell walls contained a murein structure of the A3β variation (interpeptide bridge, Gly). The clustering of the organisms into extremely and moderately thermophilic strains correlated well with molecular properties, such as the absorption spectra of their pigments and the ratio of pentadecanoic acid to heptadecanoic acid. This grouping was confirmed by the results of 16S ribosomal ribonucleic acid cataloging and deoxyribonucleic acid-deoxyribonucleic acid hybridization. Our data confirm current proposals for classification of the genus into the following two distinct species: for the extremely thermophilic strains and for the moderately thermophilic strains. Phylogenetically, representatives of the genus show a remote but significant relationship to species (similarity coefficients, 0.22 to 0.29); members of these two genera share a common peptidoglycan type.


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