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Twenty stem-and root-nodulating bacterial strains isolated from stem nodules of Sesbania rostrata were compared by numerical analysis of 221 phenotypic features with nine strains which effectively nodulate only the roots of this plant and with representative strains from the genera Rhizobium and Bradyrhizobium. Representative organisms from the different clusters were investigated further, together with possibly related organisms, by performing comparative gel electrophoresis of whole-cell proteins and by performing deoxyribonucleic acid (DNA)-DNA and DNA-ribosomal ribonucleic acid (rRNA) hybridizations. 3H-labeled rRNA was prepared from Sesbunia stem-and root-nodulating bacterial strain ORS 571T (T = type strain); [14C]rRNA from Bradyrhizobium japonicum NZP 5549T was also used. The following conclusions were drawn: (i) the Sesbania root-nodulating bacterial strains are genuine rhizobia; (ii) the Sesbania stem-and root-nodulating strains are quite different from Rhizobium and Bradyrhizobium, and thus they constitute a separate rRNA subbranch on the Rhodopseudomonas palusfris rRNA branch in rRNA superfamily IV; and (iii) the closest relative of these organisms is Xanthobacfer, but they are phenotypically and genotypically sufficiently different from the latter genus to deserve a separate generic rank. Because the feature of free-living nitrogen fixation is quite discriminative, a new genus, Azorhizobium, is proposed, with one species, Azorhizobium caulinodans. The type strain is strain ORS 571 (= LMG 6465).Bacteria which form nitrogen-fixing nodules on leguminous plants are currently divided into two genera, Rhizobium and Bradyrhizobium (17). The genus Rhizobium comprises four species, Rhizobium leguminosarum, Rhizobium meliloti, Rhizobium loti, and Rhizobium fredii. The species Rhizobium leguminosarum regroups three former species, Rhizobium trifolii, Rhizobium phaseoli, and Rhizobium leguminosarum. All species of the genus Rhizobium are fastgrowing bacteria. The genus Bradyrhizobium comprises one well-defined species, Bradyrhizobium japonicum, and includes all of the bacteria previously referred to as slowgrowing rhizobia.Nitrogen-fixing nodules are usually found on the roots of the host leguminous plants. Only some legume species belonging to the genera Neptunia, Aeschynomene, and Sesbania bear nodules on both roots and stems (9). Recently, Dreyfus et al. (8) found that the tropical legume Sesbania rostrata was actually associated with the two types of strains described below. (i) Some strains are similar to strain ORS 571T (T = type strain), which fixes atmospheric N2 in culture and grows at the expense of this fixed N,; they always nodulate both roots and stems of S. rostrata (10, 13). They are usually cited in the literature as "the Sesbania stemnodulating strains." We did not find such N,-fixing strains in stem nodules from Neptunia and Aeschynomene (B. Dreyfus, unpublished data). (ii) Some strains do not fix N, in culture and generally nodulate the roots of S. rostrata only.Recent results based on deoxyr...

A new thermophilic, xylanolytic, strictly anaerobic, rod-shaped bacterium, strain SEBR 7054T, was isolated from an African oil-producing well. Based on the presence of an outer sheath (toga) and 16s rRNA sequence analysis data, this organism was identified as a member of the genus Thermotoga. Strain SEBR 7054T possessed lateral flagella, had a G+C content of 50 mol%, produced traces of ethanol ffom glucose but no lactate, and grew optimally in the presence of 0 to 0.2% NaCl at 70°C. Its phenotypic and phylogenetic characteristics clearly differed from those reported for the five previously validly described Thermotoga species. Therefore, we propose that strain SEBR 7054T is a member of a new species of the genus Thermotoga, Thermotoga hypogea sp. nov. The type strain of T. hypogea is SEBR 7054 (= DSM 11164).Members of the order Thermotogales are rod-shaped bacteria that have a characteristic outer sheathlike structure called a toga. This order includes the following five genera: Thermotoga (18,22, 23,34,44), Thermosipho (19, 37), Fewidobacterium (1, 20, 32), Geotoga, and Petrotogu (9). It represents, along with the AquiJicales, the deepest phylogenetic branch in the domain Bacteria (45). The genus Thermotoga includes all of the hyperthermophiles (optimum temperature for growth, around 80°C) of the order Thermotogales (18, 21, 22) and the thermophiles (optimum temperature for growth, 65 to 70°C) recently isolated from oil fields (23,34). Stetter et al. (39) provided evidence of the presence of Thermotoga strains in oil fields. The isolation of Thennotoga elfii and Thermotoga subterranea from such ecosystems was reported soon thereafter (23,34). In contrast to Thermotoga maritima and i%ermotoga neapolitana, T. elfii and T. subterranea were not able to grow at temperatures above 75°C and reduced thiosulfate but not sulfur, which led to speculation that thiosulfate rather than sulfur may be an important electron acceptor in oil field ecosystems (34). The presence of thiosulfate is also thought to increase biocorrosion of oil field installations (8,27). We therefore initiated intensive studies to isolate Thermotoga strains from oil field subsurface ecosystems that grow at temperatures above 80°C and use thiosulfate as an electron acceptor. Our studies have focused on xylanolytic extremophilic microorganisms since thermostable xylanases have a potential use in paper primary-pulp manufacturing (33). In this paper, we describe the first isolation from oil field water of a xylanolytic Thermotoga species able to grow at temperatures up to 90°C. The phenotypic and phylogenetic characteristics of the new strain are consistent with its placement in a new species of the genus Thermotoga, Thermotoga hypogea sp. nov. MATERIALS AND METHODSSample collection and sample source. Strain SEBR 7054= was isolated from an oil-producing well in Cameroon in central Africa. The in situ temperature was 66"C, and the concentration of sodium chloride was 12 glliter. A 1-liter sample was collected at the wellhead as described elsewhere (3), t...

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