Five Gram-negative, motile, spiral-shaped strains were isolated from a sulfide spring (D-412^T), active sludge of wastewater (D-419^T, D-420, D-424) and industrial wastewater (D-416). Comparative 16S rRNA gene sequence analysis showed that the isolates belong to the family Comamonadaceae, within the class Betaproteobacteria, but fall into a distinct cluster. On the basis of phenotypic, chemotaxonomic and phylogenetic data, a new genus, Giesbergeria gen. nov., is proposed, including five species. The type species of the genus is Giesbergeria voronezhensis sp. nov. (type strain D-419^T=DSM 12825^T=CIP 107340^T=VKM B-2350^T) and other novel members of the genus are Giesbergeria kuznetsovii sp. nov. (type strain D-412^T=DSM 12827^T=VKM B-2352^T), Giesbergeria giesbergeri comb. nov. (basonym Aquaspirillum giesbergeri), Giesbergeria sinuosa comb. nov. (basonym Aquaspirillum sinuosum) and Giesbergeria anulus comb. nov. (basonym Aquaspirillum anulus). Using the same criteria, isolate D-416 (=DSM 12826) was identified as a strain of [Aquaspirillum] metamorphum. Strain D-416, the type strain of [A.] metamorphum and the type strain of [Aquaspirillum] psychrophilum form a distinct cluster within the family Comamonadaceae (97–97·2 % 16S rRNA gene sequence similarity) and share phenotypic and chemotaxonomic properties. Therefore, it is proposed that these strains are reclassified as members of a new genus, Simplicispira gen. nov., as Simplicispira metamorpha comb. nov. (the type species) and Simplicispira psychrophila comb. nov., respectively.

A novel nitrogen-fixing strain, designated BV-S^T, was isolated from a sulfur bacterial mat collected from a sulfide spring of the Stavropol Krai, North Caucasus, Russia. Strain BV-S^T grew optimally at pH 7.5 and 37 °C. According to the results of phylogenetic analysis, strain BV-S^T belonged to the genus Azospirillum within the family Rhodospirillaceae of the class Alphaproteobacteria. Within the genus Azospirillum, strain BV-S^T was most closely related to Azospirillum doebereinerae GSF71^T, A. picis IMMIB TAR-3^T and A. lipoferum ATCC 29707^T (97.7, 97.7 and 97.4 % 16S rRNA gene sequence similarity, respectively). DNA–DNA relatedness between strain BV-S^T and A. doebereinerae DSM 13131^T, A. picis DSM 19922^T and A. lipoferum ATCC 29707^T was 38, 55 and 42 %, respectively. Similarities between nifH sequences of strain BV-S^T and members of the genus Azospirillum ranged from 94.5 to 96.8 %. Chemotaxonomic characteristics (quinone Q-10, major fatty acid C_18 : 1 ω7c and G+C content 67 mol%) were similar to those of members of the genus Azospirillum. In contrast to known Azospirillum species, strain BV-S^T was capable of mixotrophic growth under microaerobic conditions with simultaneous utilization of organic substrates and thiosulfate as electron donors for energy conservation. Oxidation of sulfide was accompanied by deposits of sulfur globules within the cells. Based on these observations, strain BV-S^T is considered as a representative of a novel species of the genus Azospirillum, for which the name Azospirillum thiophilum sp. nov. is proposed. The type strain is BV-S^T (=DSM 21654^T =VKM B-2513^T).

A novel strain of fermenting, aerotolerant, chemo-organoheterotrophic spirochaete designated P^Twas isolated from a sulfur ‘Thiodendron’ mat in a saline spring at the Staraya Russa resort (Novgorod Region, Russia). Cells of strain P^Texhibited a helical shape. The spirochaete required sulfide in the growth medium and was able to oxidize it non-enzymically to elemental sulfur via the interaction of H₂O₂with sulfide and deposit it in the periplasmic space. Growth occurred at 4–32 °C (optimum at 28–30 °C), pH 6.0–8.5 (optimum pH 7.0–7.5), and in 0.1–1 M NaCl (optimum 0.35 M). The isolate used several sugars and polysaccharides as carbon or energy sources but did not use peptides, amino acids, organic acids or alcohols. The products of glucose fermentation were formate, acetate, ethanol, pyruvate, CO₂and H₂. The genomic DNA G+C content was 41.7 mol%. 16S rRNA gene sequence analysis showed that strain P^Tfell within a group of species in the genusSpirochaeta, includingSpirochaeta litoralis,S. isovalericaandS. cellobiosiphila, with which it shared less then 89 % sequence similarity. On the basis of its morphology, physiology and other phenotypic properties, as well as its phylogenetic position, the new isolate is considered to represent a novel species of the genusSpirochaeta, for which the nameSpirochaeta perfilieviisp. nov. is proposed. The type strain is P^T(=DSM 19205^T=VKM B-2514^T).

Five strains of filamentous, sulfur-oxidizing bacteria were isolated from sulfur mats of different sulfide springs from various regions of the Northern Caucasus, Russia. A phylogenetic analysis based on 16S rRNA gene sequence comparison showed that all of the isolates are affiliated with the filamentous, colourless, sulfur-oxidizing bacteria of the genus Thiothrix within the Gammaproteobacteria and are closely related to Thiothrix fructosivorans. All strains are capable of growing heterotrophically, lithoautotrophically with thiosulfate or sulfide as the sole energy source and mixotrophically. Strains G1T, G2, P and K2 are able to fix molecular nitrogen, but strain BLT is not. Randomly amplified polymorphic DNA (RAPD)-PCR analysis was used to assess the level of genetic relationships among the Thiothrix isolates. The Nei and Li similarity index revealed high genetic similarity among strains G1T, G2, P and K2 (above 75 %), indicating that they are closely related. In combination with physiological and morphological data, strains G1T, G2, P and K2 can be considered as members of the same species. The lowest genetic similarity (approx. 20 %) was reached between strain BLT and the other isolated Thiothrix strains. Strains BLT and G1T shared 35 % DNA–DNA relatedness and showed 51 and 53 % relatedness, respectively, to Thiothrix fructosivorans ATCC 49749. On the basis of this polyphasic analysis, strains G1T, G2, P and K2 represent a novel species within the genus Thiothrix, for which the name Thiothrix caldifontis sp. nov. is proposed, with strain G1T (=DSM 21228T =VKM B-2520T) as the type strain. In addition, strain BLT represents a second novel species, Thiothrix lacustris sp. nov., with strain BLT (=DSM 21227T =VKM B-2521T) as the type strain.

Seven strains of the genus Sphaerotilus were obtained from natural thermal sulfide (strains D-501^T, D-502, D-504, D-505 and D-507) and low-temperature ferrous (strain HS^T) springs and from an activated sludge system (strain D-380). These Sphaerotilus isolates and strains of Sphaerotilus natans obtained from the DSMZ (S. natans DSM 6575^T, DSM 565 and DSM 566) were studied using a polyphasic taxonomic approach. All strains had Q-8 as the major quinone and C_16 : 1ω7, C_16 : 0 and C_18 : 1ω7 as the major fatty acids. The DNA–DNA hybridization results and 16S rRNA, hsp60 and gyrB gene sequencing experiments showed that isolates D-501^T, D-502, D-504, D-505, D-507 and D-380 were closely related to the type strain of S. natans DSM 6575^T. However, strains D-501^T, D-502, D-504, D-505 and D-507 significantly differed from the heterotrophic strain S. natans DSM 6575^T by their capability for lithotrophic growth with reduced sulfur compounds as an electron donor for energy conservation and some other phenotypic features. For this reason, strains D-501^T, D-502, D-504, D-505 and D-507 merit a separate taxonomic classification at the subspecies level. The name Sphaerotilus natans subsp. sulfidivorans subsp. nov. (type strain D-501^T = DSM 22545^T = VKM B-2573^T) is proposed. The subspecies Sphaerotilus natans subsp. natans subsp. nov. is automatically created as a result of this proposal. Strain D-380 was phenotypically closely related to S. natans DSM 6575^T. Strains D-380 and S. natans DSM 6575^T were assigned to the subspecies Sphaerotilus natans subsp. natans subsp. nov. (type strain DSM 6575^T = ATCC 13338^T). The 16S rRNA, hsp60 and gyrB gene sequences obtained for strains HS^T and DSM 565 showed very low sequence similarity values of 97.3 %, 89.7 % and 88.4 %, respectively, with S. natans DSM 6575^T. Strain HS^T shared 99 % DNA–DNA relatedness with strain

Representatives of the Thiothrix genus are capable of denitrification as an adaptive mechanism under stress conditions.