Taxonomic investigation of representatives of the genus Sphaerotilus: descriptions of Sphaerotilus montanus sp. nov., Sphaerotilus hippei sp. nov., Sphaerotilus natans subsp. natans subsp. nov. and Sphaerotilus natans subsp. sulfidivorans subsp. nov., and an emended description of the genus Sphaerotilus.

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.

Azospirillum thiophilum sp. nov., a diazotrophic bacterium isolated from a sulfide spring.

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.

Thiothrix caldifontis sp. nov. and Thiothrix lacustris sp. nov., gammaproteobacteria isolated from sulfide springs.

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.