Two strains of curved bacteria, 7-1(T) and 7-2(T), isolated from well water, were phylogenetically examined to determine their taxonomic position. Strain 7-1(T) is a Gram-negative, slightly curved rod. Analysis of the 16S rRNA gene sequence showed that strain 7-1(T) formed a cluster with [Aquaspirillum] delicatum and [Pseudomonas] lanceolata. It has some similar characteristics to [A.] delicatum and [P.] lanceolata, but has sufficient distance to separate it from other genera. DNA-DNA hybridization analysis, as well as chemotaxonomic and morphological studies, demonstrated that strain 7-1(T), [A.] delicatum and [P.] lanceolata belong to a new genus, Curvibacter gen. nov. Strain 7-1(T) (=IAM 15033(T)=ATCC BAA-807(T)) is classified as the type strain of Curvibacter gracilis gen. nov., sp. nov., and [A.] delicatum and [P.] lanceolata are classified as Curvibacter delicatus comb. nov. and Curvibacter lanceolatus comb. nov., respectively. Strain 7-2(T) is a Gram-negative spirillum. Phylogenetic study based on the 16S rRNA gene sequences showed that it formed a cluster with the members of the genus Herbaspirillum, [Pseudomonas] huttiensis and [Aquaspirillum] autotrophicum. The classification is therefore proposed of strain 7-2(T) (=IAM 15032(T)=ATCC BAA-806(T)) as the type strain of Herbaspirillum putei sp. nov., and [P.] huttiensis and [A.] autotrophicum are transferred to the genus Herbaspirillum as Herbaspirillum huttiense comb. nov. and Herbaspirillum autotrophicum comb. nov., respectively.

Phylogenetic analysis of 15 species of the genus Aquaspirillum based on 16S rRNA gene (rDNA) sequences indicated that the genus Aquaspirillum is phylogenetically heterogeneous and the species could be divided into four groups as follows: Aquaspirillum serpens, the type species of this genus, A. dispar and A. putridiconchylium are situated in the family Neisseriaceae; members of the second group, A. gracile, A. delicatum, A. anulus, A. giesbergeri, A. sinuosum, A. metamorphum and A. psychrophilum, are included in the family Comamonadaceae; the two members of the third group, A. arcticum and A. autotrophicum, are included in the family Oxalobacteriaceae; and members of the fourth group, A. polymorphum, A. peregrinum, and A. itersonii, are included in the alpha-subdivision of Proteobacteria. Thus, phylogenetic studies indicated that all the species excepting A. serpens, the type species, should be transferred to distinct genera.