Review of the genus Methylobacterium and closely related organisms: a proposal that some Methylobacterium species be reclassified into a new genus, Methylorubrum gen. nov.

The genus Methylobacterium, when first proposed by Patt et al. in 1976, was a monospecific genus created to accommodate a single pink pigmented facultatively methylotrophic bacterium. The genus now has over 50 validly published species, however, the percentage 16S rRNA sequence divergence within Methylobacterium questions whether or not they can still be accommodated within one genus. Additionally, several strains are described as belonging to Methylobacterium, but nodulate legumes and in some cases are unable to utilize methanol as a sole carbon source. This study reviews and discusses the current taxonomic status of Methylobacterium. Based on 16S rRNA gene, multi-locus sequence analysis, genomic and phenotypic data, the 52 Methylobacterium species can no longer be retained in one genus. Consequently, a new genus, Methylorubrum gen. nov., is proposed to accommodate 11 species previously held in Methylobacterium. The reclassified species names are proposed as: Methylorubrum aminovorans comb. nov. (type strain TH-15T=NCIMB 13343T=DSM 8832T), Methylorubrum extorquens comb. nov. (type strain NCIMB 9399T=DSM 1337T), Methylorubrum podarium comb. nov. (type strain FM4T=NCIMB 14856T=DSM 15083T), Methylorubrum populi comb. nov. (type strain BJ001T=NCIMB 13946T=ATCC BAA-705T), Methylorubrum pseudosasae comb. nov. (type strain BL44T=ICMP 17622T=NBRC 105205T), Methylorubrum rhodesianum comb. nov. (type strain NCIMB 12249T=DSM 5687T), Methylorubrum rhodinum comb. nov. (type strain NCIMB 9421T=DSM 2163T), Methylorubrum salsuginis comb. nov. (type strain MRT=NCIMB 14847T=NCCB 100140T), Methylorubrum suomiense comb. nov. (type strain F20T=NCIMB 13778T=DSM 14458T), Methylorubrum thiocyanatum comb. nov. (type strain ALL/SCN-PT=NCIMB 13651T=DSM 11490T) and Methylorubrum zatmanii comb. nov. (type strain NCIMB 12243T=DSM 5688T). The taxonomic position of several remaining species is also discussed.

Phylogenetic assessment of culture collection strains of Thiobacillus thioparus, and definitive 16S rRNA gene sequences for T. thioparus, T. denitrificans, and Halothiobacillus neapolitanus.

The 16S rRNA gene sequences of 12 strains of Thiobacillus thioparus held by different culture collections have been compared. A definitive sequence for the reference type strain (Starkey; ATCC 8158(T)) was obtained. The sequences for four examples of the Starkey type strain were essentially identical, confirming their sustained identity after passage through different laboratories. One strain (NCIMB 8454) was reassigned as a strain of Halothiobacillus neapolitanus, and a second (NCIMB 8349) was a species of Thermithiobacillus. These two strains have been renamed in their catalog by the National Collection of Industrial and Marine Bacteria. The 16S rRNA gene sequence of the type strain of Halothiobacillus neapolitanus (NCIMB 8539(T)) was determined and used to confirm the identity of other culture collection strains of this species. The reference sequences for the type strains of Thiobacillus thioparus and Halothiobacillus neapolitanus have been added to the online List of Prokaryotic Names with Standing in Nomenclature. Comparison of the 16S rRNA gene sequences available for strains of Thiobacillus denitrificans indicated that the sequence for the type strain (NCIMB 9548(T)) should always be used as the reference sequence for new and existing isolates.