Expanded phylogeny of myxobacteria and evidence for cultivation of the 'unculturables'

An expanded neighbour-joining tree of myxobacteria is presented based on the analysis of 16S rRNA gene sequences of 101 strains (including types) representing 3 suborders, 6 families, 20 genera, 46 species, and 12 other novel taxa. The distinctions amongst members of the three suborders (Sorangiineae, Cytobacterineae and Nannocystineae) are reaffirmed. The positions of anaerobic myxobacteria, novel groups (Pyxidicoccus and several Cystobacter species) in Cystobacterineae, the marine genera (Plesiocystis, Haliangium, Enhygromyxa), and two additional novel taxa ('Paraliomyxa miuraensis', brackish-water isolate) were together revealed for the first time. Changes in the nomenclature of several isolates (Polyangium vitellinum Pl vt1(T), Polyangium thaxteri Pl t3, Polyangium cellulosum, NOSO-1, NOCB-2, NOCB-4) are also highlighted. Suborders Sorangiineae and Nannocystineae hold great promise for novel strain discovery. In Sorangiineae, the new family Phaselicystidaceae, with a monotypic genus, was added. Nine additional novel taxa were discovered in this suborder for which new genera or even families may be erected in the near future. These taxa appear to represent the so-called viable but not culturable (VBNC) group of myxobacteria. Based on at least 4% phylogenetic distance, new clades were formed comprising of novel Nannocystineae and Sorangiineae isolates. Overall, the myxobacteria, on the basis of bracket distance, could be divided into 16 clusters, as supported by tree topology and a morphology-based approach.