PHYLOGENY OF PROSTHECOBACTER, THE FUSIFORM CAULOBACTERS - MEMBERS OF A RECENTLY DISCOVERED DIVISION OF THE BACTERIA

Prosthecobacter fusiformis is morphologically similar to caulobacters; however, it lacks a dimorphic life cycle. To determine the relatednes s of the genus Prosthecobacter to dimorphic caulobacters and other pro sthecate members of the alpha subgroup of the Proteobacteria (alpha-Pr oteobacteria), we isolated and sequenced 16S rRNA genes from four Pros thecobacter strains. Surprisingly, the results of phylogenetic analyse s placed the fusiform caulobacters in a deeply rooted division of the Bacteria that was most closely affiliated with the Planctomyces-Chlarn ydia group and only distantly related to the alpha-Proteobacteria . Th e genus Prosthecobacter shares a common lineage in this division with Verrucomicrobium spinosum, a polyprosthecate, heterotrophic bacterium, Consistent with this phylogenetic placement, menaquinones were isolat ed from Prosthecobacter strains and menaquinones have been isolated fr om Verrucomicrobium strains and planctomycetes but not from members of the alpha-Proteobacteria. Thus, the genus Prosthecobacter is a second genus in the recently described order Verrucomicrobiales. Members of the genus Prosthecobacter are susceptible to beta-lactam antibiotics a nd contain meso-diaminopimelic acid, indicating that they, unlike memb ers of the Planctomycetales or Chlamydiales, have peptidoglycan cell w alls. This major phenotypic difference, together with the phylogenetic independence of the verrucomicrobia, indicates that these bacteria an d the sources of related 16S ribosomal DNAs obtained from soils, fresh ,vater, and the marine pelagic environment represent an unrecognized d ivision of the Bacteria.