There is no indication of a single membrane-bounded organelle not containing a nucleoid such as the anammoxosome of anaerobic ammonium-oxidizing bacteria, a group thought to represent some of the most deep-branching Planctomycetes or even a separate phylum-level lineage within the PVC superphylum [21, 22] and which share a cell plan including the pirellulosome with planctomycetes [23–25]. However, the small membrane-bounded regions

of ribosome-containing pirellulosome cytoplasm within paryphoplasm in V. spinosum resemble features of a pirellula-like planctomycete cultured from a Mediterranean sponge [26]. The cell plan determined in verrucomicrobia was revealed Smad family using a cryosubstitution method for preparation of cells before thin-sectioning for electron microscopy, a method comparable to those used previously for establishing the planctomycete cell plan [18, 27]. Cells of all

the species www.selleckchem.com/products/LDE225(NVP-LDE225).html of verrucomicrobia examined here using high-pressure freezing followed by cryosubstitution also possess condensed nucleoids, which is another feature of similarity to the ultrastructure of planctomycetes. All planctomycetes appear to possess condensed nucleoids when cryofixed cryosubstituted cells are examined [18]. Cryosubstitution, unlike conventional chemical fixation, is not expected to yield such condensation as an artifact of fixation [28–30]. This contrasts with the appearance of nucleoids in cryofixed cells of other bacterial species such as Escherichia coli and Bacillus subtilis, where a ‘coralline’ nucleoid extending through the cell cytoplasm is found [28, 29]. Chromatin-like nucleoids have been reported in “”Candidatus Xiphinematobacter”", symbionts of nematodes belonging subdivision 2 of Verrucomicrobia [4], and also in epixenosome symbionts belonging to subdivision 4 [31], although in both cases these were examined only using chemical fixation. The condensed nucleoids of all the species examined here often contained granules of

varying electron density. Such granules within nucleoids have been noted to occur within cryo-fixed cells of Deinococcus radiodurans vitreous sections examined by cryoelectron ADP ribosylation factor microscopy [32]. V. spinosum and P. dejongeii are members of subdivision 1 (class Verrucomicrobiae) of the phylum Verrucomicrobia [1]. There is another member of the phylum Verrucomicrobia, Rubritalea squalenifaciens, isolated from the marine sponge Halichondria okadai and belonging to subdivision 1 Verrucomicrobia, which seems to possess the planctomycete-like cell plan in an accompanying published figure, but this interpretation was not made by the authors [33]. The planctomycete cell plan has also been observed in symbiont bacteria studied directly in sponge tissue [34]. Some of those from the sponge Haliclona caerulea include cells with multiple prosthecae and in which both ICM and riboplasm were recognized [35].