Gemmata obscuriglobus

Gemmata obscuriglobus is a Gram-negative, aerobic, heterotrophic, motile bacterium of the phylum Planctomycetes, first described in 1984 as the only described species in the genus Gemmata. It is exceptional for its unusual morphology and for the unusual features in its genome, often considered to represent large differences in internal organization compared with most prokaryotes. G. obscuriglobus has been described as 'the platypus of microbiology'. G. obscuriglobus is a freshwater bacterium, originally described on the basis of a single strain isolated from the littoral region near the Maroon Dam in Queensland, Australia. G. obscuriglobus is a large, roughly spherical bacterium with a cell diameter of 1–2μm. It is motile and possesses multiple flagella per cell (that is, it is multitrichous). Dense, compact DNA and a deeply invaginated membrane are characteristics of the species. Among the most notable features of G. obscuriglobus is its highly complex and morphologically distinctive cell membrane system, including deep invaginations of its membrane that historically have been described as closed internal membranes that may surround the bacterium's DNA by analogy to a eukaryotic cell nucleus. The concept of membrane-bound genetic material has been described as a 'cell plan' unique to a proposed superphylum composed of the Planctomycetes, Verrucomicrobia, and Chlamydiae (PVC) and distinct from the rest of the Gram-negative bacteria. The question of whether G. obscuriglobus and other members of the PVC group possess closed internal membranes and therefore have a unique 'cell plan' is considered important in understanding the evolution of membrane-bound compartments, which are often considered a distinguishing feature between eukaryotes and prokaryotes; the lineage that gave rise to the PVC superphylum is speculated to be related to an 'intermediate' state between prokaryotes and eukaryotes. The question of how the PVC superfamily membranes are organized and how they relate to eukaryotes is an active and controversial area of research. Three-dimensional tomogram reconstructions of whole cells reported in 2013 suggest that contrary to historical belief, G. obscuriglobus membranes are continuous and do not enclose distinct cellular compartments. However, this study has been criticized for not detecting or modeling some commonly reported structural features, and a 2014 study using similar methodology was interpreted as supporting the earlier hypothesis of closed internal compartments. Early characterization of G. obscuriglobus found that it lacked a traditional Gram-negative peptidoglycan (PG) cell wall and instead possessed a proteinaceous exterior layer, later described as possibly analogous to an archaeal S-layer. More recent reports have found the exterior to more closely chemically resemble typical Gram-negative features. Compositional analysis of the membrane has been reported to find lipopolysaccharide in G. obscuriglobus, consistent with typical features of Gram-negative outer membranes. A 2015 study of several planctomycetes including G. obscuriglobus identified the presence of a PG cell wall following the typical Gram-negative structure by both biochemical and bioinformatic analysis. One or more nucleoid-like regions of densely compact DNA is commonly observed in G. obscuriglobus cells. Complex internal structure resembling a liquid crystal has been reported, with some structural similarities to the chromatin of eukaryotes such as dinoflagellates. The structure of the nucleoid has been implicated in the unusual radiation tolerance of G. obscuriglobus. Transcription and translation of genes have been reported to occur in spatially segregated locations within the cell, which is otherwise characteristic of eukaryotic but not prokaryotic cells.