Three-dimensional reconstruction of bacteria with a complex endomembrane system

Abstract

The division of cellular space into functionally distinct membrane-defined compartments has been one of the major transitions in the history of life. Such compartmentalization has been claimed to occur in members of the Planctomycetes, Verrucomicrobiae, and Chlamydiae bacterial superphylum. Here we have investigated the three-dimensional organization of the complex endomembrane system in the planctomycete bacteria Gemmata obscuriglobus. We reveal that the G. obscuriglobus cells are neither compartmentalized nor nucleated as none of the spaces created by the membrane invaginations are closed; instead, they are all interconnected. Thus, the membrane organization of G. obscuriglobus, and most likely all PVC members, is not different from, but an extension of, the "classical" Gram-negative bacterial membrane system. Our results have implications for our definition and understanding of bacterial cell organization, the genesis of complex structure, and the origin of the eukaryotic endomembrane system.

Figure 1. 3D reconstruction of bacteria with a complex endomembrane system.

(A) One slice of the tomogram is represented with the different structural features modeled. The OM is in green, the IM in cyan, the DNA in yellow, OM invaginations are in pink, and the Poly-P granule in dark blue. Scale bar is 500 nm. (B) Schematic of the cellular organization of G. obscuriglobus, not to scale. The periplasmic and cytoplasmic spaces are in dark and pale grey, respectively. Other colors are the same as in (A). (C and E) The modeled volume of one cell, sliced roughly through the middle, is represented in different orientations. Two views through the full volume are represented. (D) A slice through the same orientation as in panel (C) might give the false impression of a compartment surrounding the DNA. (F) Full volume representation without the membranes showing the five fragments of DNA, the crateriform structures, and the granule.

Figure 2. Membrane continuity between mother and daughter cells.

Electron micrograph of the neck of the bud. Mother cell (left) and daughter cell (right). Electron dense material is present in the periplasm around the neck. Outer- (OM), inner-membrane (IM), cytoplasm (C) periplasm (P), and electron dense material (EDM) are indicated by arrows. Scale bar is 100 nm.

Figure 3. Membrane organization around the neck of the bud.

(A) 3D model displaying membrane organization in proximity to the budding neck. All features are represented and color-coded as in Figure 1. The neck can be observed linking the mother cell (below) to the bud (above). (B) Only the IMs are represented in this image. The more complex organization of the IM can be observed in the mother cell; the single membrane sheet can be observed in the bud. Scale bar is 300 nm.

Figure 4. Crateriform structures.

(A) Crateriform structures seen from the outside of the cell, indicated by arrows. (B–E) Micrographs of crateriform structures seen from the side, perpendicular to the membrane, indicated by arrows. Outer- (OM), inner-membrane (IM), cytoplasm (C), and periplasm (P) are indicated. Scale bars are 50 nm.