Minimum Size for Life

Quest: what is the minimum size for life? And by cryogenic electron microscopy (cryo-TEM), what are some of the space optimization strategies used by microorganisms at the size limit for life?
By cryo-EM we found organisms about at least 100 to 300 times smaller than the common bacterium E. Coli. These nanobacteria have an average volume of 0.009 cubic microns cube, against 1-3 microns cube for E. Coli.

Images from cryo-electron tomography 3D reconstructions revealing structure morphological features of an ultra-small bacterium. 3D reconstructions reveals a very dense cytoplasmic compartment, tightly packed DNA, a conspicuous, complex cell wall enveloped by a periodic surface layer (S-layer, yellow) and pili-like structures associated with the cell surface. The high contrast sub-cellular bodies located at cell ends are putative ribosomes.

Cryo-TEM images (2D) documenting the size and some morphological features of ultra-small bacteria. The cell wall has a novel architecture, with a remarkable and distinct surface layer. Two types of pili are discernible: numerous radiating pili cover the surface of the cell in a) whereas polar pili occur on cell b), apparently connecting it to an adjacent bacterium (only top surface shown). c) A dividing ultra-small bacterium in contact with a Spirocheate cell (only small region shown). Note the contrast at the interface, suggesting cell-to-cell interaction. Three bacteriophages are associated with the surface of the cell in (d). Scale bars are all 100 nm.
Cryo-electron tomography (cryo-ET) 3D reconstructions of nanobacteria All panels are one voxel-thick slices from 3D reconstruction, revealing a very dense cytoplasmic compartment and a conspicuous, complex cell wall enveloped by a periodic surface layer. The high contrast sub-cellular bodies in left, top and bottom panels are putative ribosomes. The 3D slice shown in the top right panel contains a feature of periodic, parallel line densities spaced by 5.6 nm consistent with a tightly packed genome (pixel size 0.56 nm). Bottom right small panels show the appendages covering the cell surface and the hexagonal pattern of the surface layer. See prior and next images for scales.
Cell surface analyzes: sub-volume selection for averaging. a) A slice from a 3D reconstruction of a cell. The cartoon in light blue indicates a plane slicing the surface of the cell with a region of S-layer in-plane. b) A small region of the S-layer sliced by a plane such as in a), enabling us to visualize a hexagonal pattern distribution of repeating units (dark densities). b) A spherical shell containing large areas of the S-layer covering this quite spherical cell in plane. The regular, ordered distribution of S-layer repeating units is well appreciated. These types of manipulations with reconstructed volumes enabled the selection of S-layer units, placing model spheres centered on each unit throughout the surface as shown in d).
Tomographic sub-volumetric averaged structure of the S-layer repeating unit. a) and b) Top and bottom views of the symmetrized repeating unit of the S-layer obtained by subtomographic averaging; these are isosurface renderings obtained choosing a relatively low value of the histrogram, which means we are seeing a relatively less enclosed mass. c) and d) Top and bottom views of the same reconstruction rendered using higher histogram values, representing a larger enclosed mass than in a) and b). Top views show the surface exposed to the exterior of the cell. The bottom views show the side of the subunit attached to the cell wall. Connectors to anchor the structure to the cell wall can be clearly identified. These macromolecular components offer labeling targets for identification.
Structural features of nanobacteria. Often ultrastructural features were found that have novel, unique appearance to our knowledge. a) Putative ribosomes such as pointed by yellow arrow. Funnel-like extra-cellular structure apparently coupled to an arc of high density just inside the IM, blue arrow. b) Short, often “kinked” pili such as the one pointed at by the blue arrow. c) Several putative ribosomes such as the density pointed at by the yellow arrow; long tubular density seemingly crossing the cytoplasm from the center of the cell towards the cell wall. d) A “braided” density element crossing the cell wall.

Movies

Computational shell expanding /collapsing
and slicing the S-layer in-plane.(MPEG)