Using cryo-electron microscopy and cryo-electron tomography to visualize attack phase Bdellovibrio bacteriovorus, we have discovered that while B. bacteriovorus displays internal architectural elements that are very similar to other gram-negative bacteria, they can undergo rapid, dramatic changes in shape, as demonstrated by molding of bacterial shape to the topography of the carbon substrate in time periods less than 1-2 minutes. This is a new and unexpected discovery, and the mechanisms that lead to these shape changes remain to be determined. Coordinated changes must occur in the interior of cells in response to extracellular contact, although the nature of this molecular restructuring is unclear. The bacterial cytoskeleton is an attractive candidate that could be involved given its dynamic role in shape modulation of chemotactic eukaryotic cells. Careful examination of the cellular interior in Bdellovibrio tomograms revealed the presence of an extended network of internal filaments, with certain filaments parallel to the inner cell membrane, and others distributed in the transverse direction. In addition to describing the overall shape, cryo-electron tomographic analyses of Bdellovibrio cells have also provided considerable information about their internal architecture. Prominent dense granules are evident, which, like the acidocalcisomes of other cells, are enriched in phosphorus, oxygen and calcium relative to other regions of the same cell. These granules, which are almost always encompassed by the nucleoid, show no preferential cellular localization or an obvious correlation between granule volume and cell dimensions. The nucleoid itself occupies a relatively large volume in the cell, given that the Bdellovibrio genome (3.78 Mb) is similar in size to the genome of much larger E.coli cell. The regions of the cell between the nucleoid and the cell membrane contain abundant granular and electron dense macromolecular structures; the most prominent of these structures have dimensions consistent with ribosomes. We have also used 3D tomographic imaging to document the presence of other macromolecular assemblies constituent in each cell including the flagellar rotor, some with clearly discernible C-rings, and a single chemotaxis receptor array that localizes near the flagellum. The sheathed flagellum inserts through the inner and outer membranes at the posterior pole at a point that is off the longitudinal axis of the cell. This off-axis positioning may be essential for mono-flagellated cells such as Bdellovibrio to steer and change direction. Comparison of highly bent and moderately bent cells reveals no discernable differences in the cellular features noted above, including the nucleoid, which remains undistorted and closely follows the curvature of the bend. There was also no appreciable difference in the 25 nm spacing between inner and outer membranes at regions of the highest curvature relative to other areas of the cell, although the anterior entry pole typically had a wider (40 nm) spacing compared to the rest of the cell.
| Milne, Jacqueline L S; Subramaniam, Sriram (2009) Cryo-electron tomography of bacteria: progress, challenges and future prospects. Nat Rev Microbiol 7:666-75 |
