Over the last year we have continued to make progress using cryo-electron tomography for exploring the architecture of bacterial cells, especially with Bdellovibrio bacteriovorus. These are small delta-proteobacterial cells that feed on other gram-negative bacteria, including human pathogens. The most striking feature of attack phase B. bacteriovorus cells under the light microscope is their motility. Swimming cells resemble elongated wiggling rods but precise determination of their shape, and its influence on internal cellular architecture is difficult using light microscopy since they are very small, move rapidly through the field of focus, and turn abruptly in random directions. Our analysis of intact frozen-hydrated cells using cryo-electron tomography showed for the first time that Gram-negative bacteria can bend to extreme curvatures without suffering any obvious structural damage. We demonstrated that B. bacteriovorus cells are capable of substantial flexibility and local deformations of their outer and inner membranes without loss of cell integrity. These shape changes can occur in less than 2 minutes, and analysis of the internal architecture of highly bent cells shows that the overall distribution of molecular machines and the nucleoid is similar to those seen in moderately bent cells. B. bacteriovorus cells appear to contain an extensive internal network of short and long filamentous structures. These findings led to the novel hypothesis that rearrangements of these structures, in combination with the unique properties of the cell envelope underlie the remarkable ability of B. bacteriovorus cells to find and enter bacterial prey. Flexible shape changes also appear critical for egress from exit pores of the bdelloplast following the completion of cell replication.In addition to our discovery of the intrinsic flexiblity of B. bacteriovorus, we followed up with cryo-electron tomographic analysis of the nucleoid of these cells leading to the first in situ 3D structures of the bacterial nucleoid and direct visualization of MreB and ribosomes at the boundary of the nucleoid. Our findings provide direct structural evidence for spiral organization of the bacterial nucleoid and suggest a role for MreB in regulation of nucleoid architecture and localization of the chemotaxis apparatus. Studies are underway to improve the resolution of these studies to further explore the internal organization of the interior of these small bacterial cells.
Showing the most recent 10 out of 11 publications
