What if we want to zoom in further to examine a particular structure more closely? While in theory electron microscopy can resolve atoms, in practice the resolution is limited by many factors including the radiation sensitivity and thickness of the sample. For relatively thick samples like the (small) bacterium on the last page, we can achieve ~5 nm resolution, enough to see the shapes and arrangement of large macromolecular complexes. To boost our resolving power further, we can gather strength in numbers. By averaging multiple copies of a structure, either from the same tomogram, or from multiple cells in different tomograms, we can build up the signal relative to the noise. Here you see an example of this approach, called sub-tomogram averaging, applied to the motor that spins the flagellum. Hundreds of Campylobacter jejuni cells were imaged by cryo-ET and their individual flagellar motors were digitally extracted from the resulting tomograms and averaged to produce a higher-resolution view [10]. Note how densities that vary between motors, indicating that they are not stably associated with the structure, get washed out, while densities that appear in the same place in each motor reinforce one another. This approach only works for structures, and parts of structures, that are fairly rigid, but it can be a powerful tool to study structures inside the cell. EMD-3150
