Observation of Bacteriophage Ultrastructure by Cryo-electron Microscopy.

Transmission Electron Microscopy (TEM) is an ideal method to observe and determine the structure of bacteriophages. From early studies by negative staining to the present atomic structure models derived from cryo-TEM, bacteriophage detection, classification, and structure determination has been mostly done by electron microscopy. Although embedding in metal salts has been a routine method for virus observation for many years, preservation of bacteriophages in a thin layer of fast frozen buffer has proven to be a most convenient preparation method for obtaining images using cryo-electron microscopy (cryo-EM). In this technique, frozen samples are observed at liquid nitrogen temperature and the images are acquired using different recording media. The incorporation of direct electron detectors has been a fundamental step to achieve atomic resolution images of a number of viruses. These projection images can be numerically combined using different approaches to render a three-dimensional model of the virus. For those viral components exhibiting any symmetry, averaging procedures help to render near-atomic resolution structures.