close-up of a virus infecting a bacterium

In an article published on Jan 10, 2013 online in Science Express, we can see an incredible animation showing the T7 virus as it infects an E. coli bacterium.  This process has been visualized for the first time by Ian Molineux, professor of biology at The University of Texas at Austin and his colleagues.  

At the preferred infection site, the virus goes through a major change in structure in which it ejects some of its proteins through the bacterium's cell membrane, creating a path for the virus's genetic material to enter the host.  

After the viral DNA has been ejected, the protein path collapses and the infected cell membrane reseals.  

"Although many of these details are specific to T7," said Molineux, "the overall process completely changes our understanding of how a virus infects a cell." (Source)

Check out the animation - it's almost hard to imagine it is "real" and not purely computer animation!

The researchers used a combination of genetics and cryo-electron tomography to image the infection process. Cryo-electron tomography is a process similar to a CT scan, but it is scaled to study objects with a diameter a thousandth the thickness of a human hair." (Same source)

Cryo-electron tomography is a new-ish form of microscopy that images very small items at very low temperatures (think liquid-nitrogen cold, or 63 K to 77.2 K, or -346° to -320.44°F!) in their "natural" environment.   

An article in Nature supplies the following image:

I don't claim to totally understand this, but a sample of a suspension of cells is put in a grid, then the grid + cells are frozen in liquid ethane (which has been cooled with liquid nitrogen).  Then the whole thing imaged by an electron microscope at a variety of angles.  These tilted images are then converted to a 3D image (tomogram).  

Here's another informative link if you are interested in reading more.  And if you are really interested, you should check out this (2007) NIH article entitled, Electron Tomography of Viruses.  And, my favorite so far, a summary of a (2011) study using cryo-electron tomography to image the Marburg & Ebola viruses!"