Scientists have captured images of a virus in the very act of infecting an E. coli bug, and the structural changes that it undergoes. This is capture is the first of its kind and very rare in nature.
A virus must be able to find a suitable cell and then eject its genetic material into its host to infect it. This robot-like process has been observed in a virus called T7 and visualized by Ian Molineux, professor of biology at the University of Texas Health Science Centre-Austin, and colleagues.
Researchers used a combination of genetics and cryo-electron tomography to image the infection process. It is similar to a CT scan, but it is scaled to study objects a thousand times thinner than human hair, the journal Science Express reports.
They show that when searching for its prey, the virus briefly extends - like feelers - one or two of six ultra-thin fibres it normally keeps folded at the base of its head, according to a Texas statement.
Once a suitable host has been located, the virus behaves a bit like a planetary rover, extending these fibres to walk randomly across the surface of the cell and find an optimal site for infection.
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."
For example, researchers now know that most of the fibres are usually bound to the virus head rather than extended, as was previously thought. This is also the first time that scientists have made actual images showing how the virus's tail extends into the host - the very action that allows it to infect a cell with its DNA.
"I first hypothesized that T7 made an extended tail more than 10 years ago," said Molineux, "but this is the first irrefutable experimental evidence for the idea and provides the first images of what it looks like."