The detailed changes in the structure of a virus as it infects an E. coli bacterium have been observed for the first time.

To infect a cell, a virus must be able to first find a suitable cell and then eject its genetic material into its host.

This robot-like process has been observed in a virus called T7 and visualized by Ian Molineux, professor of biology in the College of Natural Sciences at The University of Texas at Austin, and colleagues at The University of Texas Health Science Center at Houston (UT Health) Medical School.

When searching for its prey, the virus briefly extends — like feelers — one or two of six ultra-thin fibers it normally keeps folded at the base of its head.

Once a suitable host has been located, the virus behaves a bit like a planetary rover, extending these fibers to walk randomly across the surface of the cell and find an optimal site for infection — the first experimental evidence for this.

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, the overall process completely changes our understanding of how a virus infects a cell,”.said Molineux.

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.

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.

Molineux’s co-authors are Bo Hu, William Margolin and Jun Liu from UT Health.