Scientists have discovered a trick used by some viruses to speed up the rate at which they spread - infected cells bounce incoming viruses away towards new cells.
Using video recordings, Imperial-based virologist Geoff Smith and his colleagues were able to watch as vaccinia virus, a member of the poxvirus family that was used to eradicate smallpox, 
infected a sheet of cultured cells under the microscope.
By measuring how quickly healthy cells adjacent to an infected cell began to show signs of infection, the team were able to calculate the rate at which the vaccinia virus was spreading. It turned out to be at least four times faster than it ought to be.
Previously, scientists had thought that viruses emerging from an infected cell would infect the adjacent surrounding cells, which would then in turn infect their neighbours, and so on, causing the infection to spread out radially. But the video sequences showed that this cannot be the case. In fact, infected cells were popping up several cells away from the nearest infected neighbour.
By labelling different components of the cells and the viruses the team discovered that when a cell is infected by the virus it adds two viral proteins, called A33 and A36, to its surface, which marks it as having been "hit". If a new virus then tries to enter a cell marked in this way, the two proteins trigger the cell to eject the virus using a structure known as an actin tail, which resembles a molecular spring. This bounces the virus off in the direction of other cells that may not yet have been infected.
The labelling experiments even revealed that the same virus particle can be relaunched in this way several times! The discovery is exciting because, as the researchers point out in their paper in Science, many other viruses, including human pathogens like the cold sore agent,herpes simplex, spread across cells at the same rate as vaccinia.
This suggests that other viruses might use the same trick, which might therefore be a important target for future antiviral drugs.
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