updated 5/3/2004 5:54:44 PM ET 2004-05-03T21:54:44

The protein shell that surrounds a virus is elastic but can be broken, according to scientists who repeatedly poked shells with a special microscope.

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The shell, called a capsid, is about as strong as a hard plastic, but is only about a billionth of a yard long, according to the researchers. Their findings are reported in this week’s issue of Proceedings of the National Academy of Sciences.

Tests of the shell of a virus called phi29 showed its strength was “close to that of Plexiglas,” said Gijs Wuite of the Free University in Amsterdam, Netherlands.

“At the same time, however, we found that you can deform the shell by 30 percent without breaking it. Thus the shell is quite strong and yet dynamic enough to resist cracking,” Wuite said in an interview by e-mail.

Phi29 is a bacteriophage, a virus that infects bacteria, and it has a protein shell to encase its DNA during transfer from one host cell to another.

Wuite said the researchers are using the same technique, an atomic force microscope, to study other viruses. The instrument works by scanning a fine ceramic or semiconductor tip over a surface and can measure the resistance of the surface.

“The research might reveal new insight in the transport stages of different viruses,” said Wuite. “The shell strength might also relate to the time a virus stays infectious outside the host cell, which would also be important from a medical point of view.”

Since the shell is crucial to protecting a virus when it is outside the host cell, “knowing more about the properties of these shells might help with the understanding and fighting viral infections,” Wuite said.

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