Aug. 13, 2008 -- A virus that takes advantage of other viruses for its own survival has been found by scientists in France.
"This opens up whole new avenues of scientific exploration in terms of how viruses interact," said Curtis Suttle, a scientist at the University of British Columbia who was not involved in the research, which was recently published in the journal Nature.
In 2003 scientists at the Mediterranean Institute of Microbiology in Marseilles, France, found the world's largest virus in a U.K. cooling tower, a behemoth so large scientists initially discarded it as a bacteria. (Bacteria are typically many times larger than viruses.)
The French scientists continued to study the large virus, called mimivirus, and recently found an even larger one, which they named mamavirus.
Then they found something no one had seen before -- a third virus orbiting, and actually parasitizing, the mamavirus. The scientists named it Sputnik.
Viruses are a unique form of life. Unlike most other forms, they don't have the cellular equipment to reproduce. Instead they have to infect other cells, hijacking their equipment to produce thousands of little replicas.
Unfortunately for the infected cells, those copies typically pile up inside and eventually explode. The host cells are typically killed in the process.
Sputnik, in essence, hijacks a hijacker. About 100 times smaller than mimivirus or mamavirus, it can't directly infect the other viruses. Instead, it co-infects the same cells that mimivirus and mamavirus infect, usually amoebas.
Sputnik then uses its genetic material to hijack the cellular machinery churning out copies of mimivirus and mamavirus, forcing them to make copies of Sputnik instead. As a result, the spread of mimivirus and mamavirus in the amoeba is slowed.
Two organisms crammed into another creates a lot of free-floating genetic material. When scientists sequenced Sputnik's genome, they found traces of many different organisms, said Eugene Koonin, a researcher at the National Institutes for Health who worked with the French team.
"Sputnik is a chimera. It has several genes that are distantly but clearly related to more familiar viruses, like pox viruses or the mimivirus or mamavirus," said Koonin.
"Then again, it has some genes that are strange to see," he added. "It's very peculiar."
Some of those peculiar genes come from archaea, a different branch of life altogether, but about 50 percent of Sputnik's genome was similar to marine viruses, leading scientists to speculate that Sputnik is not the only virus of its kind.
"There are on average 50 million viruses in every milliliter of sea water," said Suttle, who studies marine viruses. "It's certainly possible that these viruses are very widespread."
The viruses don't have any particular medical value yet, according to the scientists. They don't infect humans, and it is highly unlikely that doctors could modify Sputnik to parasitize viruses that attack humans, say both Koonin and Suttle.
The scientists also dismiss discussions about whether the discovery is proof positive that viruses are alive, a debate that itself has refused to die.
"It's a matter of semantics," said Koonin. "They are part of the biological world. They have their own genome. It depends on what your definition of alive is and what your preferences are."
Alive or not, the discovery "shows just how little we know about the diversity of life," said Suttle. "It's an intriguing discovery."
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