The discovery of a massive virus that suffers from another virus has reignited debate over whether the microscopic agents of infection should be considered living things rather than bags of genes. Earlier this month scientists reported a new strain of giant virus called mamavirus, which was first detected in amoebas from a water-cooling tower in Paris.
In a recent study, electron microscopy revealed a much smaller virus attached to the mamavirus, which the study authors say made the host virus grow abnormally and damaged its ability to replicate.
The tiny satellite virus, dubbed Sputnik, is the first described virophage—so named because its behavior resembles that of bacteria-targeting viruses known as bacteriophages.
A team led by microbiologist Didier Raoult from the Université de la Méditerranée in Marseille, France, published the findings earlier this month in the online edition of the journal Nature.
In an accompanying commentary, Jean-Michel Claverie, director of the Mediterranean Institute of Microbiology in Marseille, said of Sputnik's victim: "There is no doubt that this is a living organism."
But other microbiologists aren't enthusiastic about tying the discovery to a redefinition of life, with at least one expert calling speculation over whether viruses represent living organisms a red herring.
To Be, or Not To Be?
Fundamentally, viruses are bundles of genetic material surrounded by a protein shell called a capsid. To reproduce, a virus binds to a host cell, which mistakes the virus for a protein and replicates its genetic code. The cell then bursts, releasing hundreds of copies of the virus into the host.
Classic definitions hold that viruses are not life-forms, because they lack living cells of their own and must hijack those in animals, plants, and bacteria.
According to Merriam-Webster's Unabridged Dictionary, viruses "are regarded either as the smallest microorganisms or extremely complex molecules."
In 2003 Claverie and Raoult described the first known giant virus, called mimivirus, which had originally been mistaken for a bacterium because of its size.
Raoult later co-authored a paper in Nature Microbiology that challenged the cell-based definition of life and proposed redefining viruses as "capsid-encoding organisms." Claverie also put forth a new way of interpreting viruses: They are a "transitory cell into the cell, hence more like regular living organisms," he said.
"Virologists focused too much on viruses as 'particles,' where I proposed that they should be considered an intermediary stage [to cellular life]—the same role spores or seeds play for plants, for instance," he said.
Research by Claverie and his colleagues further suggests that giant viruses may predate the emergence of living organisms with a nucleus, or eukaryotes.
"In fact, there are many arguments that suggest that these giant viruses may be the ancestor of the modern cell nucleus," the French scientist added.
The virophage Sputnik apparently harms the newly described mamavirus as it exploits its reproductive mechanism, bolstering the notion that giant viruses are on par with recognized life-forms as hosts for viruses and should be reclassified.
Raoult notes, however, that such ideas might not be met with universal approval.
"People can hate … when you try to change very basic definitions," he said.
Eddie Holmes of the Center for Infectious Disease Dynamics at Pennsylvania State University said that it has long been known that viruses can infect other viruses.
"The difference here is that Sputnik is supposedly 'bad' for [the mamavirus]," Holmes wrote in an email. "However, I think this aspect of the study is rather tentative and needs more work."
Holmes added that he thinks "the debate over whether viruses are 'alive' is an entirely pointless one and of no scientific importance. It all depends on how you define life."
Eugene Koonin, a co-author of the new study at the National Institutes of Health in Bethesda, Maryland, echoed Holmes's comments that the question of whether viruses are living creatures is a nonissue.
"Is not a scientific question or, in any case, a question of any interest to scientists," Koonin stated.
"Viruses are certainly biological entities, and this is enough."