Virus-Infecting Virus Fuels Definition of Life Debate

August 22, 2008

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. (Test your virus smarts with an infectious diseases quiz.)

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.

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