Massive virus discovered in water tower

19:00 27 March 03 NewScientist.com news service

The largest virus ever discovered has been found in a water-cooling tower in Bradford, England. It was lurking inside single-celled organisms called amoebae, but its discoverers believe that it may also be capable of infecting humans.

Although the new virus is a thousand times smaller than a pinhead, its girth of 400 nanometres makes it a mammoth in the microbial world. Most viruses measure between 10 and 100 nanometres, and the newcomer is even bigger than some bacteria.

At first, Didier Raoult at the Mediterranean University in Marseille and his team assumed that was exactly what it was. The virus's outer coating even reacts to a chemical stain that is specific to some bacteria. But when the researchers tried to detect bacterial-specific genes, they could not.

Now Raoult's team have confirmed that the giant - christened Mimivirus because of its similarity to a bacterium - is a virus by comparing its genes with other viral proteins. This comparison suggests that it is closely related to other giant viruses such as smallpox.

Gene giant

But not everyone is yet convinced. Brian Austin, at Heriot-Watt University in Edinburgh, Scotland, says the researchers could be seeing a bacterium that is associated with a virus. "They've left a lot of questions open," he says.

Nonetheless, if the identity of the Mimivirus is confirmed, Austin agrees it would be the largest yet found. And its king-sized characteristics are not just physical, but genetic too.

Its genome, which is 800,000 DNA letters long, is larger than any other virus and several bacteria. It has more than 900 probable genes, easily beating its nearest competitor, called phage D, which has fewer than 300. So far, however, the researchers have little idea what these extra genes do.

Intriguingly, Raoult says that preliminary results have revealed antibodies to the virus in human patients - suggesting that Mimivirus infects humans as well. But he does not know whether the mega-virus actually causes disease.

Journal reference: Science (vol 299, p 2033)

James Randerson