THE largest virus yet discovered has been found lurking in a water cooling tower at Bradford in the north of England. It infects amoebas, but its discoverers believe it may also be capable of infecting humans.
The mammoth virus is 400 nanometres in diameter. Though that is a thousand times smaller than a pinhead, it is bigger than some bacteria.
At first, Didier Raoult at the Mediterranean University in Marseille and his team assumed that is exactly what it was. The virus鈥檚 outer coating even reacts to a chemical stain that is supposed to be bacterium-specific. But the researchers became suspicious when they failed to detect certain genes known to be present in all bacteria.
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Now Raoult鈥檚 team has confirmed that the giant is a virus by comparing its genes with those that other viruses use to make proteins. This comparison, and elements of the microorganism鈥檚 structure and development cycle, suggest that it is closely related to other large viruses such as smallpox (Science, vol 299, p 2033).
The researchers have christened it mimivirus because of its resemblance to bacteria. But not everyone agrees the evidence is watertight. 鈥淭hey鈥檝e left a lot of questions open,鈥� says Brian Austin at Heriot-Watt University in Edinburgh. He accepts that if mimivirus鈥檚 identity is confirmed it will indeed be the largest yet found. But he believes that the researchers could be seeing a bacterium that is associated with a virus.
It is not only mimivirus鈥檚 dimensions that are king-sized. Its genome is 800,000 DNA letters long, larger than any other virus and several bacteria. And it may have more than 900 genes, easily beating its nearest rival, a virus called phage D, which has fewer than 300. As yet, the researchers have no clear idea what these extra genes do or why mimivirus needs them.
Intriguingly, Raoult says that preliminary results suggest that mimivirus infects humans as well as microorganisms. He has found antibodies to the virus in human patients, but no evidence yet as to whether they actually cause disease.
