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Giant Virus Found in an Amoeba
Edward R. Winstead

A decade after a pneumonia outbreak in Britain, scientists have discovered a giant virus in an amoeba that was isolated from the water of a cooling tower in Bradford, England, following the outbreak.

Mimivirus (arrows) in cytocentrifuged A. polyphaga as Gram-positive particles.

The virus was so large that the researchers initially thought it was a bacterium. But an analysis of its genome revealed the organism to be a virus. It lacks genes found in many bacteria, and has 21 proteins similar to key viral proteins.

Viruses differ from bacteria in that they lack sufficient genes to replicate independently. Instead, they infect other cells and take over the genetic machinery, producing many copies of the virus. The host cell eventually bursts, releasing viruses into the organism.

Because it resembles a bacterium, the researchers named the newly discovered organism Mimivirus (for Mimicking microbe). The virus had infected the amoeba Acanthamoeba polyphaga.

With about 800,000 base pairs, the Mimivirus genome is larger than those of several bacteria, including Mycoplasma genitalium, Ureaplasma urealyticum, and Wigglesworthia brevipalpis.

Electronic microscopy of Mimivirus and U. urealyticum.

The researchers are sequencing the organism’s genome now. Why it has so many genes is a puzzle.

“It has more genes than several bacteria,” says Bernard La Scola of Unité des Rickettsies in Marseille, France, and a member of the research team. “We are trying to understand why a virus that is basically a cell parasite needs to have such a high number of genes.”

The virus does not appear to have contributed to the outbreak of pneumonia in the early nineties. “Nevertheless, we have some data that suggest Mimivirus could be an agent of pneumonia in humans,” says La Scola.

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La Scola, B. et al. A giant virus in amoebae. Science 299, 5615 (March 28, 2003).

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