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Research Field Microbiology

Classifying the Unclassifiable

About 20 years ago, I was investigating amoeba and the Legionella bacteria living within them, in a collection from Timothy Rowbotham. We identified five new species of Legionella and also made a surprising discovery – Gram-positive chlamydia-like bacteria living in amoeba. We tried and tried to amplify the newly discovered microbe, but all our attempts came to nothing, until eventually we started to question if it was a bacterium at all. We inspected the amoeba under an electronic microscope before and after extraction, and saw something that wasn’t bacteria-like at all, but instead looked very much like a virus. What we originally thought was a Legionella-like bacteria turned out to be mimivirus, a giant 0.4–0.8 µm virus with a 1.2 megabase genome. 

Mimivirus is a very unusual virus. In fact, it’s debatable whether it is a virus at all.  It bears more resemblance to bacteria, archaea, and eukarya than to viruses. When we investigated mimivirus further, we found that the structural motif of its DNA and RNA polymerases are very old – we suspect that their origins may date back to before the operation of ribosomes. Mimiviruses can also be infected by viruses (virophages) themselves.

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About the Author

Didier Raoult

Didier Raoult is Director of Research Unit in Infectious and Tropical Emerging Diseases, France.

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