Press release

 

Mimivirus: discovery of a giant virus

Paris, March 28, 2003

 

Mimivirus: discovery of a giant virus.A team of French researchers in Marseille has isolated and characterized a virus found in amoebas that is much bigger than any virus ever found to date. This virus, dubbed "Mimivirus" by its discoverers (Bernard La Scola et al., Université de la Méditerranée, Faculté de Médecine, Unité des Rickettsies, CNRS UMR 6020), has a diameter and genetic material about the size of that of some bacteria, much larger than any virus that has been identified as of this time. According to phylogenetic studies, Mimivirus belongs to a new family of viruses. Preliminary data suggest that it could be linked to pneumonia in humans. However, it is in no way related to the new form of pneumonia that has recently appeared: its exceptionally large size would have made it possible to identify it very quickly. This research was published in the magazine Science on March 28, 2003.

Researchers discovered the presence of Mimivirus in water free-living amoebas while looking for Legionella, the bacteria which cause of legionellosis, in the water cooling circuits of air-conditioning systems.

Although it was initially mistaken for a bacteria when it was observed under an optical microscope (Gram-positive staining of small particles resembling cocci, from which the name, Mimivirus, was derived, for Mimicking microbe), closer observation showed that these particles had the structure and morphological characteristics of a virus (icosahedral structure of the capside and particles formed around a nucleus when observed through an electronic microscope). The use of fluorescent monoclonal antibodies combined with a confocal microscopy analysis made it possible to establish a viral-type life cycle with the existence of a typical eclipse phase.

Moreover, molecular biology experiments made on these particles showed that they lack certain specific bacterial molecular components (such as those encoding ribosomal RNA, 16S, as demonstrated by PCR(*)).
Mimivirus appears to be an icosahedral-shaped particle with a diameter of 400 nm and no envelope, surrounded by 80-nm long fibrils. Mimivirus has a double-stranded DNA circular genome of about 800 kilobase pairs.

The size of its genome (the biggest of all known viruses) is greater than that of some bacteria and the size of its particles is equivalent to that of some small bacteria. It was possible to identify approximately 900 genes in its genome.

Comparative analysis of the gene coding for ribonucleotide reductase, one of Mimivirus' proteins, with those of other major DNA viruses, certainly reveals some relationship to viruses such as smallpox, but it especially makes it possible to confirm the fact that this virus separated very early in the evolutionary process and is part of a new family.

Finally, some preliminary serological evidence (antibody levels) shows that Mimivirus could be linked to pneumonia. An animal model made on mice infected by intranasal route reveals that viral particles can be isolated in a culture medium from lung tissue for at least three weeks after inoculation.

(*) Polymerase Chain Reaction


Researcher contact :
Bernard La Scola
Tel: +33 4 91 32 43 75
e-mail : bernard.lascola@medecine.univ-mrs.fr

Press contact :

Muriel Ilous
Tel : +33 1 44 96 43 09
e-mail : muriel.ilous@cnrs-dir.fr