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
Bernard La Scola
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