  The recently discovered Acanthamoeba polyphaga Mimivirus is the largest known
DNA virus. Its particle size (>400 nm), genome length (1.2 million bp) and
large gene repertoire (911 protein coding genes) blur the established
boundaries between viruses and parasitic cellular organisms. In addition, the
analysis of its genome sequence identified new types of genes not expected to
be seen in a virus, such as aminoacyl-tRNA synthetases and other central
components of the translation machinery. In this article, we examine how the
finding of a giant virus for the first time overlapping with the world of
cellular organisms in terms of size and genome complexity might durably
influence the way we look at microbial biodiversity, and force us to
fundamentally revise our classification of life forms. We propose to introduce
the word "girus" to recognize the intermediate status of these giant DNA
viruses, the genome complexity of which make them closer to small parasitic
prokaryotes than to regular viruses.
