In 1992, a pneumonia outbreak occurred in a Bradford hospital (England), and water samples from a cooling tower that contained free-living amoebae were investigated to determine the etiological agent of the pneumonia outbreak . At that time, Gram-positive cocci that were visualized by light microscopy inside Acanthamoeba polyphaga cells were named Bradford coccus. Every attempt to isolate this microorganism and amplify its 16S rDNA failed. Moreover, treatment of amoeba cultures with antibiotics to inhibit growth of this microorganism was also unsuccessful, which led doubt whether it was indeed a bacteria . After a hiatus of a few years, this organism was the subject of new studies at the Rickettsia Unit at the School of Medicine (Marseille, France) in the early 2000s. After a new series of unsuccessful characterization attempts, electron microscopy of Bradfordcoccus-infected Acanthamoeba polyphaga cells revealed icosahedral-like particles with an astonishing 750 nm diameter size . In addition to this virus-like morphology, analysis of the replication curve of this organism in amoeba cells revealed an eclipse phase, which is an almost universal feature among viruses. Finally, when the complete sequencing and analysis of its genome was finished, it became evident that this peculiar organism clustered with other giant viruses and not with bacteria . This new virus was then called Acanthamoeba polyphaga mimivirus (APMV), due its ability to infect the free-living amoebae Acanthamoeba polyphaga sp. and mimic a microbe. APMV, also known as mimivirus, had the largest viral genome known up to then, reaching approximately 1.2 Mb. Its characteristics were so different from other viruses that it was not possible to include it into any known viral family, so the Mimiviridae family was created . APMV became the first member of the Mimiviridae family, Mimivirus genus, and its prototype.
Few years before the discovery of APMV, the genomic content of known giant viruses that replicate partly or entirely in the cytoplasm of eukaryotic cells (members from Poxviridae, Asfarviridae, Phycodnaviridae, Ascoviridae and Iridoviridae families) was analyzed in depth. Several common, and supposedly essential, genes were identified, and it was suggested that the origin of these four viral families was monophyletic [21–26]. Other common characteristics, such as a large double-stranded DNA genome, the relative independence of their host transcription machinery, and a replication cycle that occurs at least partially into the cytoplasm with the formation of inclusion bodies or viral factories, were the basis for the generic name of these viral families: the nucleo-cytoplasmic large DNA viruses (NCLDVs) [22–26]. Because viruses from the Mimiviridae family also share these characteristics, they were added to the NCLDV group as well.
After APMV was described, interest in giant viruses grew, several other giant viruses were isolated, and the Mimiviridae family was expanded. Currently, the Mimiviridae family contains two genera according to the International Committee on Taxonomy of Viruses (ICTV): Mimivirus, with APMV as its only member, and Cafeteriavirus, with Cafeteria roenbergensis virus as its only member (http://www.ictvonline.org). With the increasing number of new giant virus isolates and hypothetical species, a new viral order has been proposed . The putative Megavirales order contains the Mimiviridae family and other NCLDVs, including the newly proposed Marseilleviridae family , whose founding member is Marseillevirus, another giant virus, smaller than Mimivirus, that infects amoeba isolated from cooling tower water in 2008 . Mimivirus and Marseillevirus have been primarily linked to other NCLDVs, based on a set of ≈ 50 conserved core genes shared by all or by a majority of these large and giant viruses . All these viruses were shown to compose a monophyletic group . Nucleo cytoplasmic virus orthologous groups (NCVOGs) were defined among these viruses, including 177 proteins present in > 1 NCLDV family and five common to all viruses .
The discovery of new isolates of mimiviruses of amoeba revealed the existence of three different lineages (A, B and C). Lineage A of the Mimivirus genus contains the best known mimivirus isolates, such as the APMV species . Mimivirus lineage B is represented by Acanthamoeba polyphaga moumouvirus, which was isolated from a water sample in February, 2008; genetic analysis revealed differences that placed this virus into the B lineage . The first extensively described member of lineage C was Megavirus chilensis, isolated from a water sample collected off the coast of Chile . Other previously described mimiviruses, including Courdo7, Courdo11, Terra1 and Montpellier, were also included in lineage C . Other viruses, including some that are distantly related, have been obtained from environmental and clinical samples [32, 33], and in the next years it will likely be possible to present an extensive depiction of the putative viral isolates, strains and species.