Mimivirus

Mimivirus is a genus of viruses, in the family Mimiviridae. Amoeba serve as natural hosts. This genus contains a single identified species named Acanthamoeba polyphaga mimivirus (APMV), which serves as its type species. It also refers to a group of phylogenetically related large viruses. In colloquial speech, APMV is more commonly referred to as just 'mimivirus'. Mimivirus, short for 'mimicking microbe', is so called to reflect its large size and apparent Gram-staining properties. Mimivirus has a large and complex genome compared with most other viruses. Until 2013, when a larger virus Pandoravirus was described, it had the largest capsid diameter of all known viruses. APMV was discovered accidentally in 1992 within the amoeba Acanthamoeba polyphaga, after which it is named, during research into Legionellosis by researchers from Marseille and Leeds. The virus was observed in a gram stain and mistakenly thought to be a gram-positive bacterium. As a consequence it was named Bradfordcoccus, after the district the amoeba was sourced from in Bradford, England. In 2003, researchers at the Université de la Méditerranée in Marseille, France published a paper in Science identifying the micro-organism as a virus. It was given the name 'Mimivirus' (for 'Mimicking microbe') as it resembles a bacterium on Gram staining. The same team that discovered the mimivirus later discovered a slightly larger virus, dubbed the mamavirus, and the Sputnik virophage that infects it. Mimivirus has been placed into a viral family by the International Committee on Taxonomy of Viruses as a member of the Mimiviridae, and has been placed into Group I of the Baltimore classification system. Although not strictly a method of classification, Mimivirus joins a group of large viruses known as nucleocytoplasmic large DNA viruses (NCLDV). They are all large viruses which share both molecular characteristics and large genomes. The mimivirus genome also possesses 21 genes encoding homologs to proteins which are seen to be highly conserved in the majority of NCLDVs, and further work suggests that mimivirus is an early divergent of the general NCLDV group. The mimivirus is the fourth-largest virus, preceded by the recently discovered Megavirus chilensis, Pandoravirus and Pithovirus. Mimivirus has a capsid diameter of 400 nm. Protein filaments measuring 100 nm project from the surface of the capsid, bringing the total length of the virus up to 600 nm. Variation in scientific literature renders these figures as highly approximate, with the 'size' of the virion being casually listed as anywhere between 400 nm and 800 nm, depending on whether total length or capsid diameter is actually quoted.