Marvellous mimivirus

Mimivirus is one of the largest and most complex viruses known. The virus was first isolated in 1992 from amoebae growing in a water tower in Bradford. The virus was observed in a Gram stain and mistakenly thought to be a gram-positive bacterium. In 2003, a research group in France were the first to identify this micro-organism as a virus (La Scola, B. et al. 2003 A giant virus in amoebae. Science 299: 2033). The genus Mimivirus presently contains a single virus species, Acanthamoeba polyphaga mimivirus (APMV).

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Both the particle size and the genome size of mimivirus is larger than that of some small bacteria. The 1.2 Mbp genome, which contains 911 protein coding genes, provides sufficient information to allow the virus to perform most (but not quite all) of the functions of living cells. The complexity and magnitude of the mimivirus genome, combined with the large size of the virus, calls into question some of the established divisions between viruses and single-celled organisms, as well as raising questions about their evolution (Suzan-Monti et al. 2006 Genomic and evolutionary aspects of Mimivirus. Virus Research 117: 145-155).

Examination using cryo-electron microscopy has shown that the particle has a capsid with a diameter of 750 nm, including an array of 125 nm long closely packed fibres projecting out from the capsid surface. Based on a large number of open reading frames with collagen triple helix repeats in the viral genome, these fibers might consist of collagen. The dense, 200 thick base of these fibers might be formed by cross-linking. The capsid itself appeared to contain three layers of dense matter, probably representing two successive 4 nm thick lipid membranes inside a protein shell approximately 7 nm thick (Mimivirus and the emerging concept of giant virus. Claverie JM. et al. 2006 Virus Res. 117: 133-144). Similar double lipid membrane layers have been found in some poxviruses and in African swine fever virus (ASFV), another very large virus. Mimivirus particles also have a unique protruding vertex at one corner of the particle similar to that seen in tailed bacteriophages.

Mimivirus has many characteristics which put it at the boundary between living organisms and non-living entities. It is as large as several bacteria, such as Rickettsia conorii and Tropheryma whipplei, has a genome larger than a number of bacteria, and encodes some genetic products previously not known to be possessed by any virus. In particular, mimivirus contains genes coding for nucleotide and amino acid synthesis which even some small obligate intracellular bacteria lack. This means that unlike these bacteria, mimivirus is not dependent on the host cell genome for coding the metabolic pathways for these products. It does however, lack genes for ribosomal proteins, making mimivirus dependent on a host cell for protein synthesis and energy metabolism.

So, is mimivirus alive? Like all viruses, mimivirus particles do not reproduce by division, but are replicated by the self-assembly of preformed components. This differentiates it from cellular living organisms such as bacteria.

Patients with pneumonia have shown positive serological tests for mimivirus, and a laboratory technician working with the virus developed pneumonia and seroconverted. However, neither of these observations was definitive proof that mimivirus can cause disease, so experimental infections have been carried out in mice, which also developed pneumonia (Khan M. et al. Pneumonia in mice inoculated experimentally with Acanthamoeba polyphaga mimivirus. Microb Pathog. Dec 2006).

We may be hearing a lot more about mimivirus as our knowledge of this unique virus grows.

• I am what I eat and I eat what I am: acquisition of bacterial genes by giant viruses. Filee J. et al. Trends Genet. 2007 23: 10-15