Mimiviruses and Marseilleviruses, the Largest Known Viruses


Acanthamoeba polyphaga mimivirus was discovered in 2003 but was isolated two decades ago during the investigation of a pneumonia outbreak, while trying to isolate Legionella‐like bacterial pathogens that infect and survive in amoebae. Acanthamoeba polyphaga marseillevirus was isolated five years thereafter. These two viruses are the founding members of two new viral families whose members are the largest known viruses based on the sizes of the capsid and the genome. Beyond being giant, mimiviruses and marseilleviruses exhibit several unexpected and remarkable features. Their genomes encode proteins involved in translation, these viruses harbour messenger ribonucleic acid in addition to their double‐stranded deoxyribonucleic acid (DNA) genome, and their gene repertoire indicates that they have a common ancestral origin with other large DNA viruses. Finally, mimiviruses and marseilleviruses are likely to be common in our biosphere and have begun to be isolated from humans.

Key Concepts:

  • Mimiviruses and marseilleviruses are giant viruses that infect phagocytic protists (Acanthamoeba spp. in most cases).

  • Mimiviruses are the largest known viruses, and their genomes are the largest among viruses and are larger than those of the smallest cellular life.

  • Mimiviruses and marseilleviruses differ greatly from ‘classical’ viruses in several respects.

  • Mimiviruses and marseilleviruses are linked by phylogeny based on certain genes and their core genomes to the members of the five families of nucleocytoplasmic large DNA viruses; these seven families were recently proposed to be assigned to a new viral order, Megavirales.

  • The mosaic gene repertoires of mimiviruses and marseilleviruses are likely linked to their sympatric lifestyle within phagocytic protistan grazers.

  • Mimiviruses and marseilleviruses are common in our biosphere.

  • Mimiviruses may cause pneumonia in humans.

Keywords: mimivirus; marseillevirus; megavirales; mimiviridae; marseilleviridae; giant viruses; nucleocytoplasmic large DNA viruses; fourth domain

Figure 1.

Acanthamoeba polyphaga mimivirus particle inside Acanthamoeba. Transmission electron microscopy; scale bar indicates 200 nm. A white box has been added above the scale bar, representing its whole length.

Figure 2.

Electron micrograph showing mimivirus Mont1 by negative staining. A Starfish structure, named ‘Stargate’, is indicated by an arrow on the left picture (a) and a superimposed marking on the right picture (b). Scale bar indicates 200 nm. A white box has been added above the scale bar, representing its whole length.

Figure 3.

Mimivirus factories in Acanthamoeba as visualised by Hemacolor staining: (a), immunofluorescence, (b) Gram staining, (c) Gimenez staining, and (d) Viral factory (VF), indicated by an arrow. Optical microscopy, magnification: ×400.

Figure 4.

Mamavirus during its replication cycle in Acanthamoeba showing viral at the periphery of the viral factory (a) and (b, c) mature virus particles. Transmission electron microscopy; scale bars indicate 2 μm (a), 5 μm (b), and 500 nm (c). A white box has been added above the scale bars, representing their whole length. VF, viral factory.

Figure 5.

Courdo7 virus during its replication cycle in Acanthamoeba showing the viral factory with production of viruses at the periphery of the factory (a), and mature virus particles inside Acanthamoeba before the amoebal lysis (b and c). Transmission electron microscopy; scale bars indicate 2 μm (a), 5 μm (b), and 500 nm (c). A white box has been added above the scale bars, representing their whole length. VF, viral factory.

Figure 6.

Production of Bus (mimivirus of lineage C) viral particles in Acanthamoeba: production of Bus at the periphery of the virus factory. Transmission electron microscopy; scale bar indicates 1 μm. A white box has been added above the scale bar, representing its whole length.

Figure 7.

Mont1 (mimivirus of lineage C) and Sputnik 4 virophage during their replication cycle in Acanthamoeba at a late stage of the cycle: (a) mimivirus particles disseminated in the amoebal cytoplasm; (b) higher magnification showing presence of patches of longer viral fibre named ‘cowlicks’ (indicated by an arrow). Transmission electron microscopy; scale bars indicate 2 μm (a) and 1 μm (b). A white box has been added above the scale bars, representing their whole length. Spk, sputnik; VF, viral factory.

Figure 8.

Marseillevirus particles under electron microscopy by negative staining (a and b). Scale bars indicate 100 nm. A white box has been added above the scale bar, representing its whole length.

Figure 9.

Tunisvirus fontaine2 under electron microscopy by negative staining (a, b). Scale bars indicate (a) 100 nm and (b) 200 nm. A white box has been added above the scale bars, representing their whole length.

Figure 10.

World map indicating the location of major mimiviruses and marseilleviruses isolated and described to date. Mimiviruses are in blue font and marseilleviruses are in red font.



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Further Reading

Desnues C and Raoult D (2012) Virophages question the existence of satellites. Nature Reviews Microbiology 10(3): 234.

Pagnier I, Raoult D and La Scola B (2008) Isolation and identification of amoeba‐resisting bacteria from water in human environment by using an Acanthamoeba polyphaga co‐culture procedure. Environmental Microbiology 10(5): 1135–1144.

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Boughalmi, Mondher, Colson, Philippe, and La Scola, Bernard(May 2013) Mimiviruses and Marseilleviruses, the Largest Known Viruses. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0024409]