Giant Viruses of Amoebas: An Update

Abstract

During the 12 past years, five new or putative virus families encompassing several members, namely Mimiviridae, Marseilleviridae, pandoraviruses, faustoviruses, and virophages were described. In addition, Pithovirus sibericum and Mollivirus sibericum represent type strains of putative new giant virus families. All these viruses were isolated using amoebal coculture methods. These giant viruses were linked by phylogenomic analyses to other large DNA viruses. They were then proposed to be classified in a new viral order, the Megavirales, on the basis of their common origin, as shown by a set of ancestral genes encoding key viral functions, a common virion architecture, and shared major biological features including replication inside cytoplasmic factories. Megavirales is increasingly demonstrated to stand in the tree of life aside Bacteria, Archaea, and Eukarya, and the megavirus ancestor is suspected to be as ancient as cellular ancestors. In addition, giant amoebal viruses are visible under a light microscope and display many phenotypic and genomic features not found in other viruses, while they share other characteristics with parasitic microbes. Moreover, these organisms appear to be common inhabitants of our biosphere, and mimiviruses and marseilleviruses were isolated from human samples and associated to diseases. In the present review, we describe the main features and recent findings on these giant amoebal viruses and virophages.

Morphological and Taxonomic Properties of <i>Tokyovirus</i>, the First <i>Marseilleviridae</i> Member Isolated from Japan

“…Giant viruses in the narrow sense contain several groups: Mimiviridae , Marseilleviridae , Faustovirus , and the amphora-like giant viruses Pandoravirus , Pithovirus , and Mollivirus (4). The particle sizes of these giant viruses are widely diversified, from Marseilleviridae (approximately 200 nm in diameter) to Pithovirus (1.5 μm in diameter).…”

Boiling Acid Mimics Intracellular Giant Virus Genome Release

“…GV have been isolated from a wide variety of hosts, including amoeba (Aherfi et al, 2016b), animals Andrade et al, 2015;Dornas et al, 2014a;Khan et al, 2007), as well as human and murine cells (Ghigo et al, 2008;Lusi et al, 2017). However, amoebas also infect these creatures, casting doubt on the true viral reservoir.…”

“…GV are ubiquitous (Aherfi et al, 2016b;Andrade et al, 2018) and maintain infectivity in harsh environments such as alkaline lakes , frozen permafrost (Legendre et al, 2014), 3 km deep in the ocean and dry valleys in Antarctica Kerepesi and Grolmusz, 2017). GV have retained infectivity following exposure to harsh chemicals (Campos et al, 2012), extreme pH and salinity , extreme temperatures Legendre et al, 2014), and are able to persist on hospital equipment (Campos et al, 2012;Dornas et al, 2014b).…”

‘Candidatus Cochliophilus cryoturris’ (Coxiellaceae), a symbiont of the testate amoeba Cochliopodium minus

“…Free-living amoebae are also known as hosts for diverse bacteria and giant DNA viruses 4–6 . They serve as reservoirs for a number of human pathogens such as Legionella pneumophila 7 , Pseudomonas aeruginosa 8 , Francisella tularensis 9 , Coxiella burnetii 10 , Vibrio cholerae 11, 12 , Aeromonas hydrophila 13 , and Mycobacterium species 14 , all of which escape the regular phagolysosomal pathway and transiently replicate within amoeba trophozoites.…”