Advances in Mimivirus Pathogenicity

Viral diseases in the clinical setting have been extensively investigated. Viruses are now considered as potentially responsible for nosocomial infections, especially in intensive care unit (ICU) patients. Mimivirus is the largest virus known to date. Recent studies have suggested that Mimivirus could be responsible for both community-acquired and nosocomial pneumonia. These studies were mainly based on serologic diagnosis, which showed patients with community-acquired pneumonia have more antibodies to Mimivirus than healthy controls. Serologic evidence of Mimivirus pneumonia was also found in mechanically ventilated ICU patients. In a matched-cohort study in which ICU patients with serologic evidence of Mimivirus pneumonia were matched to ICU patients remaining seronegative for Mimivirus, positive serology was associated with an increased duration of both mechanical ventilation and ICU stay. Identification by PCR techniques remains difficult, probably because of the high level of polymorphism of nucleotide sequences of giant viruses. More studies are needed to confirm the clinical impact of Mimivirus in humans. Copyright © 2010 S. Karger AG, Basel Published online: June 15, 2010 Prof. Laurent Papazian URMITE, Faculté de Médecine 27 Boulevard Jean Moulin FR–13385 Marseille Cedex 5 (France) Tel. +33 4 91 32 43 75, Fax +33 4 91 38 77 72, E-Mail laurent.papazian @ ap-hm.fr © 2010 S. Karger AG, Basel 0300–5526/10/0535–0304$26.00/0 Accessible online at: www.karger.com/int Advances in Mimivirus Pathogenicity Intervirology 2010;53:304–309 305 toms suggestive of VAP. Cytomegalovirus pneumonia was diagnosed in 25 patients. Recent reports suggest that Mimivirus could be an agent of pneumonia in critically ill mechanically ventilated patients. Mimivirus is the largest virus known to date. Its genome sequence encodes more than 900 proteins. Many of these proteins have functions that have never been encountered in a virus. While other amoeba-associated microorganisms from environmental sources, such as Legionella pneumophila , are known to cause acute pneumonia, the role of Mimivirus as a putative pneumonia agent remains a matter of debate. Mimivirus Description Mimivirus is a giant DNA virus that was isolated in 1992. An outbreak of community-acquired pneumonia in Bradford (England) triggered an investigation for Legionella -like bacteria in the water of a nearby cooling tower. Instead of the expected Gram-negative bacilluslike Legionella , a microorganism that looked like a small Gram-positive coccus was found that was able to propagate within the amoeba Acanthamoeba polyphaga . The investigator named the new intracellular parasite ‘Bradford coccus’ [13] . However, all attempts of culture on agar plates failed, as did molecular identification using universal 16S rDNA bacterial primers. Ten years later, the sample containing the microorganism was brought to Didier Raoult’s lab at the University of Marseille, France. The ultrastructure of Bradford coccus was examined by electronic microscopy. ‘Unbacterial’ bodies within infected amoebae were observed. These bodies consisted of regular icosahedral forms, much like those observed in Iridoviruses [14] . This organism contained a large double-stranded DNA chromosome and possessed an eclipse-phase replication typical of viruses. Hence, this agent was considered as a virus. It was named Acanthamoeba polyphaga mimivirus for its amoeba host and bacteria-mimicking characteristics ( fig. 1 ). Then, the analysis of several gene sequences exhibited a clear phylogenetic affinity with nucleocytoplasmic large DNA virus (NCLDV), a group of viruses including Poxviridae, Iridoviridae, Phycodnaviridae and Asfarviridae. Mimivirus is now classified as the first and prototype member species of Mimiviridae, a new family within the NCLDV.