Lorena C. F. Silva

Oysters as hot spots for mimivirus isolation

Viruses are ubiquitous organisms, but their role in the ecosystem and their prevalence are still poorly understood. Mimiviruses are extremely complex and large DNA viruses. Although metagenomic studies have suggested that members of the family Mimiviridae are abundant in oceans, there is a lack of information about the association of mimiviruses with marine organisms. In this work, we demonstrate by molecular and virological methods that oysters are excellent sources for mimiviruses isolation. Our data not only provide new information about the biology of these viruses but also raise questions regarding the role of oyster consumption as a putative source of mimivirus infection in humans.

Acanthamoeba polyphaga mimivirus and other giant viruses: an open field to outstanding discoveries

In 2003, Acanthamoeba polyphaga mimivirus (APMV) was first described and began to impact researchers around the world, due to its structural and genetic complexity. This virus founded the family Mimiviridae. In recent years, several new giant viruses have been isolated from different environments and specimens. Giant virus research is in its initial phase and information that may arise in the coming years may change current conceptions of life, diversity and evolution. Thus, this review aims to condense the studies conducted so far about the features and peculiarities of APMV, from its discovery to its clinical relevance.

A resourceful giant: APMV is able to interfere with the human type I interferon system

Acanthamoeba polyphaga mimivirus (APMV) is a giant, double-stranded virus of the Mimiviridae family that was discovered in 2003. Recent studies have shown that this virus is able to replicate in murine and human phagocytes and might be considered a putative human pathogen that causes pneumonia. However, there is little data regarding APMV and its host defense relationship. In the present study, we investigated how some components of the interferon (IFN) system are stimulated by APMV in human peripheral blood mononuclear cells (PBMCs) and how APMV replication is affected by IFN treatment. Our results demonstrated that APMV is able to replicate in human PBMCs, inducing type I Interferons (IFNs) but inhibiting interferon stimulated genes (ISG) induction by viroceptor and STAT-1 and STAT-2 dephosphorylation independent mechanisms. We also showed that APMV is resistant to the antiviral action of interferon-alpha2 (IFNA2) but is sensitive to the antiviral action of interferon-beta (IFNB1). Our results demonstrated the productive infection of professional phagocytes with APMV and showed that this virus is recognized by the immune system of vertebrates and inhibits it. It provides the first data regarding APMV and the IFN system interaction and raise new and relevant evolutional questions about the relationship between APMV and vertebrate hosts.

Mimivirus circulation among wild and domestic mammals, Amazon Region, Brazil.

To investigate circulation of mimiviruses in the Amazon Region of Brazil, we surveyed 513 serum samples from domestic and wild mammals. Neutralizing antibodies were detected in 15 sample pools, and mimivirus DNA was detected in 9 pools of serum from capuchin monkeys and in 16 pools of serum from cattle.

Acanthamoeba polyphaga mimivirus Stability in Environmental and Clinical Substrates: Implications for Virus Detection and Isolation

Viruses are extremely diverse and abundant and are present in countless environments. Giant viruses of the Megavirales order have emerged as a fascinating research topic for virologists around the world. As evidence of their ubiquity and ecological impact, mimiviruses have been found in multiple environmental samples. However, isolation of these viruses from environmental samples is inefficient, mainly due to methodological limitations and lack of information regarding the interactions between viruses and substrates. In this work, we demonstrate the long-lasting stability of mimivirus in environmental (freshwater and saline water) and hospital (ventilator plastic device tube) substrates, showing the detection of infectious particles after more than 9 months. In addition, an enrichment protocol was implemented that remarkably increased mimivirus detection from all tested substrates, including field tests. Moreover, biological, morphological and genetic tests revealed that the enrichment protocol maintained mimivirus particle integrity. In conclusion, our work demonstrated the stability of APMV in samples of environmental and health interest and proposed a reliable and easy protocol to improve giant virus isolation. The data presented here can guide future giant virus detection and isolation studies.

Amoebas as mimivirus bunkers: increased resistance to UV light, heat and chemical biocides when viruses are carried by amoeba hosts

Amoebas of the genus Acanthamoeba are protists that are associated with human disease and represent a public health concern. They can harbor pathogenic microorganisms, acting as a platform for pathogen replication. Acanthamoeba polyphaga mimivirus (APMV), the type species of the genus Mimivirus, family Mimiviridae, represents the largest group of amoeba-associated viruses that has been described to date. Recent studies have demonstrated that APMV and other giant viruses may cause pneumonia. Amoebas can survive in most environments and tolerate various adverse conditions, including UV light irradiation, high concentrations of disinfectants, and a broad range of temperatures. However, it is unknown how the amoebal intracellular environment influences APMV stability and resistance to adverse conditions. Therefore, in this work, we evaluated the stability of APMV, either purified or carried by the amoeba host, under extreme conditions, including UV irradiation, heat and exposure to six different chemical biocides. After each treatment, the virus was titrated in amoebas using the TCID50 method. APMV was more stable in all resistance tests performed when located inside its host. Our results demonstrate that Acanthamoeba acts as a natural bunker for APMV, increasing viral resistance to extreme physical and chemical conditions. The data raise new questions regarding the survival of APMV in nature and in hospital environments.

Niemeyer Virus: A New Mimivirus Group A Isolate Harboring a Set of Duplicated Aminoacyl-tRNA Synthetase Genes.

It is well recognized that gene duplication/acquisition is a key factor for molecular evolution, being directly related to the emergence of new genetic variants. The importance of such phenomena can also be expanded to the viral world, with impacts on viral fitness and environmental adaptations. In this work we describe the isolation and characterization of Niemeyer virus, a new mimivirus isolate obtained from water samples of an urban lake in Brazil. Genomic data showed that Niemeyer harbors duplicated copies of three of its four aminoacyl-tRNA synthetase genes (cysteinyl, methionyl, and tyrosyl RS). Gene expression analysis showed that such duplications allowed significantly increased expression of methionyl and tyrosyl aaRS mRNA by Niemeyer in comparison to APMV. Remarkably, phylogenetic data revealed that Niemeyer duplicated gene pairs are different, each one clustering with a different group of mimivirus strains. Taken together, our results raise new questions about the origins and selective pressures involving events of aaRS gain and loss among mimiviruses.

Modulation of the expression of mimivirus-encoded translation-related genes in response to nutrient availability during Acanthamoeba castellanii infection

The complexity of giant virus genomes is intriguing, especially the presence of genes encoding components of the protein translation machinery such as transfer RNAs and aminoacyl-tRNA-synthetases; these features are uncommon among other viruses. Although orthologs of these genes are codified by their hosts, one can hypothesize that having these translation-related genes might represent a gain of fitness during infection. Therefore, the aim of this study was to evaluate the expression of translation-related genes by mimivirus during infection of Acanthamoeba castellanii under different nutritional conditions. In silico analysis of amino acid usage revealed remarkable differences between the mimivirus isolates and the A. castellanii host. Relative expression analysis by quantitative PCR revealed that mimivirus was able to modulate the expression of eight viral translation-related genes according to the amoebal growth condition, with a higher induction of gene expression under starvation. Some mimivirus isolates presented differences in translation-related gene expression; notably, polymorphisms in the promoter regions correlated with these differences. Two mimivirus isolates did not encode the tryptophanyl-tRNA in their genomes, which may be linked with low conservation pressure based on amino acid usage analysis. Taken together, our data suggest that mimivirus can modulate the expression of translation-related genes in response to nutrient availability in the host cell, allowing the mimivirus to adapt to different hosts growing under different nutritional conditions.

Growing a giant: evaluation of the virological parameters for mimivirus production.

Acanthamoeba polyphaga mimivirus (APMV) was described in 2003, and due to its unique structural and genetic complexity, the viral family Mimiviridae was created. APMV prompted the creation of an open field of study on the function of hundreds of never-before-seen open reading frames (ORFs) and their roles in virus-host interactions. In recent years, several giant viruses have been isolated from different environments and specimens. Although the scientific community has experienced a remarkable advancement in the comprehension of the mimivirus replication cycle in the last years, few studies have been devoted to the investigation of the methodological features and conditions for mimivirus cultivation. In this work, conditions for the cultivation of mimivirus isolates were investigated to obtain relevant information about the production of infectious particles, total viral particles and viral DNA. The results suggest that low viral doses are more efficient for the production of infectious particles, yielding up to 5000 TCID50 for each inoculated TCID50. Besides methodological information, these data also reveal, for the first time, the ratio between total and infectious particles (in TCID50) that are produced during mimivirus cultivation in laboratory conditions. All of this information can be used as a worldwide guide for the production of mimiviruses and can help prompt mimivirological studies in different fields.

Detection of mimivirus genome and neutralizing antibodies in humans from Brazil

In recent years, giant viruses belonging to the family Mimiviridae have been proposed to be infectious agents in humans. In this work we provide evidence of mimivirus genome and neutralizing antibodies detection in humans.