Centro de Investigación en Enfermedades Tropicales, Facultad de Microbiología, Universidad de Costa Rica. San José, Costa Rica; libia.herrero@ucr.ac.cr
Viruses are known to be abundant, ubiquitous, and to play a very important role in the health and evolution of life organisms. However, most biologists have considered them as entities separate from the realm of life and acting merely as mechanical artifacts that can exchange genes between different organisms. This article reviews some definitions of life organisms to determine if viruses adjust to them, and additionally, considers new discoveries to challenge the present definition of viruses. Definitions of life organisms have been revised in order to validate how viruses fit into them. Viral factories are discussed since these mini-organelles are a good example of the complexity of viral infection, not as a mechanical usurpation of cell structures, but as a driving force leading to the reorganization and modification of cell structures by viral and cell enzymes. New discoveries such as the Mimivirus, its virophage and viruses that produce filamentous tails when outside of their host cell, have stimulated the scientific community to analyze the current definition of viruses. One way to be free for innovation is to learn from life, without rigid mental structures or tied to the past, in order to understand in an integrated view the new discoveries that will be unfolded in future research. Life processes must be looked from the complexity and trans-disciplinarity perspective that includes and accepts the temporality of the active processes of life organisms, their interdependency and interrelation among them and their environment. New insights must be found to redefine life organisms, especially viruses, which still are defined using the same concepts and knowledge of the fifties. Rev. Biol. Trop. 59 (3): 993-998. Epub 2011 September 01.
Key words: life organisms, viruses, definitions, new discoveries, challenges, historical views.
Resumen
Los virus son ]]>
Palabras clave: ]]>
“Viruses are Viruses”, Lwoff made the most famous definition of viruses in 1957 (Lwoff 1957). Viruses are now known to be abundant, ubiquitous, and to play a very ]]>
et al. 2005, Sullivan et al. 2005, Suttle 2005) have revealed the importance of marine viruses in oceanographic processes, since they are the most abundant and diverse “life forms” in the ocean. They are the major pathogens of planktonic organisms and consequently they are significant players in nutrient and energy ]]>
et al. 2005, Raoult & Forterre 2008). One might consider that these statements are in conflict; for this reason, viral characteristics, functions and their intimacy with the ]]>
Lwoff in 1957 wrote that the definition of viruses is somewhat arbitrary, and many definitions have been offered, in itself a proof of the difficulty of the task. He proposed to define viruses as strictly intracellular and potentially ]]>
i.e. a system of enzymes for the production of energy). In his definition, he stresses the noncellular nature of viruses, their dependence on host-cell metabolism and the fact that a specific material of a virus is reduced to an element of genetic material, its nucleic acid. ]]>
At that time, viral enzymes had not being discovered with the exception of the neuraminidase of Influenza virus (Gottschalk 1957) and it took a few more years until other viral enzymes were discovered (Baltimore & Franklin 1963, Baltimore 1970, Temin & Mizutani 1970). This situation greatly influenced how viruses were defined, since if viruses lacked enzymes it ]]>
Intracellular parasitism of viruses is shared with other parasites, including bacteria, fungi and protozoa, but the definitions of viruses stress the intimate nature of viral parasitism, which may be called parasitism at the genetic level. All other parasites have a cellular organization, their need to ]]>
et al. 1978).
Lwoff (1957) defined living organisms as independent units of integrated and interdependent structures and functions. Hence, he regarded viruses as ]]>
A few decades later, Harold (1986) stated that living things differ from non-living things in their ]]>
et al. 2005, Knoops et al. 2008).
Morowitz (1968) ]]>
Characteristics that viruses do not ]]>
The theory of Autopoiesis by ]]>
et al. 2005, Knoops et al. 2008).
Viruses rely on the host cell ]]>
et al. 2007, Miller & Krijnse-Locker 2008).
Viral factories are ]]>
et al. 2007, Miller & Krijnse-Locker 2008).
]]>
Viral factories are very dynamic structures, in which viral and cell factors move in and out depending on the step of the viral life cycle. In viral factories, mitochondria seem to provide the necessary energy for virus morphogenetic processes (Kirkegaard & Jackson 2005, Novoa et al. 2005, Kopek et al. 2007, Knoops et al. 2008). ]]>
All these processes indicate that viruses have evolved elaborate strategies to modify cellular mechanisms that are involved in vesiculation and transport for their own purposes. Although some cellular-interaction partners have been identified, a detailed understanding of the molecular mechanisms of membrane remodeling by viruses is still lacking (Kopek et al. 2007). Non-structural viral proteins can induce alterations of ]]>
These are just a few examples of how viruses interfere with very important cellular processes and therefore become an intrinsic part of the cell since they can modify cells and lead them to their death, permanently invade the cell genome, induce a persistent and a latent infection and their presence ]]>
Raoult et al. (2004) described a ]]>
et al. (2006) have recently proposed a new classification of ]]>
Häring and ]]>
There are several examples of ]]>
et al. 2004), or at the initial steps of infection (Ackermann & Bamford 2000) and they are triggered on the host-cell surface concurrently with virus budding or adsorption respectively.
Rice
et al. (2004) described the structure of a thermophilic archaeal virus with a doublestranded DNA and viral capsid that spans all domains of life. By comparing the tertiary and quaternary structures of the coat protein of this virus with those of bacterial and an animal virus, they concluded that some viruses have a common ancestor that precedes the division into the three domains of life more than 3 billion years ago. The recent discovery that the virus factory of Mimivirus can be infected by another virus (Sputnik) has also been taken as an argument in favor of ]]>
et al. 2008, Pearson 2008).
Forterre (2010) suggested to define life (an historical process) as a mode of existence of ribosome encoding organisms (cells) and capsid encoding organisms (viruses) and their ancestors; he also concluded that infected eukaryotic cells in which viral factories have taken control of the ]]>
Studies on marine viruses (Lindell ]]>
et al. 2005, Sullivan et al. 2005, Suttle 2005) have brought new discoveries, new mechanisms, an abundant genetic richness; the more research is carried out in unexplored sites and organisms, the more new viruses will be described with new characteristics that will continue to challenge viral definitions.
Conclusions
The replication of a virus is not mechanical; instead viruses have evolved to reorganize, coordinate, manipulate, and modify cell structures and processes to their own benefit in order to replicate, evolve and perpetuate. Viruses have developed the necessary enzymatic machinery to reorganize cell structures to replicate themselves and perpetuate their own kind.
Many studies (Villarreal 2004, Lindell et al. 2005, Sullivan et al. 2005, Suttle 2005) suggest that viruses are the masters of evolution and innovation in the web of life. They are the simplest organisms with the capacity of using the lowest energy to conserve life and its diversity. Life has existed for millions of years, and humans depend on newer and more sophisticated technologies to unravel life processes and mechanisms. Thus they should open their minds to the new discoveries and try to innovate definitions, according to the complexity of life itself and not from an anthropocentric point of view.
Forterre’s (2010) conclusion that infected eukaryotic cells in which viral factories have taken control of the cellular machinery became viruses themselves, is an affirmation that places itself in the center of the modern paradigm in which the analysis and study of life processes must be looked from the complexity and transdisciplinarity perspective (HerreroUribe 2008) since the analysis includes and accepts the temporality of the active processes of life organisms, their interdependency, and interrelation among themselves and with their environment.
One way to be free for innovation is to learn from life, without rigid mental structures in order to understand in an integrated view the new discoveries that will be unfolded in future research. New insights must be taken to redefine life organisms; especially viruses, which still are defined using the same concepts and knowledge of the fifties.
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Acknowledgments
The author is deeply grateful to Anne-Lise Haenni and Laya Hun Opfer for their invaluable support.
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Correspondencia a: Libia Herrero-Uribe. Centro de Investigación en Enfermedades Tropicales, Facultad de Microbiología, Universidad de Costa Rica. San José, Costa Rica; libia.herrero@ucr.ac.cr
Received 26-I-2011. Corrected 15-II-2011. Accepted 03-III-2011.