Characteristics and Host Organisms

By: Rebecca Kaplan

May 20, 2010


There are numerous species in the world that have yet to be discovered and many mysteries about known species that are waiting to be uncovered. Recently, scientists discovered a new virus, known as the Mimivirus. Research regarding the Mimivirus revealed a series of unique qualities inhabited by the species. It is known for being the largest virus ever identified. This is just one of the many characteristics obtained by the virus, those of which I will be elaborating on further. I will also be discussing possible host organisms. While only one host organism is definitely linked to the virus, it is possible that it has a broader host range, which may include humans.


The Mimivirus has been discovered fairly recently. Because of this, there is not much widespread information about it. To change that, our objective is to provide information about this mysterious virus. The virus is of particular interest because of its many unique characteristics. In addition to its remarkably large structure, the Mimivirus obtains many qualities that differentiate it from other types of viruses. Scientists have discovered many new things while researching the Mimivirus. I would like to discuss this new virus, the new findings it has brought about, as well as some questions that have yet to be answered.

Discussion: In 1992, Bradford, England had a pneumonia outbreak. To locate the source of the problem, researchers set out to find Legionella, a parasitic bacterium known to cause pneumonia. Instead, Timothy Rowbotham found what was described at the time as a new gram-positive cocci bacterium within an amoeba located in a cooling tower (Van Etten, 90). This became known as Bradfordcoccus, named after the town in which it was discovered. After a few failed tests involving bacterial primers, the sample of the new species was put aside for about ten years. In 2003, Dr.Richard Birtle brought the sample to France, where he viewed an infected Acanthamoeba polyphaga under electron microscopy (91). This revealed that what was known as Bradfordcoccus was not a bacterial species, but a giant virus. Its icosahedral symmetry along with the observation of an eclipse phase during the species replication stage as well as many genomic similarities led researchers to categorize it as a nucleocytoplasmic large DNA virus. It was then named Acanthamoeba polyphaga Mimivirus, which was classified under the new Family Mimiviridae (91).

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The name Mimivirus stands formicrobe mimicking” virus (Pommerville, 446). This is because it has many characteristics similar to single-celled organisms, which is why it was originally falsely identified as one. The most prominent characteristic that links this virus to microbes is its great size. The Mimivirus is the largest virus ever known. The size of its icosahedral capsid is approximately 400nm in diameter (Shors 71). In addition, the capsid has protein filaments extending from it that range from 200nm to 400nm in length. Thus, the virus’s full particle size ranges from approximately 600nm-800nm (Bright Hub 2010). This is larger than any of the nucleocytoplasmic large DNA viruses that the Mimivirus is grouped with. In fact, it is relative to the size of a bacterium. The Mimivirus does not have an outer viral envelope, but it believed to contain an internal lipid layer. When observed under electron microscopy, scientists described “three dense layers of matter,” which was then interpreted to be “two successive 4 nm thick lipid membranes inside a protein shell approximately 7 nm thick” (MicrobiologyBytes 2007).
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“Stargate” is the term used to describe the star-shaped structure that was examined under transmission electron microscopy (Zauberman). It is “centered at a single icosahedral vertex” where the five planes of the icoshedral capsid are said to open, resulting in the fusion of membranes (Zauberman). Thus forming the stargate, which is then further described as a “membrane tube,” that releases the viral genome into the host organism (Zauberman).
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The Mimivirus is a double-stranded DNA virus whose only known host organisms are of the
Acanthamoeba genus (Sadave, Heller, Hillis, Berenbaum 556). Since the Mimivirus was first located during a pneumonia outbreak, researchers are trying to determine if there is a link between the virus and the condition. There was a study done by Bernard La Scola that found the Mimivirus in Canadian patients diagnosed with pneumonia (Shors 71). However, another study showed no signs of the Mimivirus in young patients in an Austrian hospital (Etten 95). The CDC study also showed no presence of the virus in its population of nearly 500 patients (95). Yet again, the virus infected a lab technician and his body reacted with the viral proteins (95). Still, this is not being considered a determining link between the virus and pneumonia. To further test the theory of relation, lab tests were performed by inoculating the virus into mice. The mice reacted with definite symptoms of the condition, but there were no viral cells found in the lung tissue of the mice (95). Therefore no link was concluded. Because it has been found in humans, and there is still a possible link between the Mimivirus and pneumonia, scientists have classified the Mimivirus as a biosafety class II pathogen (95).
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DNA sequences were viewed from numerous samples of ocean waters from all over the world. This showed that, even though they have yet to be sampled, relatives of the Mimivirus can be found widely spread throughout water environments all across the world (Claverie, Abergel).
In France, a new strand of
Mimiviridae was discovered in a cooling tower. The Mamavirus shares many qualities with the Mimivirus, including its large genome and 75%-100% of the same amino acids (Claverie, Abergel). This new discovery also led researchers to reveal a previously unknown satellite virus, appropriately named Sputnik, which was found within the Mamavirus. This virus is the first satellite virus known to be related with a eukaryotic host (Claverie, Abergel). This dsDNA virus has an icosahedral caspid that is approximately 50nm large (Pommerville 446). This virus is of particular interest to scientists because it is the first known virophage, meaning it acts as a parasite to viruses. In this particular case the Sputnik interferes with replication of viruses within the Mimiviridae family, causing the viruses to “form abnormal virus particles” (446).
external image virus-virophage.jpgexternal image Sputnik_virion.jpg
The recent accidental discovery of the Mimivirus has resulted in an immense amount of research that has brought new and unique qualities of species to the attention of scientists. The discovery of the Mimivirus is a great achievement to researchers and has led, and will continue to lead, scientists to further discovery of the world around us.

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Mimivirus within an Ameoba. Digital image.
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