Viruses are obligate intracellular parasites that can infect all cellular life, including plants, animals, fungi, protozoa and bacteria. They cause many important human diseases, yet are vital components of ecosystems where they moderate the growth of many organisms.
Viruses are the most abundant biological entities, as indicated by the 100 million found in each millilitre of natural waters. Although they can have complex morphologies and replication cycles, viruses are often 100 times smaller than bacteria, the smallest cells. Their evolutionary origins are unclear, but they might have evolved from bacterial plasmids or from bacterial cells.
Discovery of a New Large Virus, Mimivirus, or APMV
During routine tests for Legionella bacteria on water from a cooling tower in Bradford, UK in 1992, a strange new type of virus was unknowingly isolated At first, due to its size, it was thought to be just another type of bacterium and was put into frozen storage. Subsequent investigations led to the discovery of the remarkable mimivirus (“mimi” from "mimicking microbe") described by Bernard La Scola and co-researchers in the March 28th 2003 Science. The virus is also known as APMV due to its ability to infect a type of protozoan called Acanthamoeba polyphaga, commonly found in cooling tower water.
APMV has the largest physical size (0.75 µm) and the largest genome (1.2 million base pairs) of any virus previously seen - larger than some bacteria. When stained, it can be visualized using a light microscope, an instrument unable to resolve almost all other viruses.
APMV was shown to have evolutionary connections with viruses infecting protists, types of phytoplankton found in marine environments, so it is likely that many more mimivirus strains will be found in the oceans. The unique features of mimivirus suggest that ancient DNA viruses could have given rise to the advanced eukaryotic cell types found in the plant and animal kingdoms. It is thought that APMV also might be a human pathogen, possibly causing pneumonia.
The Story of Large Viruses Unfolds: Mamavirus and its Satellite Virus, Sputnik
Recently, La Scola et alia published in the September 4th 2008 Nature the discovery of an even larger relative of mimivirus in a cooling tower in Paris. They named it mamavirus. This strain was shown to infect the protozoan microbe named Acanthamoeba castellanii. But remarkably, mamavirus is closely associated with another, much smaller, satellite virus that has been given the name Sputnik. And the tiny (50 nm) Sputnik actually infects the mamavirus! Sputnik alone is unable to infect and grow inside the Acanthamoeba cells. Instead it grows inside the mamavirus whilst it is inside its Acamthamoeba host.
Unlike mimivirus and mamavirus, Sputnik has no known homologues with other viruses and is likely to be a new taxonomical family. Because of its similar replication cycle to the bacteriophage (meaning bacteria-eater) it has been designated as the first discovered virophage.
Implications of The Discovery - Possible Evolutionary Precursors to Eukaryotes
There are many implications of the mimivirus, mamovirus and Sputnik story. Due to the size of mimivirus and mamavirus, which approach cellular dimensions, and the very large genomes, it is proposed that they could be evolutionary precursors to some of the eukaryotic cell lines. Their ability to infect protozoan cells might have consequences with respect to infection of human cells as they have important similarities.
The fact that the large viruses themselves can be infected supports the case for the classification of all viruses as living organisms, a suggestion that has caused great argument since their discovery in 1899. And Sputnik, as an infector of other viruses, could even be exploited in new treatments for human viral infections.
