What is a virus?
We used to think we knew. Before we discovered giant viruses, size was central to the definition of a virus: 0.3 micrometer filters were used to isolate microorganisms, and anything smaller than that which was infectious, was a virus.
So giant viruses were a bit of a surprise?
We discovered them by mistake. No biologist studying viruses could have discovered them, because the first thing they did was to discard anything that wasn’t filtered. We were working on intracellular parasitic bacteria when a 10-year old water sample arrived at our laboratory. It was thought to have been the cause of a pneumonia outbreak, but nobody had yet been able to isolate the microorganism responsible. Someone at our lab had the idea to use electron microscopy and there it was! It was infecting an amoeba but it didn’t look like a bacterium, although it was really big.
How big are giant viruses?
They can be 30 times bigger than the average virus, up to 750 nm. You can even see them with a light microscope! And their genome is larger than that of many bacteria. When we sequenced the mimivirus we were also surprised to see it was genetically very complex. It has 1018 genes, including many related to translation. These are not supposed to be present in viruses, since viruses use the host machinery to translate their genome into proteins.
That must have been a shock.
It was revolutionary, and it raised a lot of questions. With such complexity, could we still say that viruses are not alive? Recently researchers have found the first virus that can be infected by another virus – I say, if you can get sick, you are very definitely alive!
How do giant viruses change our view of evolution?
They are very old and the genomic analysis of marine viruses shows that they are very common and probably major players in the ecology. Although they are different from other viruses, if one analyses their evolutionary relationships, giant viruses fit perfectly with other viruses. However, they also have things in common with bacteria, such as the structure of the capsid, as well as with eukaryotes.
They are a real mix then?
My controversial theory is that DNA was invented by viruses to protect themselves from their host, an RNA cell. Slowly, the cell took that DNA machinery and incorporated it, and the nucleus was formed. At the same time, the viruses started to lose some of those genes that the cell had incorporated. After several cycles of infection, the cell gradually increased its complexity and the viruses reduced their genome.
Have we seen it all?
I think we will find bigger viruses, with more translation genes. One day we will have a real problem defining a virus!