Above: Image © istockphoto.com/mstay

Did you know? A nanometre (nm) is one billionth of a metre. A micrometre (µm) is 1000 nm, or one millionth of a metre.Yes, you heard that right! Although all viruses are microscopic, some are much larger than others. Recently, researchers identified viruses measuring more than 400 nanometres (nm) in diameter. By way of comparison, the human immunodeficiency virus (HIV) measures 120 nm and the hepatitis B virus (HBV) measures only 42 nm. As with many other important scientific discoveries, these large viruses were found purely by accident.

Virologists on the hunt for new viruses normally filter their samples. Since viruses are known to be tiny, they are expected to pass through the filter. Larger particles, such as bacteria, are expected to be trapped in the filter. The filtration process is a bit like rinsing vegetables in a colander. The smaller dirt particles pass through, but the vegetables remain where they are.

After filtering a sample, researchers investigating a 1992 pneumonia outbreak identified a what they thought was a bacterium using gram screening, a test used to differentiate between different classes of microorganisms. They gave it the bacterial name “Bradfordcoccus”, after the location of the outbreak.

The nanoscale

Having trouble imagining what a billionth or a millionth of a metre looks like?

This webpage from the US National Nanotechnology Initiave provides some useful comparisons, as well as a link to an online model that lets you zoom in and out from the smallest particle to the entire universe:

www.nano.gov/nanotech-101/what/nano-size

It took scientists over a decade to realize that Bradfordcoccus was actually a giant virus. Since it had been caught in their filter, the original researchers had assumed it was too large to be a virus. It was not until 2003 that a team of French researchers carrying out further tests on this particular microorganism correctly identified it as a virus with a diameter of 600 nm. It was renamed Mimivirus.

Did you know? Pithovirus sibericum, a giant virus isolated from a sample found in the Siberian permafrost, was named after the large ancient storage containers called pithoi. More discoveries of new giant viruses soon followed. Megavirus, measuring about 440nm in diameter and surrounded by thick mesh-like capsular material, was discovered in 2011 off the coast of Chile. Pandoraviruses are even larger, with a length of about 1 micrometre (µm).

Mimivirus, as seen under an electron microscope. Click image to enlarge (Wikimedia Commons/E. Ghigo et al.)

The largest virus discovered to date is Pithovirus, which has a length of 1.5 µm and a diameter of 0.5 µm. Believe it or not, it was isolated in early 2014 from a 30,000-year-old sample of permafrost found in Siberia! So not only are giant viruses more common than previously thought, they’ve also been around for a long time. They now have their very own classification group: nucleocytoplasmic large DNA viruses (NCLDV).

But what is special about these viruses? Is it just their size? Or do they have other unique characteristics?

On the one hand, there is no clear evidence that giant viruses are linked to the spread of infectious diseases in humans. In fact, they mainly infect amoebas. Although Mimivirus was first isolated from pneumonia samples, it does not appear to play a role in pneumonia infection.

Did you know? The word “serendipity” refers to the phenomenon of stumbling upon good things when you weren’t looking for them. Examples of serendipitous discoveries in science include penicillin, quinine, and virophages.On the other hand, the study of giant viruses has raised a host of new questions on the evolution of viruses. Studies on the genomes of giant viruses have identified an especially large number of potential protein coding genes. For example, Mimivirus contains approximately 911 potential protein-coding sequences and Pandoravirus contains 2556. A virus like HIV contains only 12.

More surprisingly, researchers have found gene sequences in giant viruses that are similar to genes that code for proteins involved in DNA repair, replication, translation, and metabolism. Usually, a virus depends on its host for processes like replication. So these discoveries challenge scientists’ traditional understanding of what makes a virus a virus.

Further study of these giant viruses will likely force scientists to redefine viruses and their role in evolutionary history. For example, there is debate about whether a fourth domain of life containing giant viruses should be added to the current three (eukaryotes, archaea, and bacteria).

These debates provide an excellent illustration of how our understanding of the natural world is a dynamic process. As the unknown is explored and new discoveries are made, categories and definitions continually need to be adjusted.

References

General information

Ancient "Giant Virus" Revived From Siberian Permafrost (Stefan Sirucek, National Geographic)
Discovery of the Giant Mimivirus (David Wessner, Nature Education)
Giant Viruses (James L. Van Etten, American Scientist)
Giant viruses open Pandora's box (Ed Yong, Nature)
New Giant Viruses Break Records (Jef Akst, The Scientist)
Viruses Reconsidered (Didier Raoult, The Scientist)

Scholarly publications

Arslan D, Legendre M, Seltzer V, Abergel C, Claverie JM. 2011. Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae. Proceedings of the National Academy of Sciences of the United States of America. 108(42):17486-17491.

Gokul Rajan

Gokul lives in New Delhi, India. He is a Masters Student in the Department of Biosciences & Bioengineering at the Indian Institute of Technology (IIT)- Bombay. Apart from Science, he enjoys travelling, reading and music. He is an athlete too, and loves cycling and running

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