Medical Discovery News
By Drs. David Niesel and Norbert Herzog 

From the ice, scientists hauled a monster of unimaginable size. It was larger than any of its kind, and it was alive. Luckily, it wasn’t the Yeti, but Pithovirus sibericum, an abominable snow virus of sorts.

P. sibericum is the largest virus ever discovered. It’s about 1.5 micrometers, larger than some bacterium (a single-celled organism). All things considered though, it’s still microscopic – 1,333 copies of P. sibericum would fit on top of a pin. Luckily, this gigantic virus only infects amoebas, single-celled protozoans that live in bodies of water including lakes, ponds, streams, rivers and even puddles. Some amoebas are associated with diseases such as dysentery.

This newly discovered virus was named P. sibericum because it was found in a sample of permafrost from Siberia, hence the word sibericum. The scientists who discovered it were French, and they were inspired by its shape to call it a Pithovirus from the ancient Greek word pithos, which were large containers used to store wine. They estimate the virus had been in the deep freeze for at least 30,000 years before they resurrected it this year. In 2012, the French scientists also resurrected an ancient plant from fruits buried in the same Siberian permafrost, which led them to search for the virus.

P. sibericum is unique in many ways beyond its record-breaking size. It is oval-shaped with a thick wall and a hole in one end. It has a distinctive honeycomb structure that caps the opening. Most viruses tightly pack their genetic information inside, but P. sibericum has a surprisingly small genome for a virus that big. Viruses one-third its size store two to three times more DNA. Only about one-third of its proteins have any similarity to those of other viruses.

It is not, however, the first giant virus. Mimivirus was the first large virus ever found, reported in 2003. Previously, the record for largest virus went to Megavirus chilensis, which was found in water samples from Chile.

Just like Mimiviruses and Megaviruses, Pithoviruses are taken up by their amoebic hosts and once inside, they release their proteins and their genetic information. They then commandeer the host cell to produce hundreds of new viral particles, which are released when the host cell ruptures.

Interestingly, another giant virus called Marseillevirus also infects amoebas. Its genome contains a collection of genes found in similar viruses, bacteriophage viruses that infect bacteria, amoebas and cells from the animal, plant and fungus kingdoms. This suggests that amoebas may be acting as vessels for mixing genetic information from multiple forms of life. An amoeba could simultaneously be infected with Marseillevirus and bacteria, making it possible to produce complex genomes such as those of the giant viruses.

The resurrection of P. sibericum, a DNA virus long frozen in the now-thawing permafrost, has scientists wondering about undiscovered viruses that might be future threats to human or animal health.

Medical Discovery News is a weekly radio and print broadcast highlighting medical and scientific breakthroughs hosted by professor emeritus Norbert Herzog and professor David Niesel, biomedical scientists at the University of Texas Medical Branch at Galveston. Learn more at www.medicaldiscoverynews.com.