Hi again everyone, @Stewart_Barker here with #microtwjc 37.

This week’s paper can be accessed here: http://www.unomaha.edu/toxoplasma/docs/jc20130906.pdf

In 2011 Scientists believed they had finally found the largest virus in existence, Megavirus chilensis (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3198346/). This relative behemoth stands at 1.259 megabases, encoding 1120 proteins. For comparison, one of the smallest known bacteria Hodgkinia cicadicola (a symbiont of Cicadas) has a genome of only 144 kb (read the PLOS paper here: http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1000565). Viruses of this size were an unprecedented find – one that many thought would never be topped.

For today’s session, we will be looking at one of this year’s most interesting microbiology discoveries – the Pandoravirus. Two isolates are described in this paper, Pandoravirus salinus (2.5-2.77 mb genome) and Pandoravirus dulcis (1.9 mb) – from saltwater and freshwater respectively. The two Pandovirus genomes are the largest known to date, with P. salinus being at least double the size of M. chilensis. Further to this, most viruses are measured at the nanometre scale, whereas Pandoraviruses measure 1 um long.

These statistics aren’t even the most shocking. The biggest revelation comes with the similarity of Pandoravirus to other organisms. A shocking 93% of Pandoravirus genes resemble nothing currently known, making them practically alien in nature. Their lineage has not yet been traced, but their DNA polymerase can be grouped with other giant viruses. There is even controversial talk about a fourth domain of life! This paper opens a ‘Pandora’s Box’ of possibilities, with exciting future work possible.


  1. Is the paper easy to understand?
  2. Are the methods used suitable and correct?
  3. Do you think any larger viruses are possible?
  4. What is the impact of the ‘alien’ genome?
  5. What further work can be carried out?