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

This week’s paper can be accessed here:

In 2011 Scientists believed they had finally found the largest virus in existence, Megavirus chilensis ( 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: 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?

The paper up for discussion next time is Staphylococcus aureus small colony variants are susceptible to light activated antimicrobial agents and is available from the following link



Antibiotic therapy can select for small colony variants of Staphylococcus aureus that are more resistant to antibiotics and can result in persistent infections, necessitating the development of more effective antimicrobial strategies to combat small colony variant infections. Photodynamic therapy is an alternative treatment approach which utilises light in combination with a light-activated antimicrobial agent to kill bacteria via a non-specific mechanism of action. In this study, we investigated whether the combination of 665 nm laser light and the light-activated antimicrobial agent methylene blue was able to successfully kill S. aureus small colony variants. S. aureus and isogenic stable small colony variant were exposed to varying doses (1.93 to 9.65 J/cm2) of 665 nm laser light in the presence of varying concentrations (1 to 20 μM) of methylene blue. dynamic therapy


The combination of 665 nm laser light and methylene blue was found to be an effective strategy for the killing of small colony variants. At the highest light dose (9.65 J/cm2) and methylene blue concentration (20 μM) tested, the number of viable bacteria decreased by approximately 6.9 log10 for the wild type and approximately 5 log10 for the small colony variant.


These results suggest that photodynamic therapy has potential for use in the treatment of superficial infections caused by small colony variants of S. aureus and supports further research in this field.

Discussion points
1. Views on the paper and experimental design?
2. Use of photodynamic therapy as treatment
3. Can this therapy lead to a reduction in resistant bacteria?
4. Are there other alternatives to antibiotic therapy?
5. What should be done next?