Play Ground Games: Is Lysogeny Associated with High Host Abundance

1915, the year viruses that infect bacteria were discovered. Since then the amount of research into the later coined bacteriophages has blossomed; the most likely numerous biological ‘organism’ on this planet. It is now understood that these bacteriophages have two methods of reproduction, the lysogenic cycle and the lytic cycle. The lytic cycle always ends in the hosts death which can hinder further reproduction and remove energy and materials from the food web, so for a long time it has been thought that lysogeny ensured virus survival during times of low host abundance. A recent paper has challenged this idea and suggests a new strategy.

Knowles et al., 2016 set out to understand how temperate viruses influence microbial communities using four different strains of evidence-direct counts, meta-analysis of literature, experiments and viral community metagenomics. The meta-analysis of previous independent studies showed that viruses are less abundant at high host abundance so ‘more microbes, fewer viruses’ is more common than we previously believed. This was shown in a number of different ecosystems not just the marine environment. An example of this is the cystic fibrosis lung.  It should be noted however that care should be taken when interpreting results as there is no mention if collection techniques were controlled throughout the independent studies. Differences in sampling and sequencing could result in bias. Data was also recovered from graphs using computer software in some instances where abundances were not available. Metagenomes of separate viromes from the Pacific and Atlantic were sequenced showing that viral communities cluster geographically, with viruses being more consistent with temperate life cycles at higher host abundances. The authors found that the ratio between bacteriophage and host decrease at high host abundances, they hypothesize that this is due to lysogeny at high host abundance. This evidence leads the authors to use the term ‘piggyback-the-winner’ strategy.

The current ecological models predict that lytic bacteriophages dominate at high host abundance and the lysogenic bacteriophages dominate at low host abundance. The authors suggest that these models are changed to state that lysogenic bacteriophages are favoured at high host abundance, exploiting their hosts rather than killing them. Whilst the paper only provides a starting point for this new model, it shall be interesting to see how the model advances as more evidence is gathered to support it. Bacteriophages have significant implications on microbe ecology and evolution and this model will better enable us to predict how phages are affecting microbial communities. This in turn will enable us to better understand the global carbon cycle. 

Reviewed Paper: Thingstad, T. Frede and Gunnar Bratbak. "Microbial Oceanography: Viral Strategies At Sea". Nature 531.7595 (2016): 454-455. http://www.nature.com/nature/journal/v531/n7595/pdf/nature17193.pdf