The growth in an ecosystem’s microbial population isn’t necessarily accompanied by a rise in the number of viruses who infect and destroy it, a new study has revealed. The study, published yesterday in Nature, challenges the widely held view that the greater the density of microbes in a given area, the higher the number of viruses infecting and killing off the population, the so-called ‘kill-the-winner’ model. The study was conducted by an international research team, which includes UvA ecologist Mark Vermeij.
Microbial population explosions can take on many forms and can include, for example, algal blooms in oceans and lakes, fungal blights in soil and bacterial infection in humans. Such explosions generally coincide with a rise in the number of viruses, a phenomenon that has long interested ecologists. For decades, most researchers have assumed that during microbial population booms, their viruses take advantage of the opportunity to multiply by killing the abundant microbial winners. A recent study, however, suggests that under certain conditions viruses can change their infection strategy by forgoing rapid replication and instead opting to reside peaceably inside their host microbes. This alternative model is referred to as the ‘piggyback-the-winner’ model.
For their study, the researchers decided to put this alternative model to the test by collecting and analyzing samples of microbe-rich seawater for the abundance and nature of both microbes and the viruses that infect them. What they found was that as microbial abundance increased, the virus-to-microbe ratio decreased significantly. Exploring this phenomenon further, the researchers used metagenomic analysis to determine whether the viruses in the sample showed virulent, predatory traits or the hallmarks of non-predatory lifestyles. Intriguingly, they found that in samples with a higher microbe count, viral communities became less virulent. These findings run counter to the ‘kill-the-winner’ model, in which one would expect to find more viruses per microbe in samples with a high microbial density and growth rates.
According to the researchers, a possible explanation for their surprising discovery might be that instead of multiplying and killing off their booming host population, more of the viruses instead integrate themselves into their host. They hereby replicate more slowly, but also avoid competing with other viruses and having to navigate with the host’s own immunity defenses.
B. Knowles, C.B. Silveira, B.A. Bailey, M.J.A. Vermeij, et al.: 'Lytic to Temperate Switching of Viral Communities' in Nature (16 March 2016). Doi:10.1038/nature17193