While the conventional "kill-the-winner" biological model predicts that viruses grow in number along with the density of microbes, helping it kill more and more microbes and keeping their population in check, a new study by San Diego State University researchers conflicts with this viewpoint. In the new study, the team examined virus-host dynamics near coral reefs and revealed that viruses have the ability to change their infection strategy from the standard kill-the-winner model.
The team found that when microbes increase in density, some viruses choose to inhabit their host peacefully, letting it live and ultimately reducing virus numbers. This new model has been named "piggyback-the-winner" and could have implications on the future research of phage-based medicine and ecosystem resilience.
"Kill-the-winner seems to make sense," said Ben Knowles, lead author of the study. "The logic behind it has been around for a while. The reasoning is very seductive."
Knowles and his team decided to put this model to the test and started by collecting samples of microbe-rich seawater. After gathering these samples from areas near coral reefs in both the Pacific and Atlantic Ocean, they used microscopic and genomic techniques to determine the characteristics of the microbes and viruses that infect them.
The team found - contrary to the kill-the-winner model - that as microbial density increased, the ratio of viruses to microbes significantly decreased. Intrigued at these findings, they proceeded to run another experiment that involved the monitoring of incubated seawater from a pristine coral reef for several days. They found the same results as those gained from their field sampling, suggesting the existence of a piggyback-the-winner model.
Further metagenomic analyses revealed that that viruses indeed become less virulent in samples with a higher microbe count. Instead of killing off the host population through aggressive multiplication, the viruses instead lay dormant in their hosts, replicating slower and avoiding competition with other viruses and eliminating the need to deal with their host's immune system.
The new piggyback-the-winner model offers a better explanation for virus-host dynamics during periods of fast microbial growth than the kill-the-winner model and also holds promise for improving human health through therapies for conditions such as cystic fibrosis.
"When you have a fast-growing host, if you're a virus, you profit more from integration," Knowles said. "It's just intelligent parasitism."
The findings were published in the March 16 issue of the journal Nature.
