Viruses are ancient, and despite not technically being “alive,” they’ve managed to evolve to thrive in a remarkably wide range of organisms. They’re effective killers, partly because they leave just enough hosts alive in order for them to continually replicate.
A new study in Nature has revealed that, in microbe-filled environments such as coral reefs, viruses often adopt a new infection strategy. Instead of continually replicating within and ultimately killing their hosts, they sometimes go into stealth mode, becoming dormant and non-harmful. This means they can spend time slowly integrating into the microbial community before killing again later on.
Viruses that infect bacterial hosts are known as phages, and they often “reproduce” by replicating within host cells. By producing copies of themselves, they cause the host cells to burst, destroying them. In order to maintain a population of phages, this lifestyle – known as lysis – requires that at least one of these new virus copies finds another cell to infect before it becomes inactive.
Generally speaking, these phages increase in number alongside the lifeforms they’re infecting. They can’t be too effective at replicating, or they’d kill off all their hosts, so they maintain a replication rate that’s just enough to keep up their numbers whilst continually making copies of themselves. This strategy is known as “kill-the-winner,” with the winners being the host microbes and the killers being the phages.
“Kill-the-winner seems to make sense,” said Ben Knowles, a viral ecologist at San Diego State University (SDSU) and the study's lead author, in a statement. “The reasoning is very seductive.”
An electron micrograph of phages attached to a bacterial host cell. Dr. Graham Beards/Wikimedia Commons; CC BY-SA 3.0
It’s long been known that viruses can enter dormant phases where they stop infecting other cells and instead quietly reside within them, biding their time. However, it was always assumed that during a sudden uptick in the population of the host microbe – say, in an algal bloom within marine environments, including coral reefs – the phages would take advantage of this by rapidly multiplying and killing the new microbes off as they do so.
This new study suggests that this isn’t the case. Collecting samples of microbe-rich seawater near Pacific and Atlantic coral reefs, the team at SDSU noticed that during host microbe population explosions, the virus-to-microbe ratio significantly decreased.
In order to confirm this seemingly counterintuitive finding, the researchers incubated seawater from a healthy coral reef location for several days while carefully monitoring both the microbial and viral abundance. Once again, during a microbial population spike, the virus numbers remained curiously low. By looking at the spread of viral genetic material, they confirmed that their lysis lifestyle was effectively being suppressed.
So why weren’t they killing the winners? The researchers note that by remaining dormant, the phages didn’t have to compete with other viruses for host cells, and they didn’t have to continuously navigate the host’s immunity defenses. Most importantly, they increased their numbers by stealth, replicating automatically as their host microbial cells naturally divided and multiplied. This means the phages replicate slowly, but their population would ultimately increase over time.
This behavior, technically known as lysogeny, is referred to by the team as “piggyback-the-winner,” as they’re piggybacking on their hosts’ success. In rapidly growing microbial communities, it might be more advantageous than simply killing the winners.