They can go unnoticed, silently infiltrating the body’s defenses, or go on the attack, coming out of hiding and firing in all directions. Viral attacks are almost always suicide missions, destroying the cell the virus has been depending on. The attack can only be successful if there are enough other healthy cells to infect. If the barrage of viral particles doesn’t hit anything, the virus can’t sustain itself. It doesn’t die, since viruses aren’t technically alive, but it stops working.

So for a virus, the key challenge is deciding when to go from cool down to kill mode.

Four years ago, Princeton biologist Bonnie Bassler and her then-graduate student Justin Silpe discovered that a virus has one key advantage: It can eavesdrop on communication between bacteria. Specifically, listen for the “We have a quorum!” chemical substance released by bacterial cells when they have reached a critical number for their own purposes. (The original discovery of this bacterial communication process, called quorum sensing, has led to a number of awards for Bassler and her colleagues.)

Now Bassler, Silpe and their research colleagues have discovered that dozens of viruses respond to quorum sensing or other chemical signals from bacteria. His work appears in the current issue of Nature.

“The world is full of viruses that can police the host for proper information,” said Bassler, Princeton’s Squibb Professor of Molecular Biology and chair of the department of molecular biology. “We don’t know what all the stimuli are, but we show in this paper that this is a common mechanism.”

They not only demonstrated the abundance of the strategy, but also discovered tools that control it and send signals that tell viruses to switch from cold mode to lethal mode.

The type of virus that attacks bacterial cells, known as bacteriophages, or phages for short, land on the surface of a bacterial cell and deliver their genes into the cell. More than one type of phage can infect a bacterium at the same time, as long as they are all in cold mode, which biologists call lysogeny. When it comes to multiple phages chilling in a single bacterium, it is called polylysogeny.

In polylysogeny, phages can coexist, allowing the cell to copy itself over and over again as healthy cells do, the viral DNA or RNA hidden inside the bacterium replicating along with the cells.

But the infiltrating phages aren’t exactly peaceful; it’s more like mutual assured destruction. And the tenuous détente lasts only until something activates one or more of the phage to switch to kill mode.

Scientists who study phage warfare have long known that a major disruption to the system, such as a high dose of ultraviolet radiation, cancer-causing chemicals, or even some chemotherapy drugs, can cause all resident phages to go into mode. of death.

At that point, the scientists thought, the phages begin to race for the bacterium’s resources, and the fastest phage will win, firing off its own viral particles.

But that’s not what Bassler’s team found.

Grace Johnson, a postdoctoral research associate in Bassler’s research group, used high-resolution imaging to observe individual bacterial cells that were infected with two phages while flooding them with one of these universal kill signals.

Both phages went into action and destroyed the host cell. To see the result, Johnson “painted” the genes of each phage with special fluorescent tags that light up in different colors depending on which phage was replicating.

When they turned on, he was surprised to see that there was no clear winner. It wasn’t even a draw between the two. Instead, he saw that some bacteria glowed with one color, others with the second color, and still others were a mixture, simultaneously producing both phages at the same time.

“No one imagined that there would be three subpopulations,” Bassler said.

“That was a really exciting day,” Johnson said. “I could see the different cells doing all the possible combinations of phage production: inducing one of the phages, inducing the other, inducing both. And some of the cells were inducing neither phage.”

Another challenge was finding a way to activate only one of the two phages at a time.

Silpe, who had returned to Bassler’s lab as a postdoctoral research associate after doing postdoctoral studies at Harvard, had taken the lead in finding the triggers. While the team does not yet know what signals these phages respond to in nature, Silpe has designed an artificial chemical trigger specific to each phage. Grace Beggs, another postdoctoral fellow in Bassler’s group, was instrumental in molecular analyzes of artificial systems.

When Silpe exposed polylysogenic cells to his signal, only the phage that responded to his artificial trigger replicated, and in all cells. The other phage remained completely in cold mode.

“I didn’t think it would work,” he said. “I was hoping that because my strategy didn’t mimic the authentic process found in nature, both phages would replicate. It was a surprise that we saw only one phage. No one had done that before, to my knowledge.”

“I don’t think anyone even thought to ask a question about how phage-phage warfare occurs in a single cell because they didn’t think they could, until Grace J. and Justin did their experiment,” Bassler said. “Bacteria are very small. It’s hard to image even individual bacteria, and it’s really, In fact it is difficult to image phage genes within bacteria. We’re talking smaller than small.”

Johnson had been adapting the imaging platform (fluorescence in situ hybridization, often called FISH) for another quorum sensing project involving biofilms, but when he heard Silpe share his research in a group meeting, he realized that FISH could reveal what he was doing. that point were intractable secrets about spying phages from him.

Most of the world’s bacteria have more than one phage cooling inside of them, “but no one has been able to manipulate or image them like these two did,” Bassler said. “The clever strategy where they could induce one phage, the other phage, or both phages on demand, that was Justin’s coup, and then be able to see it actually happen in a single cell? That’s never been done either. That was Grace J. We can see phage warfare at the level of a single cell.”

Almost all of the genes in viral genomes remain mysterious, Bassler added. We just don’t know what most viral genes do.

“Yes, here we uncover the functions of some phage genes, and show that their jobs are to enable this completely unexpected cold-kill switch and that the switch dictates which phage wins during phage-phage warfare. That discovery suggests that there are potentially still processes left.” exciting things yet to be found,” he said. “Phages ushered in the era of molecular biology 70 years ago and are coming back into vogue as therapies and also as this incredible repository of molecular tricks that have unfolded over evolutionary time. It’s a treasure trove and it’s almost completely unexplored.”