Some of the greatest scientific discoveries have been accidental. To that list, Israeli scientists have added one more. They’ve discovered for the first time an instance of viruses leaving messages for other viruses.

What makes the discovery remarkable is that scientists expect such communication systems to exist among other kinds of viruses. If true, we’ll have one more route to attack devastating viruses, such as HIV or herpes.

The search began when Rotem Sorek of Weizmann Institute of Science and his colleagues were looking for “bacterial chatter.” They were studying how viruses attacked a bacterial species called Bacillus subtilis and they knew that, under certain situations, these bacteria communicate. The phenomenon, called quorum sensing, helps bacteria control their behavior based on the number of other bacteria around. It’s crucial in deciding, say, when a pathogenic bacteria decides to launch an attack on its host which could develop into a disease. What they found, however, was that the viruses were chattering too.

Viruses attack bacteria in two ways. Most of the time they enter the bacterial cell and take over its machinery to multiply until the cell explodes and dies. Sometimes, however, they simply inject their genome into the bacteria, waiting for an environmental cue to reawaken and multiply later.

Sorek’s hypothesis was that, given how frequently viruses attack B. subtilis, perhaps the bacteria have developed a way of warning others when an attack begins. So he took a virus called phi3T and added that to a flask full of B. subtilis. As expected, viruses killed the bacteria in large numbers.

To find if there was any chemical signaling going on, Sorek filtered the mixture to remove bacteria and viruses, leaving behind only proteins. He then added the protein mixture to a fresh flask containing B. subtilis. To his surprise, this time when he added phi3T viruses to that flask, the virus did something else. Instead of killing its hosts like last time, it slipped a part of its genome into the bacteria.

That means, instead of bacteria talking to each other, something in the protein mixture had made viruses change their mode of attack. Sorek had a mystery to solve.

After more than two years of searching through the protein mess, Sorek reports in the journal Nature that his team has found the protein that viruses used to communicate. His team has called the protein arbitrium, which is Latin for “decision.”

Sorek believes that when the levels of arbitrium build up, viruses switch their strategy from killing their host cells to injecting their genome. “It does make a lot of sense,” says Peter Fineran of the University of Otago told Nature. “If the phage is running out of hosts, it would try and limit its destruction, and sit quiet and wait for the host to re-establish growth.”

What’s more intriguing, however, is that Sorek found signs of many more types of arbitrium-like proteins. When phi3T viruses inject their genomes into B. subtilis bacteria, they also copy the DNA coded needed to produce arbitrium. When Sorek looked at genomes of other Bacillus bacteria, he found more than 100 unique DNA-snippets that were similar to the arbitrium DNA snippet. Thus, it’s quite likely that other viruses also use chemical signaling to communicate messages.

Even though viruses are the most primitive form of life, they infect and harm millions of people every year. The possibility of tapping into viral communication has many scientists excited, because it offers new ways to build drugs that could defeat viruses.

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