The Eavesdropping Virus


Ed Yong wrote this fascinating article on this phenomenon called quorum sensing, “in which bacteria release molecules that indicate how many of their peers are around”. If you thought the bacteria are literally doing a head-count, you’d be wrong. Instead, each bacterium releases the signaling molecule. The concentration of the overall signal indicates the head-count. A derivative signal.

But why do bacteria signal at all? It allows them to sit dormant until enough of them are around and only then launch certain actions. Like an infectious attack. And if the concentration of the signal becomes too high, then they switch from infect mode to scatter mode. This explains why diseases like cholera are so problematic. They wait and wait and spread only when the numbers are right.

Fascinating, right? Next up is the virus. Turns out viruses too can do the quorum sensing of the bacteria’s signaling molecule. Just as the bacteria infers its own head-count via the concentration, so too does the virus.

Why does the virus care about the bacteria head-count? Aha, if the concentration of the signal is high enough, the virus does two things:
1)      It multiples within the bacterial host, and breaks out of its host’s body… in the process killing the host. But with enough other hosts around (something the virus “knew”, thanks to the signal concentration), the virus spreads onto the other live bacteria. As Bonnie Bassler, a biologist, says:
“At high densities, cholera, a parasite, wants to leave its host and get into another host. And at high densities, the virus, a parasite of a parasite, wants to leave its host and get into another host. They’re doing the same thing [using the same signal molecule].”
2)     But before that, the virus also messes up the host’s genes, misleading the bacteria to switch from infect to scatter mode.
Yong summarizes the two points succinctly:
“The phage (virus) not only ensures that its progeny have plenty of hosts to infect, but also ensures that those hosts spread far and wide.”

Next up in this story is the human! Bassler’s student Justin Silpe was able to engineer a virus to listen to the signaling molecule of bacteria different from its “regular” host:
“And when it detects molecules that are present only in its targets, it kills them. This random virus is now a programmable assassin that Silpe can set to go after particular targets.”

So is this the beginning of a new era in phage therapy (using a virus instead of antibiotics to kill disease causing bacteria)? Not yet. Silpe’s experiment has yet to be confirmed independently. But it’s certainly an interesting possibility that possibly lies in the horizon.

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