I’m reposting this from 2011, as it seems relevant to current interest in our gut microbiome and plant microbiomes.

“Do you expect me to talk?”

“No, Mr. Bond, I expect you to die!”

Sometimes, there are more than two possibilities.

Many bacteria make antibiotics which, in high doses, kill other bacteria. But microbiologists have noticed that, at lower doses, antibiotics may change gene expression or alter behavior, without killing. So, they suggest, maybe antibiotics are mainly “tools of communication,” rather than weapons.

Maybe, but those are not the only possibilities. Will Ratliff and I have just published a short Perspective in Science suggesting that two other possibilities are more likely. A PDF is freely available here, courtesy of the journal.

If communication (or “signaling”) implies mutual benefit, what about nonlethal effects of antibiotics that benefit receiver or sender, but not both? For example, a bacterium that detects an antibiotic may hide in a biofilm to escape from the antibiotic, just as a zebra that smells lions hides in a herd to escape predators. If hiding in the biofilm benefits the bacterium that detected the antibiotic, but not the bacterium that made it, we would call the antibiotic a “cue.”

Or, suppose a bacterium making antibiotics benefits by scaring competitors into dispersing (or hiding in biofilms)? This could benefit the producer, by reducing competition, but harm the receiver. We would call that “manipulation.”

To distinguish among these possibilities, we need to measure actual fitness consequences, to producer and receiver, of bacterial responses to antibiotics.

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