David’s comment: note the bold. Potassium is in high concentration within cells. This appears to be a release of some ionized potassium, which has an electrical charge to do the signaling. I suspect the bacteria receiving the electric signal then respond automatically to it.

dhw: I suspect that the bacteria deliberately and intelligently use their available means of communication (electrical waves) to recruit new members in order to strengthen their community, as the researchers tell us, and I suspect that the bacteria receiving the message deliberately and intelligently work out the meaning of the signals and deliberately and intelligently respond to them. Worth repeating the quote here: “Think bacteria aren’t sophisticated? Think again.”

They certainly appear more sophisticated than suspected. That doesn't mean the whole process isn't automatic