concept

Quorum sensing

How bacteria "take a headcount": each cell releases a small signaling molecule, and when cell density is high enough the signal builds to a threshold that switches on a batch of genes at once. This lets a whole population act only when it is numerous enough for the action to matter. The classic case is the glowing bacterium *Vibrio fischeri*: diffuse in open seawater it stays dark, but packed densely inside a squid's light organ the signal accumulates and only then do they all switch on the light.