Quorum sensing
How bacteria count their own numbers with chemical signals and switch on group behaviours, such as glowing, only when crowded.
Quorum sensing lets bacteria act together once there are enough of them. Each cell continually releases small signalling molecules, called autoinducers, into its surroundings. When the bacteria are few and scattered, the signal simply drifts away. When they are packed together it builds up, and once it passes a threshold it switches on a set of genes in every cell at once.
It was first discovered in glowing marine bacteria such as Vibrio fischeri. Free in seawater, where a young squid can pick them up from water holding as few as a thousand cells per millilitre, they stay dark; a lone bacterium's light would be wasted. Inside the light organ of the Hawaiian bobtail squid they can reach more than ten billion cells per millilitre. At that density the signal turns on the genes for luciferase, the light-making enzyme, and the colony glows. The squid uses the light to cancel out its own shadow in moonlit water; the bacteria get a safe, well-fed home.
Activated cells also make more of the signal, a positive feedback loop that flips the switch decisively.
Quorum sensing is now known to control many group behaviours, including forming slimy biofilms and, in some disease-causing bacteria, releasing toxins only when there are enough cells to overwhelm the host's defences.
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