Bacteria live in and on us in complex communities that outnumber the cells and genes of our own tissues. These bacteria possess a communication mechanism that allows them to coordinate their activities. This mechanism, called quorum sensing, was first described in bacteria living symbiotically in a squid. The bacteria produce bioluminescence which simulates moonlight and camouflages the squid. The key to quorum sensing is a molecular signal released by the bacteria that is monitored by receptors, which in turn modulate gene expression. Bioluminescence genes are only turned on when the population density—and therefore the signal concentration—is high.