BACTERIA float around in the ambient fluid. It is a dangerous life, where many perils are encountered. If a bacterium bumps into a surface, it grabs it and holds on with tiny appendages called pili, exchanging a drifting, random-encounter environment for a two-dimensional local-interaction one. The bacteria aggregate in microcolonies, perhaps moving by twitching the pili. When there are enough microcolonies in an area, the bacteria change their behavior. They build an external polysaccharide matrix of some complexity, within which they live as a community, largely protected from bacteriocidal agents in the surrounding environment.

Such biofilms are responsible for persistent middle-ear infections in children and for pneumonia in patients with cystic fibrosis. They grow on catheters and artificial heart valves. They form in your own mouth every night, as gram-positive cocci labor to construct cities of dental plaque, only to have their handiwork destroyed by the morning toothbrushing. The discoverer of bacteria, Anton van Leeuwenhoek, found them in his own dental plaque and reported the results to the Royal Society of London in a letter of September 17, 1683.

Efforts of isolated bacteria toward constructing the polysaccharide matrix would have been to no avail. How do bacteria know when the community is large enough to initiate the plaque-building behavior? The fortuitous timing of the change in behavior is achieved by a process known as quorum signaling.