Quorum sensing

Quorum sensing is a form of chemical communication that lets bacteria, and some other microbes, act as a group. Every cell releases small signal molecules called autoinducers. While cells are few, the signal spreads out and stays weak, but as the population grows it builds up. Once it passes a threshold, receptors in the cells change which genes are expressed, across the whole population at about the same time. Sensing the signal often raises its own production, a feedback loop that keeps the cells in step.

The signals differ between groups. Many Gram-negative bacteria make acyl-homoserine lactones (AHLs), which cross the cell membrane and bind receptor proteins inside the cell that act as gene switches. Gram-positive bacteria usually release short peptides, detected by sensor proteins in the membrane. A signal called autoinducer-2 is made by many species and may let different kinds of bacteria sense one another.

Quorum sensing controls behaviors that pay off only when many cells act at once, such as making light, forming biofilms and releasing toxins. Vibrio fischeri glows only when densely packed in the light organ of the Hawaiian bobtail squid, and pathogens such as Pseudomonas aeruginosa use it to time their attack. Blocking these signals, called quorum quenching, is being studied as a way to disarm bacteria without killing them.