Quorum Sensing
Bacteria communicate by releasing signaling molecules into their environment. When enough accumulate, behavior switches collectively — a phase transition from individual to group identity.
About this lab
Quorum sensing: bacterial democracy
Quorum sensing is a system of stimulus and response correlated to population density. Bacteria release chemical signal molecules called autoinducers into their environment. As the bacterial population grows, so does the concentration of autoinducer. When it reaches a threshold, the bacteria collectively switch on genes for behaviors that are only useful at high density — bioluminescence, biofilm formation, virulence factor production, sporulation. It is, in essence, a voting system: each bacterium casts a chemical ballot, and collective action requires a quorum.
Bonnie Bassler and the universal language
The pioneering work of Bonnie Bassler at Princeton revealed that quorum sensing is not a curiosity of one species but a universal phenomenon. She discovered that Vibrio harveyi uses multiple autoinducer signals simultaneously, and that a molecule called AI-2 serves as an interspecies communication signal — a kind of bacterial “lingua franca.” Her work opened the possibility of disrupting quorum sensing as an alternative to antibiotics: rather than killing bacteria, you jam their communications so they never coordinate an attack.
The public goods game and cheaters
Quorum sensing creates a public goods problem. Producing autoinducer costs energy, but the collective behavior it triggers benefits everyone nearby — producers and non-producers alike. This creates an opportunity for “cheaters”: mutant bacteria that sense the signal but don’t produce it, free-riding on the cooperative investment of others. Toggle cheaters in this simulation to see the dilemma. If cheaters dominate, the population never reaches quorum. In nature, spatial structure and kin selection help keep cheaters in check.
Phase transitions in biology
The switch from individual to collective behavior in quorum sensing is a genuine phase transition — a discontinuous change in system behavior at a critical parameter value. Below threshold, each bacterium is on its own. Above it, the colony acts as a superorganism. This is the same mathematics that governs magnetization in the Ising model, percolation in random networks, and synchronization in coupled oscillators. Biology discovered phase transitions long before physics named them.