Bacteria communicate via autoinducer molecules, switching collective behavior at a population threshold
Each bacterium continuously produces autoinducer (AI) molecules that diffuse through the environment. When the local AI concentration exceeds a threshold K, the cell switches to an "active" phenotype — coordinating bioluminescence (V. fischeri), biofilm formation, or virulence factor release. The autocatalytic feedback (active cells produce more AI) creates a sharp switch: below quorum, cells remain individual; above it, the entire colony synchronizes. Mathematically, this is a bistable switch driven by a Hill function: dA/dt = ρ·(1+H(A,K,n)) − δA + D∇²A.