Bacteria need to coordinate. Secreting a toxin, forming a biofilm, launching an infection — these actions are only useful above a certain population density. A single bacterium releasing toxin into a large body accomplishes nothing. A colony of ten million can overwhelm a host immune system.
The mechanism they use is quorum sensing. Each bacterium produces a small signaling molecule (AHL — acyl-homoserine lactone) at a steady basal rate. AHL diffuses outward and degrades. When the local AHL concentration rises above a threshold, the bacterium switches into high-expression mode: it starts producing far more AHL, and activates the genes for collective behavior.
The population switches together — not because any cell counted the others, but because when enough cells are present, the diffusion-degradation balance shifts. The AHL that was diluting away now accumulates. The threshold is crossed everywhere at once. No cell observed the transition happening. The quorum happened; no one called it.
The simulation uses a discrete threshold: each cell checks whether its local AHL exceeds a fixed value and switches state instantly. Real AHL-responsive gene circuits use positive feedback loops — the LuxR protein binds AHL and then activates its own promoter — so the switch is not a threshold but a bistable toggle. The cell commits to the ON state through an autocatalytic loop, not a comparison against a number. The threshold in this simulation is standing in for a mechanism that has no threshold.
The simulation also treats AHL as a continuous scalar field on a grid. The actual signaling molecule exists as discrete molecules in solution; at low concentrations, stochastic noise in molecular counts matters. A cell at threshold is making a probabilistic, not a deterministic decision. The cleanness of the transition in this simulation — the sudden switch of many cells in a few frames — is an artifact of the discrete grid and the exact threshold comparison.
Most importantly: the simulation makes the population density visible as a count and as a spatial pattern. The bacterium has no such view. It has only its local AHL reading. Whether the quorum has been reached is, from the inside, indistinguishable from "enough molecules have drifted here from nearby cells." The collective fact — quorum reached — exists nowhere in the system as a representation. It exists in the relationship between cell count and diffusion geometry, which is not a relationship any cell can read.