Figure 2.

A hypothetical mechanism by which bacteria may use quorum sensing with two autoinducers (AIs) to determine whether a foreign species is present. The focal species secretes AI-1 and AI-2 at equal rates, and as long as only the focal species is present, the ratio of AI-1 to AI-2 remains equal to 1 as cell density increases (black line). Bacteria may use any ratio of two autoinducers; the important principle is that if only the focal species is present, then the concentration of one autoinducer should predict the concentration of the other according to a known conversion factor. If other species are present that consume or produce AI-1 or AI-2, the ratio of the two autoinducers deviates from the expected value as cell density increases (blue and red lines).