FIG 7.

Asymmetric communication in a biofilm. Gradients of nutrients (or other environmental factors) can affect AI production and perception. Emerging AI gradients can themselves affect AI synthase or receptor activities, generating feedback. Often, these impacts are nonmonotonous. For example, nutrients may upregulate AI at low concentrations and downregulate AI at high concentrations. Depending on the environmental conditions and the intracellular regulation architecture, this can result in different spatial patterns of components involved in AI production (AI synthase) and/or AI sensing (receptor). These impacts might be strongest at the upper or lower biofilm surface or somewhere in between, which is depicted by scenarios a to c (126). In other words, different layers in the biofilm talk or listen with different intensities. Asymmetric communication can be further complicated by stochastic effects in communication, the development of subpopulations, heterogeneity of biofilm morphology, or the presence of other species.