Bacterial quorum-sensing in spatially complex communities

Bacterial quorum sensing (QS) allows the group-wide coordination of cellular responses through the secretion and detection of small diffusible signal molecules. While QS is crucial for biofilm development in many species, the rules which govern its dynamics in such structurally complex and genetically heterogeneous communities are not well understood as well as the way by which those depend on the QS system architecture. Specifically, Gram positive bacteria has two main types of peptide-based QS architectures – type I based on external sensing of the signal while type II acts by intracellular sensing of irreversibly imported signals. Here we combine experiments and theory to show that the two Gram-positive QS types lead to very different length-scales of action in dense communities. Using a microfluidics microscopy setup, we experimentally examine multiple representatives of the two designs in Bacillus subtilis. We find that signal propagation in type I systems has a long length-scale of interaction. In contrast, we find that type II systems are self-limited in their range to ~10 microns. We demonstrate that this self-limit is due to the irreversible uptake of the short peptide signal of these systems by all cells . We show that the existence of a length-scale allows QS bacterial cluster to probe their size and prevent interaction between distant clusters. Finally, we show that the type II quorum-sensing systems carried by B. subtilis phages and conjugative elements allow those elements to adaptively tune their mobility, based on micron-scale estimation of their frequency within a recipient.