Bacterial biofilms are sophisticated structures which are the end result of cell-cell communication, reception of signals from the proximate environment, and complex gene expression responses. One of the key events being investigated is the switch between the motile-planktonic and the biofilm state. Single bacteria that swim freely in the liquid phase can attach to a surface and gradually suppress motility so they could start producing the extracellular matrix (ECM) and build the biofilm structure. However, while this switch is well described for single motile cells converting to ECM production, the fate of the surrounding cells exposed to the produced ECM is unknown. In this study, we asked whether the ECM has a role in regulation of motility. We examined the biofilm forming bacterium Bacillus subtilis wild type strain, and mutants that are incapable of producing different key ingredients of the ECM. Surprisingly, we saw a subpopulation of motile cells in the wild type biofilm, which was diminished in the ECM mutants strain. Expression of motility in the mutant strains was restored when ECM from the wild type strain biofilm was introduced. Together these results suggest an ECM-originated chemical cue that appears in early stages of B. subtilis biofilm formation and promotes, rather than inhibits, motility. Thus, in the biofilm, a subpopulation of motile cells co-exists with the ECM producing cells, and is maintained by signals from the ECM itself.
