Editors' ChoiceMicrobiology

Ionic Signals to Community Formation

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Sci. Signal.  13 Jan 2009:
Vol. 2, Issue 53, pp. ec10
DOI: 10.1126/scisignal.253ec10

Bacteria are single-celled organisms, but under certain conditions they form multicellular communities. One such community, the biofilm, forms on surfaces when the density of bacteria becomes high enough that the bacteria detect each other's presence. Bacillus subtilis forms a biofilm when cultured on a minimal medium, but not when cultured on rich medium. López et al. tested several structurally unrelated, but natural, bacterial products, such as nystatin, amphotericin, gramicidin, and surfactin, for the ability to trigger biofilm formation of B. subtilis in rich medium. The common property of the molecules that did so was their ability to elicit cation leakage. Surfactin is a natural product of wild B. subtilis, and bacteria lacking the ability to produce this molecule do not form biofilms. Surfactin has surfactant properties, but these do not appear to be required for induction of the biofilm, because nystatin, which lacks surfactant properties, stimulated biofilm formation in strains of surfactin-deficient B. subtilis. Induction of biofilm formation by surfactin was prevented if the bacteria were cultured in high concentrations of potassium, but not sodium or lithium, which is consistent with cation leakage, specifically potassium leakage, as the signal for biofilm formation. Reporter assays demonstrated that surfactin and nystatin stimulated the expression of the operons involved in the production of extracellular matrix components required for biofilm formation. B. subtilis deficient in the histidine-sensor kinase KinC failed to form biofilms in response to surfactin. Introduction of KinC, the transcription factor and putative kinase target Spo0A, and Spo0A reporter gene into Listeria monocytogenes resulted in induction of the reporter gene in response to surfactin, which was inhibited by culturing the cells in high concentrations of potassium. Thus, bacteria appear to have evolved a mechanism to sense each other's presence that does not require a specific ligand-receptor interaction but instead triggers a change in cellular state (reduced potassium concentration) that activates a kinase to initiate a change in gene expression.

D. López, M. A. Fischbach, F. Chu, R. Losick, R. Kolter, Structurally diverse natural products that cause potassium leakage trigger multicellularity in Bacillus subtilis. Proc. Natl. Acad. Sci. U.S.A. 106, 280–285 (2009). [Abstract] [Full Text]

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