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Reference

Bacillus subtilis spreads by surfing on waves of surfactant

Angelini T.E., Roper M., Kolter R., Weitz D.A., Brenner M.P.

PNAS 106: 18109-18113 (2009) PMID: 19826092 (Pubget)

Motivation

Gram-stained B. subtilis

In times of stress Bacillus subtilis differentiate into spores, which helps them survive but at a significant energy cost. B. Subtilis try to keep this by three known mechanisms: 1) a sub-population of cells differentiations into cannibal cells, which selectively lyse non-cannibal cells; 2) the cells form a dense mat of exopolysacharides, called a biofilm; and 3) the cells spread out in search of a more hospitable environment. All three mechanisms appear to be controlled by the same regulatory protein, Spo0A. This protein is part of a quorum sensing system. The lipopetide surfactin secreted by the bacteria activates the membrane kinase KinC, which, in turn, phosphorylates and thereby activates Spo0A. It has been shown that surfactin activates both biofilm-building matrix production and cannibal differentiation in the same cells.

How cannibalism and biofilm formation work is now broadly understood, but the mechanism by which colonies migrate outwards remains an open question. This paper proposes that surfactin gradients lead to gradients in surface tension, which leads to the colony spreading.

Experimental Observations

Hypothesis

Conceptual Explanation

Imaging the 'B. subtilis' biofilm shows that the colony is thicker in the center that at the edges. It has also been previously shown that 90% pure surfactin can reduce water surface pressure from 72 mN/m to 27 mN/m.

If we assumed that:

• Surfactin secreted by cells moves quickly to the air-biofilm interface.
• Surfactin reduces the film surface tension.
• All cells secrete surfactin at the same rate.
• The bacteria are homogeneously distributed throughout the film.

Scaling Analysis

Mathematical Model

Relationship to Soft Matter Physics

References

López D, Vlamakis H, Losick R, Kolter R., Cannibalism enhances biofilm development in Bacillus subtilis. Mol Microbiol (2009). Pubget

Yeh MS, Wei YH, Chang JS., Enhanced Production of surfactin from Bacillus subtilis by addition of solid carriers. Biotechnol Prog (2005). Pubget