Bacillus subtilis spreads by surfing on waves of surfactant

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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)


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

Angelini2009 fig2c.jpg

Bacteria were inoculated into 4 mL of medium that is conducive to biofilm formation, and grown in a humidified chamber at 30 C. Cells formed a biofilm on the surface of the medium once the diffuse oxygen in the medium had begun to be depleted. About 12 hours after biofilm formation the biofilm began to climb the walls of the tube. The surface tension at the base of the climbing film was measured by placing a capillary tube sealed with a dialysis membrane. The height of the fluid in the capillary tube reaches an equilibrium when the capillary and hydrostatic pressures are equal:

<math> \gamma \cos{\theta} = \frac{\rho g d h_r}{4} </math>


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


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