Cell Syst. 7, 137–145 (2018)

When a bacterium in the middle of a biofilm feels starved of nutrients, it releases a potassium signal that impacts nutrient uptake in its neighbours, starting a chain reaction that propagates all the way to the edge of the film. The peripheral cells respond by reducing consumption so the interior cells have access to nutrients. This display of altruism incurs a high cost to the individual — and cell-to-cell heterogeneity makes it a nontrivial problem in signal transmission. Now, Joseph Larkin and colleagues have shown that the benefit outweighs the individual cost when the system is poised near a percolation threshold.

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The authors’ model predicts a power-law decay in the distribution of cluster sizes and puts the fraction of cells required for signalling connectivity at a little less than half. By incorporating excitable dynamics into their framework, they found a minimum in the cost–benefit ratio near the percolation threshold. Experiments placed Bacillus subtilis biofilms near this threshold, whereas mutant strands with modified structure and dynamics strayed away, suggesting the biofilm’s spatial heterogeneity is tuned to ensure efficient transmission.