Cooperation of motility- and growth-driven activity in biofilm dynamics*

Bacterial communities have often been investigated as active nematic systems in which activity is driven either by cell motility or cell growth. Although it is known that both mechanisms are present in a single community, it remains unclear what roles these coexisting sources of activity play in cellular interactions and collective behaviors. We have explored this at the single cell level using experimental and computational approaches.

We used confocal fluorescence microscopy to capture the spatiotemporal behavior and patterning of Bacillus subtilis biofilms, focusing on the specific contributions of flagella-propelled motile cells and growing chains of matrix-producing cells. These experiments showed that cell type proportion and spatial distribution are maintained during biofilm expansion. The contributions of the two cell types on dominant loop motifs, topological defects, local velocity and order were also explored. We probed these dynamics further using a finite element modeling framework in which cells are represented as elastic spherocylinders with the ability to self-propel or chain. By varying key parameters, such as activity level, adhesion, and cell type proportion, we saw that both sources of activity are important for pattern formation, although their heterogeneous interactions are more influential than their activity levels.