(566f) Localization of Matrix Production Reveals B. subtilis Biofilm Growth Mechanics

Bacterial biofilms are communities of microbial organisms that are encased in self-produced extracellular polymeric substances (EPS). The EPS matrix facilitates cellular signaling that leads to coordinated behavior at scales that are much larger than the single bacterium. This gives rise to sophisticated collective behavior that is absent in planktonic cells, as evidenced by the enhanced mechanical rigidity, advanced architecture for water retention and uptake of nutrients, and the elevated antibiotic resistance in biofilms. Existing studies have mainly focused on the regulatory networks involved in biofilm formation at the single-cell level. The relation between colony-scale physiological growth and behavior, with spatial and temporal patterns cell-differentiation within the colony remains under-explored.

Here, using distinct fluorescent transcriptional reporters and in situ imaging of biofilm growth, I demonstrate that considering the spatial and temporal distribution of the matrix-producing, sporulating and motile cellular phenotypes is critical in resolving B. subtilis biofilm development. Our key result is that, beyond an initial transient phase, the production of the EPS matrix and the onset of sporulation is localized to a radially propagating front at the biofilm exterior. The width of the matrix producing front approaches a constant value at late times. By using a correlation analysis in the fluctuations of fluorescence reporter activity, we reveal that the propagating fronts correspond to a pair of travelling waves of tapA gene expression (matrix-related) and sspB gene expression (sporulation-related) of cells that are immobilized within the biofilm matrix. Importantly, we show that the spatiotemporal propagation of both traveling waves are coupled by a single length scale (~ 500 μm) and time scale (~1.4 h) throughout development. Furthermore, the spatial gene expression profiles of matrix production and sporulation exhibit a data collapse into a self-similar asymptotic shape, and the front displacement exhibits a square-root scaling - hallmarks of universal dynamics that governs matrix production and sporulation of cells within the front.

What are the implications of our discovery towards quantitatively characterizing physical mechanisms that give rise to the observed phenotypic diversity and physiological heterogeneity? In a single cell, it is well-known that nutritional stresses and starvation triggers a transition from matrix production to the transcription of genes involved in sporulation. Additionally, the physical expansion and spreading of the biofilm colony must result from the mechanical forces associated with EPS matrix production within the localized propagating front. Therefore, a full biomechanical model of biofilm shape and morphology requires coupling nutrient uptake and consumption that gives rise to localized patterns of matrix gene expression, with mechanical forces that govern colony expansion.

In the second part of the talk, I present a mathematical model that combines nutrient transport, a concentration dependent biomass production rate and a thin-film equation that balances biomass production and biofilm advection driven by mechanical forces due to osmotic pressure and cell growth. I show that the model reproduces both the shape and morphology of the biofilm colony, observed expansion rates, as well as the experimentally measured matrix-production and sporulation gene expression profiles. Finally, I discuss the broader implications of using this combined experimental and modeling approach to probe various aspects of understanding biofilm colony development, phenotypic diversity and growth strategies.