N. Ducharme: POE Viscoelastic shear moduli increase with Ɣ-PGA production and nutrient concentration in Bacillus subtilis biofilms

Bacterial biofilms are living, adaptive materials composed of cells embedded in a self-produced extracellular matrix (ECM). Biofilms adapt to changes in the environment by modifying the polymers and other matrix components of which they are composed. Bacillus subtilis is a well-established model organism for studying biofilms. B. subtilis produces several extracellular polymers. One such polymer of particular interest is poly-γ-glutamic acid (γ-PGA), which exhibits high water absorbance and crosslinks with other matrix components. Using shear rheology, we observe that after 24 hours of growth in the linear viscoelastic region the storage and loss moduli are significantly higher for γ-PGA producing biofilms than for γ-PGA non-producers; however, this difference is negligible after 72 hours of growth. Additionally, the moduli for biofilms grown with a lower starting nutrient concentration are all statistically similar irrespective of their potential to produce γ-PGA and the time for which they are grown. These moduli are also significantly less than those of the 24-hour, standard nutrient biofilms. However, the moduli of standard nutrient biofilms at 72 hours are statistically similar to those of low-nutrient biofilms at both time points. These observations suggest that biofilms decrease their stiffness and viscosity in response to the depletion of nutrients, likely in preparation for dispersal.