Osmotic pressure gradients in E. coli biofilms revealed by in-situ sensors

Osmotic pressure has been known to play essential roles in living systems from single cells to complex tissues. However, direct in-situ measurements of osmotic pressures in biosystems have remained challenging, especially in complicated heterogeneous systems in which osmotic pressure gradients could exist and induce directed forces. Bacterial biofilms -- organized communities of bacteria encased in a self-produced extracellular matrix -- are a major mode of bacterial life. It has, however, remained unexplored how the osmotic pressure is distributed in the biofilm and how this distribution contributes to biofilm growth and activity. Here, liposomal nano-sensors are developed for the in-situ mapping of osmotic pressures at an unprecedented microscale resolution in real time using Escherichia coli. biofilm as a model system that develops at the surface of a hydrogel containing the nutrients. The measurements reveal osmotic pressure gradients with a radially increasing trend from the inner regions to the outer regions of the biofilm, which is associated with biofilm formation, morphology, and metabolism. The gradients likely contribute to mechanical properties, internal stresses, and nutrient transport. The sensor readouts also show that there is an osmotic pressure difference between the biofilm and the adjacent medium, which may promote biofilm expansion through matrix swelling and bacteria growth via water and nutrient uptake from the surroundings. Our novel approach based on in-situ osmotic pressure mapping in a growing biofilm reveals a sophisticated spatial regulation of physical forces, which may inspire new models and approaches in the field of mechanobiology.