Charged agar surfaces affect E. coli biofilm properties by balancing curli amyloid quantity and quality

Biofilm extracellular matrix (ECM) varies with environmental conditions and substrate properties. Understanding the surface-biofilm relationship helps to perfect antibacterial strategies and to design new engineered living materials (ELMs). In this work, we studied how cationic and anionic polyelectrolyte coatings affect macroscopic features of Escherichia coli curli-producing biofilms, as well as the properties of their curli amyloid fibers. Cationic coatings limited biofilm spreading, increased their surface density and water absorption, which correlated with a higher yield of curli amyloid fibers with looser structure. In contrast, anionic surfaces allowed for standard biofilm spreading, with a lower fiber yield but a more compact and chemically stable fiber structure. Higher biofilm rigidity and adhesion were measured on both types of charged surfaces. Thus, we propose that the differences in biofilm macroscopic properties result from a trade-off between curli quantity and quality in the ECM, namely fiber density and molecular packing, as well as their interaction with water. Our findings provide insights on how the biophysical properties of the ECM can be controlled by tuning the substrate physico-chemical characteristics with charged coatings. This work opens up new avenues for developing antimicrobial strategies, as well as tailoring the properties of amyloid-based ELMs.