Polymer-induced aggregation of Escherichia coli is reduced by fimbriation

In their natural habitats, bacteria are often surrounded by polymers and other macromolecules (e.g., mucin and F-actin in the airways of patients with cystic fibrosis and exopolymers in the oceans). Hence, understanding the effects of extracellular polymers on bacterial behavior is likely to provide insight into the role of environmental conditions on bacterial physiological characteristics. Non-motile bacteria lacking flagella behave like passive colloids and can aggregate upon addition of non-adsorbing polymers due to polymer-induced depletion attractions. Many bacteria bear a variety of surface-anchored appendages, including fimbriae and curli, which play roles in virulence and adhesion. Type 1 fimbriae, straight, hair-like structures radiating from the cell surface, help bacteria attach to cells and evade antibiotics during initial infection and hence are widely studied as a critical factor in pathogenic virulence. Here, we explore how fimbriae expressed on the surface of Escherichia coli affect aggregation induced by the non-adsorbing polymer sodium polystyrene sulfonate (NaPSS). Using plasmid-based tunable expression of type 1 fimbriae in a fimA-fliC deletion mutant (lacking native expression of fimbriae and flagella), we study the effect of fimbrial expression on depletion aggregation induced by polymers using sedimentation assays and confocal microscopy. We find that fimbriae reduce polymer-induced depletion aggregation in E. coli and act as steric stabilizers under non-growth conditions. These results provide insight into the mechanisms of chronic bacterial infections and flocculation processes relevant to wastewater treatment.