Curli fibers in Escherichia coli biofilms: the influence of water availability on amyloid structure and properties

Microbial biofilms are multicellular communities where bacteria produce an extracellular matrix mainly consisting of proteins and polysaccharides. These biofilms not only confer resistance against external stresses (e.g. antibiotics), but their physical and chemical properties can also adapt to environmental conditions (e.g. temperature, humidity, etc.). Gaining insight on how such cues affect the molecular structure of biofilms would greatly benefit to the emerging field of engineered living materials. In this work, we studied the physical-chemical properties of curli amyloid fibers extracted from biofilms grown on substrates with different water contents. Electron microscopy, FT-IR and fluorescence spectroscopy were used to characterize the conformation and physical-chemical properties of these fibers as a function of the biofilm growth conditions. The results show that varying the water content of the substrate leads to differences in the yield of curli fibers purified from the biofilms and to differences in the packing, hydrophobic character and chemical stability of the fibers. Fibers from biofilms grown on lower water content substrates presented a higher hydrophobicity and chemical stability than those from biofilms grown on higher water content substrates. Such fundamental knowledge on the biophysical features of amyloid fibrils sheds light on the influence of water on biofilm behavior and how the molecular structure of biofilm matrix defines biofilm macroscopic properties. Understanding the molecular behavior of the biofilm matrix formed in various growth conditions may inspire future strategies to engineer biofilm-based materials. Graphical Abstract