Purely Polysaccharide-Based Biofilm Matrix Provides Size-Selective Diffusion Barriers for Nanoparticles and Bacteriophages

Biofilms are complex mixtures of proteins, DNA, and polysaccharides surrounding bacterial communities as protective barriers that can be biochemically modified during the bacterial life cycle. However, their compositional heterogeneity impedes a precise analysis of the contributions of individual matrix components to the biofilm structural organization. To investigate the structural properties of glycan-based biofilms, we analyzed the diffusion dynamics of nanometer-sized objects in matrices of the megadalton-sized anionic polysaccharide, stewartan, the major biofilm component of the plant pathogen, Pantoea stewartii. Fluorescence correlation spectroscopy and single-particle tracking of nanobeads and bacteriophages indicated notable subdiffusive dynamics dependent on probe size and stewartan concentration, in contrast to free diffusion of small molecules. Stewartan enzymatic depolymerization by bacteriophage tailspike proteins rapidly restored unhindered diffusion. We, thus, hypothesize that the glycan polymer stewartan determines the major physicochemical properties of the biofilm, which acts as a selective diffusion barrier for nanometer-sized objects and can be controlled by enzymes.