AbstractsBiology & Animal Science

In this dissertation, we characterize the polymeric and rheological properties of polysaccharide intercellular adhesin (PIA) present within the matrix of extracellular polymers (EPS) in biofilms formed by Staphylococcus epidermidis. Biofilms are viscoelastic soft matter consisting of bacterial aggregates embedded within the EPS. The EPS predominantly contains polysaccharides, in addition to proteins and DNA. S. epidermidis biofilms frequently contaminate medical implants resulting in blood stream infections. In such cases, biofilm formation by S. epidermidis and resistance to blood shear stresses is attributed to the presence of PIA. Using techniques of chromatography, light scattering, microrheology and colloidal physics, we understand the contribution of PIA towards biofilm viscoelasticity. We identified that PIA exhibits self-associations and complexation with proteins in dilute solutions. At concentrations found within shaker grown biofilms, PIA displayed a viscoelastic rheology. We extracted concentration dependent scaling laws for zero-shear viscosity and the creep compliance of PIA. A polymeric composite consisting of PIA, bovine serum albumin and lambda DNA, simulating the biofilm EPS, was 50-fold more elastic than PIA. However, we found that the EPS polymers, on their own, do not generate the elasticity of mature biofilms. To understand this gap, we report the self-assembly of artificial biofilms using planktonic cells and the polysaccharide chitosan as a proxy for PIA. We report that the elasticity or the long time plateau in creep compliance, in the artificial biofilms is mediated by pH induced phase instability of chitosan. Using this finding, we showed that increasing the pH of the solvent environment resulted in softening of a S. epidermidis biofilm within 4 hours. To support development of medical procedures aimed at biofilm softening, we provide a theoretical derivation to extend the applicability of cavitation rheometry, to measure elastic modulus of materials as small as few microliters, similar to biofilms formed in infection sites. Collectively, the study provides structural and rheological characterization of biofilm exopolymers that supports multi-component constitutive modeling of biofilms and development of treatment techniques that target biofilm disassembly through physicochemical changes to the growth environment.