Mechanical and functional characterization of extracellular polymeric substances of bacterial biofilms

161 p.Microbial cells secrete extracellular polymeric substances (EPS) to encase themselves in a matrix and form assemblages known as biofilms. Microbial biofilms are notoriously difficult to eradicate, and can cause fouling of technical systems and be detrimental to health when they are formed by pathogenic bacteria in our bodies to cause infection. Despite the EPS forming the major constituent of most biofilms (approximately 50-90% total organic matter), relatively less is known about the EPS as compared to cell signaling and molecular pathways involved in biofilm formation. As EPS components define the mechanical properties of biofilms, they impact on multicellular structural organization and have consequences for biofilm stability, dissemination and downstream colonization in flow environments. Furthermore, EPS components can be heterogeneously distributed in the biofilm, highlighting the need to carry out biofilm investigations at the micro-scale. In this thesis, particle tracking microrheology and micropillars were used to quantify the mechanical properties of specific EPS components at microscale resolution, and how these properties related to biofilm functions were investigated. In opportunistic pathogen P. aeruginosa, which uses Psl and Pel exopolysaccharides as major EPS components, it was found that Psl contributed to a more elastic and effectively crosslinked matrix that resisted spreading at the surface but facilitated microcolony formation. In contrast, Pel contributed to a more viscoelastic and loose matrix that facilitated surface spreading, lateral growth and streamer formation. Psl and Pel also contributed differentially but synergistically to heterogeneous lateral growth. The rheological roles of Psl and Pel were largely maintained in P. aeruginosa-S.aureus community biofilms, where Psl increased the crosslinking of the matrix and was required for the community microcolony formation, and Pel loosened and increased particle diffusivity in the matrix, which likely facilitated microcolony expansion.Singapore Centre for Environmental Life Sciences Engineering (SCELSE), whose research is funded by the National Research Foundation Singapore (NRF), Ministry of Education Singapore (MOE), Nanyang Technological University (NTU) and National University of Singapore (NUS) , under its Research Centre of Excellence (RCE) program; Singapore MIT Alliance for Research and Technology’s (SMART) research program in BioSystems and Micromechanics supported by the National Research Foundation Singapore.Doctor of Philosophy (IGS