Online

Jin, Xing

Mechanical Engineering

2020

Ph. D.

Bacterial biofilms play a critical role in environmental processes, water treatment, human health, and food processing. They exhibit highly complex dynamics due to the interactions between the bacteria and the extracellular polymeric substance (EPS), water, nutrients, and minerals that make up the biofilm. In the current dissertation, a hybrid computational model was proposed for simulation of biofilm growth processes using a multiphase continuum for the transport of water and EPS, as well as nutrient diffusion, and discrete phase particles for simulation of bacterial cells and their interactions. Mass and momentum conservations of each phase and bacterial motion, rotation, growth, division, and EPS production were all included in the model governing equations. The model was demonstrated to be capable of capturing both the heterogeneous structure of bacterial colonies and the opposing directions of water and EPS velocities for both spherical and spherocylindrical bacterial cells in 2D and 3D. The biofilm structure was observed to depend on the pore spacing between bacteria, which controls the percolation rate of water and nutrient to the bacterial colony, as well as the details of the various intercellular forces acting between the bacterial cells. Simulations performed using this hybrid model found that four intercellular forces had the most significant impact on biofilm development. These forces include van der Waals adhesion between cell surfaces, fimbriae tension force, lubrication force between the cells, and drag force on the cells from the outward moving EPS flow. The first two forces act to hold the bacterial colony together in a tight unit, whereas the last two forces act to separate the cells and pull the bacterial colony apart. These forces were found to be critical for setting both the pore size and the overall shape of the colony. For non-spherical cells, these different forces were also found to have an important effect on the cell rotation rate and degree of alignment with neighboring cells. A careful examination of the effects of EPS drag and fimbrial force was made for cases with different EPS production rates and different numbers of fimbriae appendages per cell. As these parameters were varied, the spatial pattern of bacterial colony changed from a single tightly-packed colony (for low EPS production rate and high numbers of fimbriae per cell) to a system with loosely-connected small clusters of suspended cells (for high EPS production rate and small number of fimbriae per cell). In-between these extremes, the bacterial colony was observed to exhibit a state with an asymmetric structure with multiple clusters of cells, connected by thinner strands. The balance between outward drag force on the cells due to the EPS flow away from the bacterial colony and the inward tensile fimbrial force acting on chains of cells connected by adhesive fimbriae appendages was identified as the dominant mechanism controlling these structural transitions.