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Influence of cell interaction forces on growth of bacterial biofilms

A hybrid computational method was proposed for simulation of biofilm growth processes using a continuum model for transport of water and extracellular polymeric substance (EPS) and a discrete model for simulation of bacterial cells. The current paper focuses on development of accurate models for different forces acting between bacterial cells, which are represented by spherocylinder particles. The major forces acting on the bacterial cells include drag from flow of EPS generated by the bacterial colony, adhesion forces (e.g., van der Waals adhesion and ligand–receptor binding) between colliding cell surfaces, lubrication force due to cell growth and EPS production, and tension from the fimbria appendages that project outward from many types of bacterial cells. The lubrication force and drag force act to separate the cells and expand the bacterial colony, whereas the adhesion and fimbria forces act to pull the bacterial colony together. Simulations are performed to examine the effect on biofilm development of each of these forces individually. The significance of different forces depends on the cell shape and other specifics of the given computation. However, there appears to be an opposing influence at the scale of the bacterial colony between the outward-oriented EPS drag on cells and the inward-oriented fimbria force. These two forces were particularly found to be important for determining the degree of orientation alignment of the cells. On the smaller scale of individual cells, the actions of the cell surface adhesion force and the lubrication force similarly oppose each other, with the balance influencing cell clustering and the degree of contact.