Quantitative modeling of heterogeneous biofilms using cellular automata.

摘要

A fully quantitative two-dimensional (2D) Cellular Automaton (CA) biofilm model was developed. The model describes substrate and biomass as discrete particles existing and interacting in a specified physical domain. Substrate particles move by random walks, simulating molecular diffusion. Microbial particles grow attached to a surface or to other microbial particles, consume substrate particles, and duplicate if a sufficient amount of substrate is consumed. The dynamics of the system is simulated using stochastic processes that represent the occurrence of specific events within the biofilm. The ability of the CA model to predict substrate gradients and fluxes was evaluated by comparing model simulations to predictions from a traditional differential equations model.; A bitwise implementation of the CA biofilm model was developed to take advantage of the architecture of single-processor computers. The results of 2D simulations demonstrated that the CA model was able to reproduce heterogeneous biofilm structures. At the same time, the model produced quantitative results of substrate flux, concentration gradients, and cell distribution within the biofilm. The heterogeneous biofilm simulations showed that the predictions of substrate flux and biofilm thickness made with the 2D model were considerably different than the predictions of the 1D model using the same set of parameters, indicating the importance of biofilm microstructure on biofilm activity.; To incorporate the effect of biofilm heterogeneity into 1D model simulations, two parameters usually considered constant, biomass density and thickness of the boundary layer, are proposed to be replaced by the biomass per unit area and the thickness of the mass transfer limited zone, which lumps biofilm and boundary layer thickness. In this way, the restrictions typically imposed to 1D models are relaxed and an iterative procedure can be used to calculate steady-state substrate flux for heterogeneous biofilms where biomass density and boundary layer thickness are not constant.