Time-resolved study of biofilm architecture and transport processes using experimental and simulation techniques: the role of EPS

Cellular material and extracellular polymeric substances are the basic stru ctural elements in biofilm systems. The structure and role of EPS for biofi lm development and metabolic processes have not been precisely determined a nd, therefore, have not yet been included as a necessary element in modelli ng and simulation studies. This is due to the difficulty of experimentally detecting the extracellular polymeric substances in situ and differentiatin g them from cellular material on the one hand, and to the subsequent uncert ainty about appropriate models - e.g. rigid hindrances, porous microstructu re or visco-elastic structure- on the other hand. In this work, we report o n the use of confocal laser scanning microscopy to monitor the development of a monoculture biofilm of Sphingomonas sp. grown in a flow cell. The bact erial strain was genetically labelled resulting in strong constitutive expr ession of the green fluorescent protein. The development of extracellular p olymeric substances was followed by binding of the lectin concavalin A to c ell exopolysaccharides. The growth of the resulting strain was digitally re corded by automated confocal laser scanning microscopy. In addition, local velocity profiles of fluorescent carboxylate-modified microspheres were obs erved on pathlines throughout the biofilm. The CLSM image stacks were used as direct input for the explicit modelling and three-dimensional numerical simulation of flow fields and solute transport processes based on the conse rvation laws of continuum mechanics. At present, a strongly simplifying EPS -model is applied for numerical simulations. The EPSs are preliminarily ass umed to behave like a rigid and dense hindrance with diffusive-reactive sol ute transport.