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Abstract

Microbial communities are the common mode of bacterial existence in natural habitats. As such complex systems become more and more of interest for biotechnology as well as medical and ecological research, there is an increasing need for further knowledge of their behaviour. Three medically relevant bacteria causing lung diseases in cystic fibrosis patients (Burkholderia cepacia, Pseudomonas aeruginosa, Staphylococcus aureus) and a chemically defined medium with glucose as carbon source were chosen as a model system. Pure culture growth characteristics and kinetic parameters were determined using 250 mL shake flasks. During growth of S. aureus acetate and pyruvate accumulated in culture medium. Biomass formation in B. cepacia and P. aeruginosa cultivations could be increased by addition of sodium acetate as an additional carbon source. A scale-up for the model organisms was carried out from shake flask scale into a 2 L stirred tank reactor (STR) using exhaust gas analysis for calculation of oxygen transfer rate (OTR), carbon dioxide transfer rate (CTR) and respiratory quotient (RQ). Pure culture cultivations in the STR showed less biomass formation of P. aeruginosa and S. aureus compared to shake flask experiments. OTR- and RQ-characteristics indicated metabolic differences between the model organisms. A continuous cultivation set up was established as a standard procedure for cultivation of a defined mixed culture. The inoculation strategy showed reproducible results for cultivations. RQ-behaviour indicated metabolic changes for mixed cultures at different dilution rates. Using kinetic parameters derived form pure culture experiments, a computer based simulation for competition of three species growing on one limiting substrate in a chemostat showed similar results compared to experimental data of mixed cultivations.