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Abstract
Microbial biofilms have been proposed as robust, self-immobilized and self-regenerating catalysts. As a model study, this thesis provides an example for the design and operation of a catalytic biofilm process for the production of chemicals.
The capacity to form single-species biofilms was evaluated for 68 strains of microorganisms to estimate the scope of biofilms for catalytic application. By changing substratum characteristics, inoculum density and nutrient availability, 66 strains (97%) demonstrated biofilm formation and 36 strains (53%) were classified as strong biofilm formers. The abundance of biofilm forming microbes demonstrates a broad potential for biofilm application in chemical production processes.
With the aim to develop a biofilm process for ethylene glycol biotransformation, 62 bacterial and yeast strains were screened for ethylene glycol conversion. Pseudomonas putida JM37 displayed the highest substrate conversion rate and was selected as a potential catalyst for the production of glyoxylic acid. Based on published metabolic pathways for P. putida, tartronate semialdehyde synthase (gcl), malate synthase (glcB) and isocitrate lyase (aceA) were identified as targets for gene disruption to block the further conversion of glyoxylic acid. Single and double knockout mutants of gcl, glcB and aceA were generated by transposon and site-directed mutagenesis. Glyoxylic acid conversion was not affected by these mutations, which indicates that gcl, glcB and aceA are not essential for glyoxylic acid metabolism in P. putida JM37. As a biofilm former with good glycolic acid productivity, Pseudomonas diminuta was chosen for the evaluation of a trickle-bed biofilm reactor with structured packing.
Structured packing is efficient for gas-liquid exchange in chemical catalysis. The current study is the first to employ structured packing as biofilm substratum, and an aerated continuous biofilm reactor system was designed. P. diminuta established an active biofilm and catalyzed the oxidation of ethylene glycol to glycolic acid for over two months. A steady-state productivity of up to 1.6 gl-1h-1 was achieved, with excellent process robustness and reproducibility. The results demonstrate the potential of structured packing as biofilm substratum for the production of chemicals. Implementation is recommended for whole-cell processes which require improved catalyst stability, catalyst retention for continuous operation, or efficient gas-liquid exchange.