Physiology of Caulobacter crescentus grown on different carbon sources

Abstract: Freshwater, obligate aerobic, Gram-negative bacterium Caulobacter crescentus strain CB2 is able to metabolise various types of carbon sources. In this degree project, growth characterization of C.crescentus was made on different substrates including glucose, xylose, and mannose using shake-flask cultivations and bioreactor experiments. Special emphasis was put on the xylose degradation pathway, the Weimberg pathway in C. crescentus. This pathway has recently been of interest for introduction into other hosts. In shake-flask cultivations of C. crescentus strain CB2, the highest specific growth rates were found in 10 g/L glucose (0.142 h-1) followed by 10 g/L xylose (0.109 h-1). The organism also grew successfully in 10 g/L mannose but with a lower specific growth rate (0.053 h-1). The main product in cultivations in xylose was α-ketoglutaric acid, the last metabolite in the Weimberg pathway. The strain CB2 showed incomplete substrate consumption whereas another strain tested, CB15, showed complete substrate consumption in shake-flask cultivations using 5 g/L glucose or xylose substrates. Bioreactor experiments with Caulobacter crescentus strain CB2 indicated that the optimal pH for growth was 6.5. The incomplete substrate consumption suggested that growth was limited (or inhibited) by sensitivity to aeration, limiting nutrients (present in yeast extract), or sensivity to some products formed. Enzyme assays were made for three of the enzymes in the Weimberg pathway (xylose dehydrogenase (xylB), xylonate dehydrogenase (xylD), and α-ketoglutaric semialdehyde dehydrogenase (xylA). Activities of these enzymes were all found, 38.2, 25.4, and 50.9 μmol/(min'mg) protein, respectively, in medium containing only xylose as the carbon source. No activities were found in medium containing glucose as the carbon source.