Browsing by Subject "Wood-Werkman"

Propionibacterium freudenreichii is a common bacterium used in food industry. Despite of its wide use it has not been extensively studied yet. P. freudenreichii is usually considered as an anaerobe although it carries all the genes required for aerobic respiration in its genome. In my study, I examined the effect of oxygen on cell physiology and metabolism of P. freudenreichii. Cells were grown in bioreactors under anaerobic and microaerobic conditions as well as using a two-step cultivation with first an anaerobic and then a microaerobic phase. The partial pressure of oxygen was 0 kPa in anaerobic conditions and 4 kPa in microaerobic conditions. The growth rates and amount of ATP were studied, the metabolic activity of the cells was analysed, production of acetic, pyruvic, lactic and propionic acid was examined and finally I studied the expression of the atpA and recA genes. According to the results, P. freudenreichii grows with about two times shorter generation time in microaerobic conditions than in anaerobic conditions. The cells were metabolically most active in exponential phase when conditions were microaerobic. The amount of ATP was found to be rather static under the conditions used. Propionic acid was produced in anaerobic conditions but when conditions switched to microaerobic, its concentration diminished. Pyruvic acid was found to accumulate in microaerobic conditions. The atpA gene was expressed more efficiently in microaerobic conditions than in anaerobic conditions. P. freudenreichii probably utilizes reversible Wood-Werkman -fermentation route to produce ATP by oxidative phosphorylation when oxygen serves as an electron acceptor. Substrate-level phosphorylation did not increase in microaerobic conditions. The growth rate of P. freudenreichii and the final cell densities it reaches can be increased by cultivation in microaerobic conditions instead of anaerobic conditions.