The bacterial stress phenomenon is remarkable considering the industrial production organisms but also in prevention of the pathogen growth. Stress can be caused for example by abnormal temperature, pH, metabolic product, osmotic or hydrostatic pressure, lack of substrate, shearing forces, and oxidative radicals. The stress effects are diverse. The stresses occurring in a process can affect productivity, yield, and product quality. A sublethal stress can also improve the strain robustness towards subsequent stresses. In this dissertation the subject of bacterial stress was approached through perspective of anaerobic Clostridium acetobutylicum and facultative anaerobe Lactobacillus rhamnosus. C. acetobutylicum is known of acetone-butanol-ethanol fermentation already from the beginning of the 20th century. The process shows potential as a source to produce butanol in response to renewable fuel demand. L. rhamnosus is widely utilized as a probiotic, in tablets or supplemented in functional foods. The stress-related gene expression of L. rhamnosus was studied with scale-down methodology. In the scale-down the gradients of industrial scale processes were simulated in smaller scale using plug flow reactor in continuous cultivation and oscillating pH control in batch process. In the plug flow reactor pH and temperature were used as changing environmental variables. According to the gene expression results especially heat-shock response and phosphate uptake system were sensitive even to small scale pH changes. The pH change of 0.3 unit was enough to affect the expression of heat shock related genes. No relation between the expressions of the studied genes and freeze stability or acid tolerance was found. The stress effects on C. acetobutylicum metabolism was investigated applying constraint-based genome-scale metabolic modelling. The study on metabolism was carried using data obtained from continuous cultivations. Excess butanol and glucose limitation were used as sources for stress in the cultivations. The solution space of the metabolic model for the studied cases was narrowed with additional constraints from experimental measurements and 13C-metabolic flux analysis results for internal metabolic fluxes. The solutions of flux space were investigated using different optimization objectives for the flux distribution of the model, such as maximization of the growth rate and maximization of the ATP maintenance. The flexibility of single fluxes with respect to different optimization objectives was identified for each case applying flux variance analysis.
|Publication status||Published - 2017|
|MoE publication type||G5 Doctoral dissertation (article)|
- Lactobacillus rhamnosus, Clostridium acetobutylicum, bacterial stress, ABE, gene expression, metabolic modelling