CFD predicted pH gradients in lactic acid bacteria cultivations.

The formation of pH gradients in a 700 L batch fermentation of Streptococcus thermophilus was studied using multi-position pH measurements and computational fluid dynamics (CFD) modeling. To this end, a dynamic, kinetic model of S. thermophilus and a pH correlation were integrated into a validated one-phase CFD model, and a dynamic CFD simulation was performed. First, the fluid dynamics of the CFD model were validated with NaOH tracer pulse mixing experiments. Mixing experiments and simulations were performed whereas multiple pH sensors, which were placed vertically at different locations in the bioreactor, captured the response. A mixing time of about 46 s to reach 95% homogeneity was measured and predicted at an impeller speed of 242 rpm. The CFD simulation of the S. thermophilus fermentation captured the experimentally observed pH gradients between a pH of 5.9 and 6.3, which occurred during the exponential growth phase. A pH higher than 7 was predicted in the vicinity of the base solution inlet. Biomass growth, lactic acid production, and substrate consumption matched the experimental observations. Moreover, the biokinetic results obtained from the CFD simulation were similar to a single-compartment simulation, for which a homogeneous distribution of the pH was assumed. This indicates no influence of pH gradients on growth in the studied bioreactor. This study verified that the pH gradients during a fermentation in the pilot-scale bioreactor could be accurately predicted using a coupled simulation of a biokinetic and a CFD model. To support the understanding and optimization of industrial-scale processes, future biokinetic CFD studies need to assess multiple types of environmental gradients, like pH, substrate, and dissolved oxygen, especially at industrial scale.

Response of Corynebacterium glutamicum exposed to oscillating cultivation conditions in a two- and a novel three-compartment scale-down bioreactor.

The oscillatory conditions in substrate and oxygen supply that typically occur on a large (industrial) scale are usually simulated in two-compartment scale-down reactors. In this study, the performance of nutrient-limited fed-batch cultivations of Corynebacterium glutamicum in a standard two-compartment reactor (two-CR) is compared to the performance in a novel three-compartment reactor (three-CR). The three-CR is designed to mimic three distinct zones of an industrial scale bioreactor that occur if the feed addition is installed at the bottom of the fluid phase. Our findings show that lactate and succinate appear in concentrations two-fold higher in the three-CR cultivation than in the two-CR cultivation. Similar results are revealed for the amino acids glycine, threonine, glutamate, and glutamine. In contrast to the two-CR cultivation, no intracellular accumulation of pyruvate is observed in the three-CR cultivation, since the carbon fluxes are directed toward lactate. As previously reported, the expression of lactate dehydrogenase (LDH) is increased in the context of oxygen deprivation. Thus, C. glutamicum adapts to the oscillating environment in the three-CR. This successful adaptation is revealed by a flow cytometric analysis of BOX-stained cells and a series of electrooptical at line measurements of cell polarisability. Both methods indicate a higher polarisability of cells in the three-CR cultivation. PI-staining does not indicate any membrane damage or accelerated cell death in either system. However, although the strain shows robustness, the product yield of lysine is reduced in scale-down cultivations as compared to cultivations at homogeneous conditions, which underlines the relevance of process optimization.

Assessment of robustness against dissolved oxygen/substrate oscillations for C. glutamicum DM1933 in two-compartment bioreactor.

Corynebacterium glutamicum is an important organism for industrial biotechnology; particularly, in amino acid production (e.g. L-lysine). Production scales often reach reactor working volumes of several hundred cubic meters, which triggers inhomogeneous distribution of substrates and dissolved gasses due to increasing mixing times. Individual cells which follow the flow profile through the reactor are experiencing oscillating microenvironments. Oscillations can have an influence on the process performance, which is a subject of scale-down experiments. In this work, L-lysine-producing C. glutamicum DM1933 was assessed for its robustness against continuous dissolved oxygen and substrate supply oscillation in two-compartment scale-down bioreactors. Aerobic, substrate-limited stirred tank and non-aerated, substrate-excess plug flow compartments were applied for oscillation. Inhomogeneity of substrate and oxygen supply was observed to cause rapid side product turnover, redistribution of oxygen uptake from oxygen limited into fully aerobic zones, and intermediate medium acidification. However, process inhomogeneity did not impair productivity or growth at plug flow residence times of several minutes. In a focused analysis of proteome, metabolome, transcriptome, and other physiological parameters, no changes were identified in response to process inhomogeneity. In conclusion, fed-batch processes with C. glutamicum DM1933 possess remarkable robustness against oxygen and substrate supply oscillation, which is a unique property in the field of published scale-down studies. Microbial physiology of C. glutamicum appears to be ideally adapted to both homogeneous and inhomogeneous conditions. This ensures exceptional suitability for cultivation at increased mixing times, which is suggested to constitute an important basis for the long-lasting success in large scale bioprocess application.