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.

Carboxylic acid consumption and production by Corynebacterium glutamicum.

Corynebacterium glutamicum is well-known as an industrial workhorse, most notably for its use in the bulk production of amino acids in the feed and food sector. Previous studies of the effect of gradients in scale-down reactors with complex media disclosed an accumulation of several carboxylic acids and a parallel decrease of growth and product accumulation. This study, therefore, addresses the impact of carboxylic acids, for example, acetate and l-lactate, on the cultivation of the cadaverine producing strain C. glutamicum DM1945Δact3:Ptuf -ldcCopt and their potential role in scale up related performance losses. A fluctuating power input in shake flask and stirred tank cultivations with mineral salt was applied to mimic discontinuous oxygen availability. Results demonstrate, whenever sufficient oxygen was available, C. glutamicum recovered from previously occurring stressful conditions like an oxygen limiting phase. Reassimilation of acids was detected simultaneously. In cultures, which were supplemented with either acetate or l-lactate, a rapid cometabolization of both acids in presence of glucose was observed, showing conversion rates of 7.8 and 3.8 mmol gcell dry weight-1 hr-1 , respectively. Uptake of these acids was accompanied by increased oxygen consumption. Proteins related to oxidative stress response, glycogen synthesis, and the main carbon metabolism were found in altered concentrations under oscillatory cultivation conditions. (Proteomics data are available via ProteomeXchange with identifier PXD012760). Virtually no impact on growth or product formation was observed. We conclude that the reduced growth and product formation in scale-down cultivations when complex media was used is not caused by the accumulation of carboxylic acids.

Tools for the determination of population heterogeneity caused by inhomogeneous cultivation conditions.

Population heterogeneity among a microbial culture can occur in various stages of a bioprocess. When a nutrient-limited fed-batch process is applied in the large scale, gradient formation (e.g. at the substrate concentration or the pH-value) can have a tremendous impact on the process performance. It might also have an impact on population heterogeneity, as the cells are opposed to stressful conditions that is an oscillating environment, when they pass the different zones evolved in the liquid phase. Nevertheless, the question whether these conditions support heterogeneity of a culture has not been clearly answered so far. Furthermore, if such a heterogeneity affects product formation, the usual analysis tools, which do not rely on a single-cell basis, certainly do not gain sufficiently suitable information for process optimization. This overview on the one hand provides information about the contribution of an oscillating environment to the formation of heterogeneities within a population and on the other hand a summary of tools, which can be used to investigate physiologic and morphologic heterogeneity during process development, scale up and production. If these techniques are considered, the identification of targets for accelerated process optimization on a single-cell basis becomes easier and faster.