Cells oxygen-limited

R. eutropha is actually an autotrophic hydrogen-oxidizing bacterium which can also produce poly(3HB) from C02, H2, and 02 [34]. The critical factor in such autotrophic cultivation processes is to avoid possible gas explosions. Therefore, a recycled gas, closed circuit culture system equipped with several safety features was developed and the oxygen concentration in the substrate gas phase was kept below the lower limit for gas explosions. A bacterial biomass of 91.3 g 1 1 has been achieved and the poly(3HB) content reached up to 67% per cell dry weight under these oxygen-limited conditions [35]. 

This example treats a diffusion-reaction process in a spherical biocatalyst bead. The original problem stems from a model of oxygen diffusion and reaction in clumps of animal cells by Keller (1991), but the modelling method also applies to bioflocs and biofilms, which are subject to potential oxygen limitation. Of course, the modelling procedure can also be applied generally to problems in heterogeneous catalysis. 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

Reuveny et al. [81] suggested that lowering the incubation temperature from 27°C to 25°C may alleviate eventual oxygen limitations without compromising the maximum cell yield. The expression level of the recombinant proteins at 27°C was similar to that obtained at 22 °C and 25 °C but lower protein yields were obtained at 30 °C. An increase in temperature from 22 °C to 27 °C led to earlier production of the proteins and to an increase in the proportion of the product released by the cells [81]. 

Prior to the fermentations, the pH of the different hydrolysate samples was adjusted to 5.5 with NaOH (5 M). All fermentations were carried out under oxygen-limited conditions in 55-mL glass vessels containing 50 mL of medium of which 47.5 mL was hydrolysate (or, alternatively, an aqueous glucose solution for reference fermentations). The vessels were sealed with rubber stoppers and equipped with cannulas for outlet of C02. The hydrolysates were supplemented with nutrients as previously described (20). Fermentations of 35 g/L of glucose and nutrients but no hydrolysate were used for reference. The flasks were inoculated to an initial cell mass concentration of 2.0 g/L dry wt and incubated at 30°C with stirring. The fermentations were run for 36 h. Samples of 200 pL were taken after 0,2,4, 6, 8,10, 24, and 36 h. 

Methods for chemical analysis of NAD(P)H include enzymatic methods [37] and bioluminiscence methods ([38] by Grupe and Gottschalk using a commercial test kit from Boehringer Mannheim). Increased NADH levels often indicate oxygen limited states of aerobic organisms (well known for baker s yeast, and also observed with hybridoma cultures by Zupke et al. [39], where the situation is more complex due to cell compartments). 

T. acidophilum contains a menaquinone [123], and spectral data suggests the presence of c-type and o-type cytochromes [124]. The respiratory chain of T. acidophilum has been studied in early stationary cells in aerobic and oxygen-limited cultures. Aerobic cells contain a bands located at 554nm and 619nm, and shoulders at 559 and 549nm. The 549 nm and 554 nm bands are associated with c-type cytochromes while the 559 nm and 619 nm bands are ascribed to b and d(a2) cytochromes, respectively. In oxygen-limited cultures, the spectral features associated with the c-type cytochrome and the cytochrome oxidase decrease and those related to the b-type cytochromes increase [125]. 

The uptake of aminoglycosides into proximal renal tubular epithelial cells is limited to the luminal cell border and is saturable. Less frequent administration of doses larger than needed for saturation of this uptake may therefore reduce drug accumulation in the renal cortex (98). In vitro and in vivo data have provided evidence that partially reduced oxygen metabolites (superoxide anion, hydrogen peroxide, and hydroxyl radical), which are generated by renal cortical mitochondria, are important mediators of aminoglycoside-induced acute renal insufficiency (99). 

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