Metabolic flux analysis of Escherichia coli expressing the Bacillus subtilis acetolactate synthase in batch and continuous cultures.

Metabolically engineered Escherichia coli expressing the B. subtilis acetolactate synthase has shown to be capable of reducing acetate accumulation. This reduction subsequently led to a significant enhancement in recombinant protein production. The main focus of this study is to systematically examine the effect of ALS in the metabolic patterns of E. coli in batch and continuous culture. The specific acetate production rate of a strain carrying the B. subtilis als gene is 75% lower than that of the control strain (host carrying the control plasmid pACYC184) in batch cultures. The ALS strain is further demonstrated to be capable of maintaining a reduced specific acetate production rate in continuous cultures at dilution rates ranging from 0.1 to 0.4 h-1. In addition, this ALS strain is shown to have a higher ATP yield and lower maintenance coefficient. The metabolic flux analysis of carbon flux distribution of the central metabolic pathways and at the pyruvate branch point reveals that this strain has the ability to channel excess pyruvate to the much less toxic compound, acetoin.     

Control of mixed-substrate utilization in continuous cultures of Escherichia coli

The chemostat culture technique was used to study the control mechanisms which operate during utilization of mixtures of glucose and lactose and glucose and l-aspartic acid by populations of Escherichia coli B6. Constitutive mutants were rapidly selected during continuous culture on a mixture of glucose and lactose, and the beta-galactosidase level of the culture increased greatly. After mutant selection, the specific beta-galactosidase level of the culture was a decreasing function of growth rate. In cultures of both the inducible wild type and the constitutive mutant, glucose and lactose were simultaneously utilized at moderate growth rates, whereas only glucose was used in the inducible cultures at high growth rates. Catabolite repression was shown to be the primary mechanism of control of beta-galactosidase level and lactose utilization in continuous culture on mixed substrates. In batch culture, as in the chemostat, catabolite repression acting by itself on the lac enzymes was insufficient to prevent lactose utilization or cause diauxie. Interference with induction of the lac operon, as well as catabolite repression, was necessary to produce diauxic growth. Continuous cultures fed mixtures of glucose and l-aspartic acid utilized both substrates at moderate growth rates, even though the catabolic enzyme aspartase was linearly repressed with increasing growth rate. Although the repression of aspartase paralleled the catabolite repression of beta-galactosidase, l-aspartic acid could be utilized even at very low levels of the catabolic enzyme because of direct anabolic incorporation into protein.     

On-line monitoring of growth of Escherichia coli in batch cultures by bioluminescence

Bioluminescence was used to monitor growth of Escherichia coli in batch cultures on-line. Light emission of a strain engineered for constitutive bioluminescence was monitored with a simple set-up consisting of a photodiode, a photodetector amplifier and a recorder. Bioluminescence and colony forming units (CFU) of the cultures increased and decreased proportionally and were correlated during every growth phase at temperatures between 28 °C and 40 °C. Up to the late log (deceleration) phase, both light emission and CFU increased rapidly. Beyond the stationary phase these characteristics decreased very slowly at lower temperatures, while at higher ones they declined more rapidly. Towards the end of the cultivation, light emission of the cultures dropped to undetectable levels, even though CFU were recovered. This was particularly marked at lower temperatures where non-luminescent cultures retained very high CFU. This indicates that the actual metabolism of cells in a culture can be at a very low level or completely shut down, yet cells retain their capability to be culturable. The on-line technology described here has a number of potential uses in the laboratory and industry. Received: 30 September 1999 / Received revision: 29 November 1999 / Accepted: 3 December 1999     

Comparison of growth, acetate production, and acetate inhibition of Escherichia coli strains in batch and fed-batch fermentations

The growth characteristics and acetate production of several Escherichia coli strains were compared by using shake flasks, batch fermentations, and glucose-feedback-controlled fed-batch fermentations to assess the potential of each strain to grow at high cell densities. Of the E. coli strains tested, including JM105, B, W3110, W3100, HB101, DH1, CSH50, MC1060, JRG1046, and JRG1061, strains JM105 and B were found to have the greatest relative biomass accumulation, strain MC1060 accumulated the highest concentrations of acetic acid, and strain B had the highest growth rates under the conditions tested. In glucose-feedback-controlled fed-batch fermentations, strains B and JM105 produced only 2 g of acetate.liter-1 while accumulating up to 30 g of biomass.liter-1. Under identical conditions, strains HB101 and MC1060 accumulated less than 10 g of biomass.liter-1 and strain MC1060 produced 8 g of acetate.liter-1. The addition of various concentrations of sodium acetate to the growth medium resulted in a logarithmic decrease, with respect to acetate concentration, in the growth rates of E. coli JM105, JM105(pOS4201), and JRG1061. These data indicated that the growth of the E. coli strains was likely to be inhibited by the acetate they produced when grown on media containing glucose. A model for the inhibition of growth of E. coli by acetate was derived from these experiments to explain the inhibition of acetate on E. coli strains at neutral pH.     

Comparison of growth, acetate production, and acetate inhibition of Escherichia coli strains in batch and fed-batch fermentations.

The growth characteristics and acetate production of several Escherichia coli strains were compared by using shake flasks, batch fermentations, and glucose-feedback-controlled fed-batch fermentations to assess the potential of each strain to grow at high cell densities. Of the E. coli strains tested, including JM105, B, W3110, W3100, HB101, DH1, CSH50, MC1060, JRG1046, and JRG1061, strains JM105 and B were found to have the greatest relative biomass accumulation, strain MC1060 accumulated the highest concentrations of acetic acid, and strain B had the highest growth rates under the conditions tested. In glucose-feedback-controlled fed-batch fermentations, strains B and JM105 produced only 2 g of acetate.liter-1 while accumulating up to 30 g of biomass.liter-1. Under identical conditions, strains HB101 and MC1060 accumulated less than 10 g of biomass.liter-1 and strain MC1060 produced 8 g of acetate.liter-1. The addition of various concentrations of sodium acetate to the growth medium resulted in a logarithmic decrease, with respect to acetate concentration, in the growth rates of E. coli JM105, JM105(pOS4201), and JRG1061. These data indicated that the growth of the E. coli strains was likely to be inhibited by the acetate they produced when grown on media containing glucose. A model for the inhibition of growth of E. coli by acetate was derived from these experiments to explain the inhibition of acetate on E. coli strains at neutral pH.     

The effect of acetate on population heterogeneity in different cellular characteristics of Escherichia coli in aerobic batch cultures

Acetate as the major by-product in industrial-scale bioprocesses with Escherichia coli is found to decrease process efficiency as well as to be toxic to cells, which has several effects like a significant induction of cellular stress responses. However, the underlying phenomena are poorly explored. Therefore, we studied time-resolved population heterogeneity of the E. coli growth reporter strain MG1655/pGS20PrrnBGFPAAV expressing destabilized green fluorescent protein during batch growth on acetate and glucose as sole carbon sources. Additionally, we applied five fluorescent stains targeting different cellular properties (viability as well as metabolic and respiratory activity). Quantitative analysis of flow cytometry data verified that bacterial populations in the bioreactor are more heterogeneous in growth as well as stronger metabolically challenged during growth on acetate as sole carbon source, compared to growth on glucose or acetate after diauxic shift. Interestingly, with acetate as sole carbon source, significant subpopulations were found with some cells that seem to be more robust than the rest of the population. In conclusion, following batch cultures population heterogeneity was evident in all measured parameters. Our approach enabled a deeper study of heterogeneity during growth on the favored substrate glucose as well as on the toxic by-product acetate. Using a combination of activity fluorescent dyes proved to be an accurate and fast alternative as well as a supplement to the use of a reporter strain. However, the choice of combination of stains should be well considered depending on which population traits to aim for.     

Dynamics of growth in batch and continuous cultures of Saccharomyces cerevisiae during shifts from aerobiosis to anaerobiosis and reverse

Summary Saccharomyces cerevisiae H 1022 was cultivated in batch and continuous culture on a glucose substrate. The yeast was subjected to a sudden change from aerobic to anaerobic growth conditions by switching the inlet-gas stream from air to dinitrogen. The dynamics of growth and product formation during the periods of adaptation were studied. A structured growth model based on Monod-Blackman-kinetics was applied to simulate these shift-experiments. The immediate switch of the yeast to maximum anaerobic growth and ethanol production predicted by this model was confirmed very well in the chemostat-experiments. However, a slow adaptation was evident for the switch from anaerobiosis back to aerobiosis.Gesellschaft für Biotechnologische Forschung mbH, Mascheroder Weg 1, D-3300 Braunschweig-Stöckheim, Federal Republic of Germany     

Effect of acetate on the growth of Escherichia coli during aerobiosis and anaerobiosis

The dynamics of acetate accumulation was studied in Escherichia coli K-12 batch cultures with a substrate addition. At pH 7.0, the growth stopped at 150 mmoles of acetate per litre of the medium under the aerobic conditions or at 35 mmoles of acetate per litre of the medium under the anaerobic conditions. Experiments with extraneous addition of acetate suggest that acetic acid plays a key role in inhibiting the growth of E. coli by acid metabolites. The authors propose a hypothetical mechanism to account for the inhibiting action of acetate.     

New patterns of mixed-substrate utilization during batch growth of Escherichia coli K12

Microbial growth on mixtures of substrates is of considerable engineering and biological interest. Most of the work until now has dealt with microbial growth on binary mixtures of sugars or polyols. In these cases, it is often found that no matter how the inoculum is precultured, only one of the two substrates is consumed in the first growth phase, leading to the diauxic growth pattern. The goal of the experiments reported here is to investigate growth on mixtures containing at least one organic acid. These experiments show that the substrate utilization patterns in such mixtures are qualitatively different from the diauxic growth pattern. For instance, during growth of Escherichia coli K12 on certain binary mixtures of organic acids, the two substrates are utilized simultaneously, and the mixed-substrate maximum specific growth rate exceeds the single-substrate maximum specific growth rate on either one of the two constituent substrates. Furthermore, the very same mixed-substrate maximum specific growth and substrate uptake rates are observed no matter how the inoculum is precultured. On the other hand, in a mixture of glucose and pyruvate, the maximum specific growth rate seems to depend on the preculturing conditions, thus suggesting the existence of multiple physiological quasi-steady states. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 747-757, 1997.     

Control of pH and inorganic carbon in batch cultures of cyanobacteria

Summary Synechococcus leopoliensis was grown in batch cultures gassed with air or CO₂ in air to test for effects of gassing on pH drift and growth. A method is described whereby pH and inorganic carbon are held constant during rapid growth.     

A metabolic model of cellular energetics and carbon flux during aerobic Escherichia coli fermentation

An integrated metabolic model for the production of acetate by Escherichia coli growing on glucose under aerobic conditions was presented previously (Ko et al., 1993). The resulting model equations can be used to explain phenomena often observed with industrial fermentations, i.e., increased acetate production which follows from high glucose uptake rate, a low dissolved oxygen concentration, a high specific growth rate, or a combination of these conditions. However, several questions still need to be addressed. First, cell composition is growth rate and media dependent. Second, the macromolecular composition varied between E. coli strains. And finally, a model that represents the carbon fluxes between the Embden-Meyerhof-Parnas (EMP) and the hexose monophosphate (HMP) pathways when cells are subject to internal and/or external stresses is still not well defined. In the present work, we have made an effort to account for these effects, and the resulting model equations show good agreement for wild-type and recombinant E. coli experimental data for the acetate concentration, the onset of acetate secretion, and cell yield based on glucose. These results are useful for optimizing aerobic E. coli fermentation processes. More specifically, we have determined the EMP pathway carbon flux profiles required by the integrated metabolic model for an accurate fit of the acetic acid profile data from a wild-type E. coli strain ML308. These EMP carbon flux profiles were correlated with a dimensionless measurement of biomass and then used to predict the acetic acid profiles for E. coli strain F-122 expressing human immunodeficiency virus-(HIV(528)) beta-galactosidase fusion protein. The effect of different macromolecular compositions and growth rates between these two E. coli strains required a constant scaling factor for improved quantitative predictions.     

Flow cytometry studies of recombinant Escherichia coli in batch and continuous cultures: DNA and RNA contents; light-scatter parameters

Flow cytometry has been used to study the contents of macromolecular compounds and light-scatter parameters in batch and continuous cultures of a recombinant Escherichia coli strain that forms protein inclusion bodies. Changes in relative DNA and RNA contents and cell mass as estimated by forward-angle light scatter were detected and tightly correlated in batch culture. In addition, heterogeneity of wide-angle light scatter (WALS), which we related to the presence of cellular inclusion bodies, was observed. In contrast, the relative RNA content and cell mass did not change during continuous culture, and homogeneity of WALS was found. In addition, unexpected changes in relative DNA content were observed after 67 h of culture, indicating a change in bacterial physiology.     

Responses in continuous cultures of lysogenic Escherichia coli following induction

Bacteriophages are widely distributed in nature and may be important factors in regulating populations of their hosts. Model continuous culture systems of a single bacterial species and a temperate parasitic phage have been studied. Steady state cultures of lysogenic Escherichia coli 159T^? (λcts) produced a small quantity of free λ cts phage. Temperature shocking such a culture resulted in a sharp increase in phage concentration with a concomitant fall in cell population. With time the system returned to a steady state condition.     

Dynamics of glutamate synthesis and excretion fluxes in batch and continuous cultures of temperature-triggered Corynebacterium glutamicum

Corynebacterium glutamicum 2262 strain, when triggered for glutamate excretion, experiences a rapid decrease in growth rate and increase in glutamate efflux. In order to gain a better quantitative understanding of the factors controlling the metabolic transition, the fermentation dynamics was investigated for a temperature-sensitive strain cultivated in batch and glucose-limited continuous cultures. For non-excreting cells at 33°C, increasing the growth rate resulted in strong increases in the central metabolic fluxes, but the intracellular glutamate level, the oxoglutarate dehydrogenase complex (ODHC) activity and the flux distribution at the oxoglutarate node remained essentially constant. When subjected to a temperature rise to 39°C, at both high- and low-metabolic activities, the bacteria showed a rapid attenuation in ODHC activity and an increase from 28% to more than 90% of the isocitrate dehydrogenase flux split towards glutamate synthesis. Simultaneously to the reduction in growth rate, the cells activated a high capacity export system capable of expelling the surplus of synthesized glutamate.     

The application of a novel heat flux calorimeter for studying growth of Escherichia coli W in aerobic batch culture

The modification and principle of a novel heat flux calorimeter for the in situ, on-line measurement of the heat generated during microbial growth is described. Data concerning the physical characterization of the calorimeter as a fermentor, including stability and sensitivity of the heat signal, are presented. The calorimeter has been successfully applied to the study of the aerobic batch culture of Escherichia coli W on glucose under carbon and nitrogen limitation. A direct correlation between growth and heat evolution was obtained. Quantitative analysis of the data suggests that the new calorimetric technique could be used for monitoring growth and specific metabolic events, for convenient medium optimization, and as a basis for a novel fermentation process control system.