A model for energy-sufficient culture growth

Pirt's maintenance model has been widely accepted for the effects of growth rate and maintenance on growth yield. However, the interpretation of parameters in Pirt's model as biological constants is difficult for energy-sufficient culture growth. In this study, a mechanistic model for the growth energetics of energy-sufficient chemostat cultures is proposed and verified with literature data. In the model, the overutilization of the energy substrate in energy-sufficient culture growth is attributed to the defective regulation of the energy substrate metabolism and energy uncoupling. The model also uses an "energy surplus" concept to collectively represent the effects of energy excessiveness. The proposed model provides a better quantitative understanding of the maximum growth yield and maintenance of energy-sufficient cultures. It also explains the glucose concentration effect reported in the literature.     

A discrete model of bacterial metabolism

This paper describes a computer model of the intermediary metabolismof bacteria during steady-state growth and during adaptations,e g. to new carbon sources. Metabolic regulation is representedas a process of optimisation, in which the trend is towardsimproved metabolic performance. The model uses linear programmingtechniques for the optimisation The implementation falls intofour phases: (i) assembly of model parameters; (ii) calculations;(iii) storage of solutions and (iv) projection of solutions.The use of a commercial database and a commercial spreadsheethas proved to be of great assistance in the first and thirdphases. A metabolic map format, with the optional addition ofconversion values, names of enzymes or co-factors has been usedto project the results in a form convenient for inspection. Received on August 22, 1985; accepted on January 3, 1985     

Maintenance energy: a general model for energy-limited and energy-sufficient growth

The new model proposed to account for the energy requirement for growth includes both a constant maintenance energy term (m) independent of the specific growth rate and a term (m′) which decreases linearly with increase in specific growth rate and becomes zero at the maximum specific growth rate. The available data for testing the model do not deviate significantly from the relations predicted. Consistent values of the maximum growth yield (Y _G) can be derived, irrespective of whether the cultures are energy limited or energy sufficient. Attention is drawn to the possibility that the constant maintenance energy term may be estimated from the maximum specific growth rate.     

Experimental identification and quantification of glucose metabolism in seven bacterial species

The structurally conserved and ubiquitous pathways of central carbon metabolism provide building blocks and cofactors for the biosynthesis of cellular macromolecules. The relative uses of pathways and reactions, however, vary widely among species and depend upon conditions, and some are not used at all. Here we identify the network topology of glucose metabolism and its in vivo operation by quantification of intracellular carbon fluxes from 13C tracer experiments. Specifically, we investigated Agrobacterium tumefaciens, two pseudomonads, Sinorhizobium meliloti, Rhodobacter sphaeroides, Zymomonas mobilis, and Paracoccus versutus, which grow on glucose as the sole carbon source, represent fundamentally different metabolic lifestyles (aerobic, anaerobic, photoheterotrophic, and chemoheterotrophic), and are phylogenetically distinct (firmicutes, gamma-proteobacteria, and alpha-proteobacteria). Compared to those of the model bacteria Escherichia coli and Bacillus subtilis, metabolisms of the investigated species differed significantly in several respects: (i) the Entner-Doudoroff pathway was the almost exclusive catabolic route; (ii) the pentose phosphate pathway exhibited exclusively biosynthetic functions, in many cases also requiring flux through the nonoxidative branch; (iii) all aerobes exhibited fully respiratory metabolism without significant overflow metabolism; and (iv) all aerobes used the pyruvate bypass of the malate dehydrogenase reaction to a significant extent. Exclusively, Pseudomonas fluorescens converted most glucose extracellularly to gluconate and 2-ketogluconate. Overall, the results suggest that metabolic data from model species with extensive industrial and laboratory history are not representative of microbial metabolism, at least not quantitatively.     

Modulation of glucose metabolism by sulfonylureas in primary cultures of adult rat hepatocytes

Addition of tolbutamide (0.1-5 microM) or glipizide (0.05-5 microM) to primary cultures of adult rat hepatocytes caused a dose-dependent increase of fructose 2,6-bisphosphate concentration. This effect was accompanied by a stimulation of the rate of L-lactate production and by an acceleration of the metabolic flux through the reaction catalysed by 6-phosphofructo 1-kinase. Moreover, the continuous presence of tolbutamide during the first 26 hours of culture mimicked long-term insulin effects by raising fructose 2,6-bisphosphate levels and the rate of L-lactate formation. Glucokinase, 6-phosphofructo 1-kinase and total 6-phosphofructo 2-kinase activities were not found to be significantly different in hepatocytes cultured either in the presence or in the absence of sulfonylurea.     

A cellular automaton model for microcarrier cultures

In order to achieve high cell densities anchoragedependent cells are commonly cultured on microcarriers, where spatial restrictions to cell growth complicates the determination of the growth kinetics. To design and operate large-scale bioreactors for microcarrier cultures, the effect of this spatial restriction to growth, referred to as contact inhibition, must be decoupled from the growth kinetics. In this article, a cellular automaton approach is recommended to model the growth of anchorage-dependent cells on microcarriers. The proposed model is simple to apply yet provides an accurate representation of contact-inhibited cell growth on microcarriers. The distribution of the number of neighboring cells per cell, microcarrier surface areas, and inoculation densities are taken into account with this model. When compared with experimental data for Vero and MRC-5 microcarrier cultures, the cellular automaton predictions were very good. Furthermore, the model can be used to generate contact-inhibition growth curves to decouple the effect of contact-inhibition from growth kinetics. With this information, the accurate determination of kinetic parameters, such as nutrient uptake rates, and the effects of other environmental factors, such as toxin levels, may be determined. (c) 1994 John Wiley & Sons, Inc.     

Utilization of dimeric lignin model compounds by mixed bacterial cultures

Summary The degradation of dimeric phenylpropanoid lignin model compounds using mixed bacterial cultures was studied. The six model compounds contained the most common linkages of lignin: -O-4, -, -5, and -1. The results indicate that it is possible to enrich bacteria which are able to degrade all these compounds. Bacteria were also able to use these dimers as the sole source of carbon for growth. In view of these results it seems probable that bacterial inability to degrade polymeric lignin is due to the physical properties such as the molecular size of lignin.     

Regulation of hepatic glucose metabolism by leptin in pig and rat primary hepatocyte cultures

Direct effects of leptin on gluconeogenesis in rat hepatocytes are equivocal, and model systems from other species have not been extensively explored in assessing the regulation of glucose metabolism by leptin. Therefore, the goal of the present study was to compare the effects of leptin on gluconeogenesis in pig and rat hepatocyte cultures as well as to investigate an underlying mechanism of action at the level of phosphoenolpyruvate carboxykinase (PEPCK). In rat hepatocytes, leptin exposure (3 h, 50 and 100 nM) attenuated glucagon-stimulated hepatic gluconeogenesis by 35 and 38% (P < 0.05), respectively. However, leptin did not produce any significant acute effect in pig hepatocytes. Leptin exposure for 24 h failed to produce any significant effect on gluconeogenesis in either rat or pig hepatocytes cultured in the presence of glucagon or dexamethasone. Mechanistically, there was a 25-35% decrease (P < 0.05) in glucagon-induced PEPCK mRNA levels in rat but not pig hepatocytes cultured with leptin. This effect on PEPCK mRNA was not due to an alteration in the relative abundance of the leptin receptor or the ability of PEPCK to respond to cAMP. The nonuniformity of the effects of leptin on gluconeogenesis in pig and rat hepatocytes indicates differences in leptin action between species. Furthermore, the unique action of leptin in porcine hepatocytes points to the utility of this model system for biomedical research and also underscores the value of comparative studies.     

A model for bacterial activity in the lung

A bivariate model is proposed for the analysis of data from experiments concerning bacterial activity in the mouse lung. The parameters of the model are discussed and estimated. Using the likelihood ratio statistic, simultaneous confidence intervals are constructed. The improvements over previously used models are then presented.     

Microcalorimetric investigations of the metabolism of yeasts. Growth in chemostat cultures on glucose

Summary As part of a project on the production of penicillin, the penicillin production of two strains of Penicillium chrysogenum which have a different penicillin productivity was investigated in bubble column bioreactors and for comparison in stirred fermenters. The main interest of this study were the complicated interrelations between the stirrer speed, the stirrer type, the shear stress, the morphology of the mycelium and broth viscosity as well as the effect of the oxygen transfer behavior on antibiotic productivity.Stirred tank reactors with different turbine stirrers as well as with a draught tube and propeller were employed.The main variable investigated was the stirrer speed. At low stirrer speeds, gas dispersion is inadequate and the insufficient oxygen transfer rate is a limiting factor. At higher stirrer speeds, the oxygen supply of pulpy mycelia is improved and more cell mass is formed. This result is the same for both strains in all three reactors.If the oxygen partial pressure is near the lower cirtical value, a high percentage of the carbon source is converted into penicillin but the penicillin productivity is low due to a low percentage of penicillin producing cells. At oxygen partial pressures just above 8% saturation, the absolute penicillin productivity is maximal. At higher stirrer speeds and dissolved oxygen concentrations the penicillin production phase is shorter, cell growth is higher and a higher percentage of the carbon source is converted into CO₂.In reactors with a draught tube and propeller, a lower productivity is attained than in those with turbine stirrers.The behavior of the two strains is fairly similar. The higher producing strain, however, has a more distinct separation between its periods of growth and production than does the low producing one. At high stirrer speeds the increase in the cell growth rate is less significant and the substrate yield coefficients are higher for the high producing strain than for the low producing one.Symbols C Dissolved oxygen concentration (mg l^–1) - C^* C at saturation (mg l^–1) - k_La Volumetric mass transfer coefficient (h^–1) - OTR Oxygen transfer rate (mg l^–1 h^–1) - OUR Oxygen uptake rate (mg l^–1 h^–1) - rpm Impeller speed (min^–1) - X (Dry) biomass concentration (g kg^–1) - V_g Volumetric gas flow rate (Nl min^–1) - CMC Carboxymethyl cellulose