Fed-batch culture: Modeling and application in the study of microbiol energetics

The continuously fed batch reactor for measuring microbial growth rates

The contiuously fed batch reactor (CFBR) is proposed as an alternative technique to the traditional chemostat and batch cultures, for measuring microbial growth rates. After reviewing the pitfalls which plague the conventional growth measurement techniques, the methodology for operating the CFBR to generate specific growth-rate-versus-substrate-concentration data is detailed. This information is extracted from the transient state of the CFBR where both the biomass and substrate concentration show extrema in time. It is suggested that the CFBR can be used for measuring microbial growth rates at low rates at low substrate concentrations where the chemostat method normally encounters difficulties.

Modeling of bacterial growth; formulation and evaluation of a structured model

Models which consider changes in the composition of biomass in response to environmental changes are called Structured models. They provide a more comprehensive description of microbial behavior than unstructured models. Compared with the unstructured modeling efforts, very little has so far been done on the theory and practice of structured model building. In most of the works reported so far, no experimental data were provided, and hence no means of testing the proposed models were offered. Others only reported macroscopic response data and not the cellular composition. In an attempt to fill some of the gaps in this field, in this work, first the general formal approach to structured modeling is developed in matrix notation. Then, a simple two-compartmental model, i.e., a structured model describing the activity of the biomass with two variables, is described. The cell is divided into two fractions, one of which relates to the RNA fraction. The proposed model was then critically evaluated with experimental data, including the RNA data, obtained from fed-batch and continuous-culture experiments. The importance of using cellular structure data for model verification, i.e., RNA data in this case, is shown. Shortcomings and capabilities of the developed model are discussed.

Structured modeling of a microbial system: II. Experimental verification of a structured lactic acid fermentation model

A two-compartment model for the lactic acid fermentation with Streptococcus cremoris is experimentally verified. The seven parameters of the model are determined using steady-state chemostat data at varying values of dilution rate, D, but with a constant feed concentration, s(f), of a single carbohydrate source (glucose, lactose, or galactose), and a constant feed concentration of s(Nf) of the N source. Steady-state measurements of the RNA content at different exit concentrations, s, of the carbohydrate are included to calculate kinetic parameters that determine the cell composition for varying operating conditions. The model is tested using data from a large set of steady-state and non-steady-state experiments: batch fermentations and step and pulse experiments in a chemostat. Both qualitatively and quantitatively the major features of the model are confirmed: the external substrates enter into intracellular high-energy building blocks, and lactic acid is formed as a by-product of these reactions. Cell growth depends on the fraction of active components (X(A)) of the cell and is not accompanied by lactic acid production. Possible model modifications are discussed, primarily to obtain a better description of lactic acid fermentation at nongrowth conditions.

Application of bioenergetics to modelling the microbial conversion of D-xylose to 2,3-butanediol

During the oxygen limiting growth of Klebsiella oxytoca, the xylose metabolism may be considered as consisting of three components: conversion to 2,3-butanediol by "fermentation," oxidation to carbon dioxide by respiration, and assimilation to cell mass. The amount of energy required for the assimilation of cell mass is assumed to determine the extent to which the two energy producing reactions occur. The activity of each energy producing pathway is also determined by the availability of oxygen and by the energy yield of each pathway. These relationships can be quantified by equating the ATP required for growth and maintenance to the ATP produced by the energy producing reactions. The resulting equation for butanediol production appears similar to the Luedeking and Piret model where the parameters alpha and beta are related to the maximum cell yield from ATP and the maintenance energy requirement. These parameters were estimated from 14 batch fermentations, and the resulting simulation was used to describe the effects of the oxygen transfer rate and the initial xylose concentration on the yields and rates of the 2,3-butanediol fermentation.

Microbial maintenance: a critical review on its quantification

Microbial maintenance is an important concept in microbiology. Its quantification, however, is a subject of continuous debate, which seems to be caused by (1) its definition, which includes nongrowth components other than maintenance; (2) the existence of partly overlapping concepts; (3) the evolution of variables as constants; and (4) the neglect of cell death in microbial dynamics. The two historically most important parameters describing maintenance, the specific maintenance rate and the maintenance coefficient, are based on partly different nongrowth components. There is thus no constant relation between these parameters and previous equations on this subject are wrong. In addition, the partial overlap between these parameters does not allow the use of a simple combination of these parameters. This also applies for combinations of a threshold concentration with one of the other estimates of maintenance. Maintenance estimates should ideally explicitly describe each nongrowth component. A conceptual model is introduced that describes their relative importance and reconciles the various concepts and definitions. The sensitivity of maintenance on underlying components was analyzed and indicated that overall maintenance depends nonlinearly on relative death rates, relative growth rates, growth yield, and endogenous metabolism. This quantitative sensitivity analysis explains the felt need to develop growth-dependent adaptations of existing maintenance parameters, and indicates the importance of distinguishing the various nongrowth components. Future experiments should verify the sensitivity of maintenance components under cellular and environmental conditions.

Thermodynamics of organisms in the context of dynamic energy budget theory

We carry out a thermodynamic analysis to an organism. It is applicable to any type of organism because (1) it is based on a thermodynamic formalism applicable to all open thermodynamic systems and (2) uses a general model to describe the internal structure of the organism--the dynamic energy budget (DEB) model. Our results on the thermodynamics of DEB organisms are the following. (1) Thermodynamic constraints for the following types of organisms: (a) aerobic and exothermic, (b) anaerobic and exothermic, and (c) anaerobic and endothermic; showing that anaerobic organisms have a higher thermodynamic flexibility. (2) A way to compute the changes in the enthalpy and in the entropy of living biomass that accompany changes in growth rate solving the problem of evaluating the thermodynamic properties of biomass as a function of the amount of reserves. (3) Two expressions for Thornton's coefficient that explain its experimental variability and theoretically underpin its use in metabolic studies. (4) A mechanism that organisms in non-steady-state use to rid themselves of internal entropy production: "dilution of entropy production by growth." To demonstrate the practical applicability of DEB theory to quantify thermodynamic changes in organisms we use published data on Klebsiella aerogenes growing aerobically in a continuous culture. We obtain different values for molar entropies of the reserve and the structure of Klebsiella aerogenes proving that the reserve density concept of DEB theory is essential in discussions concerning (a) the relationship between organization and entropy and (b) the mechanism of storing entropy in new biomass. Additionally, our results suggest that the entropy of dead biomass is significantly different from the entropy of living biomass.

Degradation of Toluene and Trichloroethylene by Burkholderia cepacia G4 in Growth-Limited Fed-Batch Culture

Burkholderia (Pseudomonas) cepacia G4 was cultivated in a fed-batch bioreactor on either toluene or toluene plus trichloroethylene (TCE). The culture was allowed to reach a constant cell density under conditions in which the amount of toluene supplied equals the maintenance energy demand of the culture. Compared with toluene only, the presence of TCE at a toluene/TCE ratio of 2.3 caused a fourfold increase in the specific maintenance requirement for toluene from 22 to 94 nmol mg of cells (dry weight)(sup-1) h(sup-1). During a period of 3 weeks, approximately 65% of the incoming TCE was stably converted to unidentified products from which all three chlorine atoms were liberated. When toluene was subsequently omitted from the culture feed while TCE addition continued, mutants which were no longer able to grow on toluene or to degrade TCE appeared. These mutants were also unable to grow on phenol or m- or o-cresol but were still able to grow on catechol and benzoate. Plasmid analysis showed that the mutants had lost the plasmid involved in toluene monooxygenase formation (pTOM). Thus, although strain G4 is much less sensitive to TCE toxicity than methanotrophs, deleterious effects may still occur, namely, an increased maintenance energy demand in the presence of toluene and plasmid loss when no toluene is added.

Exponentially fed-batch cultures as an alternative to chemostats: the case of penicillin acylase production by recombinant E. coli

Exponentially fed-batch cultures (EFBCs), fed with medium containing a highly concentrated carbon source, are commonly employed for attainment of high cell densities. However, large variations in environmental conditions occur, and quasi-steady-state is usually achieved only for the limiting substrate concentration, restricting the use of such cultures in kinetic characterization studies. In this work we report the production of recombinant penicillin acylase (PA) in EFBC of an E. coli JM101 transformed with the pPA102 plasmid, which includes the PA gene under regulation of the lacZ gene promoter and using isopropyl-beta-thio-galactopyranoside (IPTG) as inducer. The culture was fed with nonconcentrated complete medium, resulting in the attainment of quasi-steady-state conditions not only in substrate concentration, but also in cell concentration, and in the specific rates of growth, product production, and substrate consumption. Similar transient behavior was observed between EFBC and chemostat results. At quasi-steady-state, the dilution rate in the EFBC equaled the growth rate. Specific PA production rate during the fed-batch phase remained relatively constant at each dilution rate and followed typical Luedeking-Piret kinetics, with growth-associated and non-growth-associated constants of 142 U gDCW-1 and 7.2 U gDCW-1 h-1, respectively. Specific glucose consumption rate linearly increased from 0.025 to 0.6 g gDCW-1 h-1 as the dilution rate increased from 0.01 to 0.35 h-1. The maximum specific PA activity increased with decreasing dilution rate, reaching its highest value of 2.0 U mg-1 at a dilution rate of 0.01 h-1, the lowest dilution tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Modeling of high cell density fed batch cultivation

Escherichia coli was grown in carbon- and energy source-limited fed batch cultures to study the effect of osmotic stress and different feed rates on the growth kinetics. An unstructured model based on the linear equation for substrate consumption provided an adequate description of the bacterial growth during the first phase of biomass production (20 h), except for cultures exposed to osmotic stress by the addition of 0.5 M NaCl. The addition of salt to the culture media had a large effect on the energetics, that could not simply be described in terms of an increased maintenance requirement. In the later phase of growth, an extensive decline in viability for all cultures was observed. Coincidentally, the specific sugar uptake rate approached a lower limit. It is concluded that the total obtainable biomass in a fed batch culture is strongly affected by the magnitudes of the substrate feed rate and the ionic strength of the culture medium.

Metabolic and kinetic studies of hybridomas in exponentially fed-batch cultures using T-flasks

Exponentially fed-batch cultures (EFBC) of a murine hybridoma in T-flasks were explored as a simple alternative experimental tool to chemostats for the study of metabolism, growth and monoclonal antibody (MAb) production kinetics. EFBC were operated in the variable volume mode using an exponentially increasing and predetermined stepwise feeding profile of fresh complete medium. The dynamic and steady-state behaviors of the EFBC coincided with those reported for chemostats at dilution rates below the maximum growth rate. In particular, steady-state for growth rate and concentration of viable cells, glucose, and lactate was attained at different dilution rates between 0.005 and 0.05 h-1. For such a range, the glucose and lactate metabolic quotients and the steady-state glucose concentration increased, whereas total MAb, volumetric, and specific MAb production rates decreased 65-, 6-, and 3-fold, respectively, with increasing dilution rates. The lactate from glucose yield remained relatively constant for dilution rates up to 0.03 h-1, where it started to decrease. In contrast, viability remained above 80% at high dilution rates but rapidly decreased at dilution rates below 0.02 h-1. No true washout occurred during operation above the maximum growth, as concluded from the constant viable cell number. However, growth rate decreased to as low as 0.01 h-1, suggesting the requirement of a minimum cell density, and concomitant autocrine growth factors, for growth. Chemostat operation drawbacks were avoided by EFBC in T-flasks. Namely, simple and stable operation was obtained at dilution rates ranging from very low to above the maximum growth rate. Furthermore, simultaneous operation of multiple experiments in reduced size was possible, minimizing start-up time, media and equipment costs.

Modeling of microbial substrate conversion, growth and product formation in a recycling fermentor

Paracoccus denitrificans and Bacillus licheniformis were grown in a carbon- and energy source-limited recycling fermentor with 100% biomass feedback. Experimental data for biomass accumulation and product formation as well as rates of carbon dioxide evolution and oxygen consumption were used in a parameter optimization procedure. This procedure was applied on a model which describes biomass growth as a linear function of the substrate consumption rate and the rate of product formation as a linear function of the biomass growth rate. The fitting procedure yielded two growth domains for P. denitrificans. In the first domain the values for the maximal growth yield and the maintenance coefficient were identical to those found in a series of chemostat experiments. The second domain could be described best with linear biomass increase, which is equal to a constant growth yield. Experimental data of a protease producing B. licheniformis also yielded two growth domains via the fitting procedure. Again, in the first domain, maximal growth yield and maintenance requirements were not significantly different from those derived from a series of chemostat experiments. Domain 2 behaviour was different from that observed with P. denitrificans. Product formation halts and more glucose becomes available for biomass formation, and consequently the specific growth rate increases in the shift from domain 1 to 2. It is concluded that for many industrial production processes, it is important to select organisms on the basis of a low maintenance coefficient and a high basic production of the desired product. It seems less important that the maximal production becomes optimized, which is the basis of most selection procedures.

Fed-batch culture technology

The use of fed-batch procedures offers distinct advantages over other modes of operation of bioreactors, and is a widely researched technique. These advantages are discussed; some uses of fed-batch procedures and the associated methods of modelling and control are reviewed.