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Schrödinger stated in his landmark book, What is Life?, that life feeds on negative entropy. In this contribution, the validity of this statement is discussed through a careful thermodynamic analysis of microbial growth processes. In principle, both feeding on negative entropy, i.e. yielding products of higher entropy than the substrates, and generating heat can be used by microorganisms to rid themselves of internal entropy production resulting from maintenance and growth processes. Literature data are reviewed in order to compare these two mechanisms. It is shown that entropy-neutral, entropy-driven, and entropy-retarded growth exist. The analysis of some particularly interesting microorganisms shows that enthalpy-retarded microbial growth may also exist, which would signify a net uptake of heat during growth. However, the existence of endothermic life has never been demonstrated in a calorimeter. The internal entropy production in live cells also reflects itself in the Gibbs energy dissipation accompanying growth, which is related quantitatively to the biomass yield. An empirical correlation of the Gibbs energy dissipation in terms of the physico-chemical nature of the growth substrate has been proposed in the literature and can be used to predict the biomass yield approximately. The ratio of enthalpy change and Gibbs energy change can also be predicted since it is shown to be approximately equal to the same ratio of the relevant catabolic process alone.

The bioconversion of L-phenylalanine (L-Phe) to 2-phenylethanol (PEA) by the yeast Saccharomyces cerevisiae is limited by the toxicity of the product. PEA extraction by a separate organic phase in the fermenter is the ideal in situ product recovery (ISPR) technique to enhance productivity. Oleic acid was chosen as organic phase for two-phase fed-batch cultures, although it interfered to some extent with yeast viability. There was a synergistic inhibitory impact toward S. cerevisiae in the presence of PEA, and therefore a maximal PEA concentration in the aqueous phase of only 2.1 g/L was achieved, compared to 3.8 g/L for a normal fed-batch culture. However, the overall PEA concentration in the fermenter was increased to 12.6 g/L, because the PEA concentration in the oleic phase attained a value of 24 g/L. Thus, an average volumetric PEA production rate of 0.26 g L(-1) h(-1) and a maximal volumetric PEA production rate of 0.47 g L(-1) h(-1) were achieved in the two-phase fed-batch culture. As ethanol inhibition had to be avoided, the production rates were limited by the intrinsic oxidative capacity of S. cerevisiae. In addition, the high viscosity of the two-phase system lowered the k(l)a, and therefore also the productivity. Thus, if a specific ISPR technique is planned, it consequently has to be remembered that the productivity of this bioconversion process is also quickly limited by the k(l)a of the fermenter at high cell densities.

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In this work, dielectric spectroscopy was used to monitor two CHO perfusion culture experiments (B14 and B16). The capacitance of the cell suspension was recorded every 20 minutes over an excitation frequency range of 0.2 MHz to 10.0 MHz. A phase plot of the capacitance at a low excitation frequency vs. the value at a higher frequency proved to be an accurate indicator of the major transition points of the culture, i.e., maximum cell viability, end of lactate consumption, point of zero viability. For both experiments, the capacitance signal correlated very well (R(2) >0.98) with viable cell number up to concentrations of 1 x 10(7) cells/mL. Visual observation of the capacitance spectra indicated that changes in the capacitance relative to frequency were related to the cellular morphology. A multivariate model was developed using off-line data that could predict the median cell diameter within a single experiment (B14) with an error of 0.34 microm (2%). Upon extension to a subsequent experiment (B16), the predicted error was 1.18 microm (9%).

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Aerobic chemostat cultures of Saccharomyces cerevisiae were performed under carbon-, nitrogen-, and dual carbon-and nitrogen-limiting conditions. The glucose concentration was kept constant, whereas the ammonium concentration was varied among different experiments and different dilution rates. It was found that both glucose and ammonium were consumed at the maximal possible rate, i.e., the feed rate, over a range of medium C/N ratios and dilution rates. To a small extent, this was due to a changing biomass composition, but much more important was the ability of uncoupling between anabolic biomass formation and catabolic energy substrate consumption. When ammonium started to limit the amount of biomass formed and hence the anabolic flow of glucose, this was totally or at least partly compensated for by an increased catabolic glucose consumption. The primary response when glucose was present in excess of the minimum requirements for biomass production was an increased rate of respiration. The calculated specific oxygen consumption rate, at D = 0.07 h-', was more than doubled when an additional nitrogen limitation was imposed on the cells compared with that during single glucose limitation. However, the maximum respiratory capacity decreased with decreasing nitrogen concentration. The saturation level of the specific oxygen consumption rate decreased from 5.5 to 6.0 mmol/g/h under single glucose limitation to about 4.0 mmol/g/h at the lowest nitrogen concentration tested. The combined result of this was that the critical dilution rate, i.e., onset of fermentation, was as low as 0.10 h-1 during growth in a medium with a low nitrogen concentration compared with 0.20 hobtained under single glucose limitation.During growth of Saccharomyces cerevisiae in a chemostat with glucose as the sole limiting substrate, energy rather than carbon limits biomass formation (7,22,29), i.e., it can be regarded as a catabolic limitation. If, on the other hand, a nitrogen source such as ammonium is used as the limiting substrate, this can instead be viewed as an anabolic limitation. Several studies, concerning bacteria, have shown that anabolism and catabolism do not seem to be tightly coupled (3,5,12,(16)(17)(18)(19)(20)25). The catabolic activity is often higher than what can be explained by the anabolic requirements for ATP when the energy source is present in excess. When glucose-grown chemostat cultures of the bacterium Klebsiella aerogenes were subjected to a limitation other than a glucose limitation, it was found that the specific respiration rate increased compared with that of glucose-limited cells. In addition, when the energy source was in excess, the cells responded by excreting a number of products more oxidized than the carbon source, i.e., not a fermentative metabolism, a phenomenon named overflow metabolism (3,12,(17)(18)(19)(20)25). K aerogenes is the most extensively studied in this respect, but several other bacteria seem to respond in a similar way (16,20). In regards to the yeast S. cerevisiae, uncoupling between c...