Thermodynamic efficiency of bacterial growth calculated from growth yield of Pseudomonas oxalaticus OX1 in the chemostat.

In order to determine the thermodynamic efficiency of bacterial growth, Pseudomonas oxalaticus OX1 was grown in carbon-limited continuous cultures. 11 different carbon sources, ranging from oxalate (most oxidised component) to ethanol (most reduced component), were used as limiting substrate in these experiments. From the experimental yield values (expressed as C-mol dry weight produced per C-mol carbon substrate consumed) the thermodynamic efficiencies were calculated. On substrates more reduced than biomass (such as ethanol and glycerol) the thermodynamic efficiency of growth of P. oxalaticus was negative but it reached a maximum of 23 +/- 3% with substrates with a degree of reduction of 3 (citrate) and lower. The actual concentrations of the components involved were incorporated into the calculations but this affected the overall thermodynamic efficiency only to a small extent. This result strengthens the conclusion of Westerhoff et al. (Westerhoff, H.V., Hellingwerf, K.J. and Van Dam, K. (1983) Proc. Natl. Acad. Sci. 80, 305-309) that bacteria have been optimised towards a theoretical thermodynamic efficiency of 24%, corresponding with maximisation of growth rate at optimal efficiency, with highly oxidised substrates.

Effect of macromolecular composition of microorganisms on the thermodynamic description of their growth.

Most models describing bacterial growth, including the original mosaic non-equilibrium thermodynamic (MNET) description, do not take into account that the macromolecular composition of the cells varies with growth rate. The MNET description of bacterial growth is extended to account for such a variation in macromolecular composition of the cells in order to make the MNET description more generally applicable. Klebsiella aerogenes NCTC 418 was cultured in a chemostat under glucose- or ammonia-limited conditions to determine the macromolecular composition at varying growth rate. The dilution rate has a strong influence on the macromolecular composition of the cells. Under glucose-limited conditions an increase of the RNA content of the cells was observed with increasing growth rate. The RNA content of the cells was much lower under ammonia-limited conditions of the cells than under glucose-limited conditions but also showed an increase with increasing growth rates. Under ammonia-limited conditions, the polysaccharide content strongly decreased with increasing growth rate. The other cellular components changed relatively less with changing growth rate. It is shown that the slope of the line relating catabolism to anabolism varies very little due to variation of the macromolecular cell composition with growth rate, at least under the tested conditions.

Energetic consequences of two mutations in Escherichia coli K+ uptake systems for growth under potassium-limited conditions in the chemostat.

The energetics of growth of two Escherichia coli strains (TK 2240 and TK 2242) differing in Km of the high-affinity potassium uptake system and lacking the low-affinity system were studied in the chemostat under potassium-limited conditions. The results were compared with the results obtained previously (Mulder, M.M., Teixeira de Mattos, M.J., Postma, P.W. and Van Dam, K. (1986) Biochim. Biophys. Acta 851, 223-228) with the wild-type FRAG-1, having two potassium uptake systems, and FRAG-5, a mutant which lacks the high-affinity potassium uptake system. We postulated that the high-affinity potassium uptake system was able to generate such a steep gradient across the membrane that the low-affinity system would act in reverse, thus creating a futile cycle of potassium ions at the cost of energy. As a result, FRAG-1 would show a higher ATP turnover at all growth rates tested than the mutant FRAG-5, in which strain the proposed futile cycle is interrupted because of the lack of the high-affinity system. It is shown here that the results obtained with TK 2240 and TK 2242 are in line with our hypothesis of futile potassium cycling. Under our experimental conditions, the yield on potassium was not dependent on the kinetic parameters of the uptake systems. The (thermodynamic) energy demand of the uptake systems determined the carbon substrate conversion required to achieve this yield.

Continued growth of Escherichia coli after stopping medium addition to a K+-limited chemostat culture.

The steady-state bacterial dry wt of Escherichia coli, growing under K+-limited conditions in the chemostat, was inversely dependent on the growth rate. This phenomenon was more carefully investigated in medium-flow stop experiments. Growth did not stop immediately but continued for a time, initially at the same rate as before. The dry wt increased to a value corresponding to a steady-state growth rate near zero, independent of the initial specific growth rate. This was observed in both the wild-type strain and a mutant that lacked the high-affinity K+ uptake system. The wild-type strain maintained a low extracellular K+ concentration both in the chemostat under steady-state conditions and after stopping the medium flow. The mutant, on the other hand, maintained a much higher extracellular K+ concentration in the steady state, which decreased to much lower values after stopping the medium flow. From the increase in bacterial dry wt and the low external K+ concentration after stopping the medium flow it is concluded that the intracellular K+ is redistributed among the cells, including new cells. The growth yield on K+ was highest in the stationary growth phase of a batch culture and all steady-state cultures converged ultimately to this yield value after the medium flow had been stopped. It is proposed that the growth rate of E. coli under K+-limited conditions is determined by the intracellular K+ concentration.