In glucose-limited continuous culture the minimum substrate concentration for growth, s min, is crucial in the competition between the enterobacterium Escherichia coli and Chelatobacter heintzii, an environmentally abundant bacterium

摘要

The competition for glucose between Escherichia coli ML30, a typical copiotrophic enterobacterium and Chelatobacter heintzii ATCC29600, an environmentally successful strain, was studied in a carbon-limited culture at low dilution rates. First, as a base for modelling, the kinetic parameters max and K s were determined for growth with glucose. For both strains, max was determined in batch culture after different precultivation conditions. In the case of C. heintzii, max was virtually independent of precultivation conditions. When inoculated into a glucose-excess batch culture medium from a glucose-limited chemostat run at a dilution rate of 0.075 h 1 C. heintzii grew immediately with a max of 0.170.03 h 1. After five transfers in batch culture, max had increased only slightly to 0.180.03 h 1. A different pattern was observed in the case of E. coli. Inoculated from a glucose-limited chemostat at D0.075 h 1 into glucose-excess batch medium E. coli grew only after an acceleration phase of 3.5 h with a max of 0.52 h 1. After 120 generations and several transfers into fresh medium, max had increased to 0.800.03 h 1. For long-term adapted chemostat-cultivated cells, a K s for glucose of 15 g l 1 for C. heintzii, and of 35 g l 1 for E. coli, respectively, was determined in 14 C-labelled glucose uptake experiments. In competition experiments, the population dynamics of the mixed culture was determined using specific surface antibodies against C. heintzii and a specific 16S rRNA probe for E. coli. C. heintzii outcompeted E. coli in glucose-limited continuous culture at the low dilution rates of 0.05 and 0.075 h 1. Using the determined pure culture parameter values for K s and max, it was only possible to simulate the population dynamics during competition with an extended form of the Monod model, which includes a finite substrate concentration at zero growth rate (s min). The values estimated for s min were dependent on growth rate; at D0.05 h 1, it was 12.6 and 0 g l 1 for E. coli and C. heintzii, respectively. To fit the data at D0.075 h 1, s min for E. coli had to be raised to 34.9 g l 1 whereas s min for C. heintzii remained zero. The results of the mathematical simulation suggest that it is not so much the higher K s value, which is responsible for the unsuccessful competition of E. coli at low residual glucose concentration, but rather the existence of a significant s min.