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.