  When steady state chemostat cultures are abruptly exposed to substrate-excess
conditions, they exhibit long lags before adjusting to the new environment. The
identity of the rate-limiting step for this slow response can be inferred from
the initial yields and specific growth rates measured by exposing steady state
cultures at various dilution rates to substrate-excess conditions. We measured
these parameters for glucose-limited cultures of E. coli ML308 growing at
various dilution rates between 0.03 and 0.6 1/hr. In all the cases, the initial
yields were 20-30% less than the steady state yields. The decline of the yield
implies that overflow metabolism is triggered in response to excess glucose. It
is therefore unlikely that the initial response of the cells is limited by
substrate uptake. The initial specific growth rates of cultures growing at low
dilution rates (D = 0.03, 0.05, 0.075, 0.1, 0.3 1/hr) were significantly higher
than the steady state specific growth rates. However, the increment in the
specific growth rate decreased with the dilution rate, and at D=0.6 1/hr, there
was no improvement in the specific growth rate. The initial specific growth
rates varied hyperbolically with the dilution, decreasing sharply at dilution
rates below 0.1 1/hr and saturating at D=0.6 1/hr. This is consistent with a
picture in which the initial response is limited by the activity of glutamate
dehydrogenase.
