Purpose: The current study evaluated whether “nutritional-shift” trials could be used to distinguish and quantify the adjustment and metabolic periods of the lag phase of Escherichia coli.
Methods: BHI-grown E. coli K12 cells were cultured in tryptic soy broth without a carbohydrate source (TSB-G) to early stationary phase and then transferred to TSB-G and TSB + 0.5% lactose (TSB+L). Growth was followed by viable counts and absorbance, and growth curves fitted using a 2-phase linear model. Cultures were assayed for lactase activities using the AOAC ONPG method. The trials were run at 15, 20, 25, 30, 35, and 40°C.
Results: The growth kinetics of lag and log phases and their standard deviations were similar in both cultures, and displayed the expected temperature dependency. Lag times and specific growth rates were used to estimate the adjustment period value, which displayed a similar temperature dependency. Detectable lactase activity was limited to TSB+L cultures, with first appearance occurring slightly after the lag phase, indicating that the expression of lactase didn't occur until the completion of lag phase. Using the standard deviations from the trials it was possible to effectively describe the shape of the growth curve using Monte Carlo simulation software.
Significance: The use of nutritional-shift appears to be an effective tool for developing more mechanistic growth models, however, measurements of gene up-regulation are needed to better map the timing of the initial adjustment period.