Behaviour of log-phase Escherichia coli at temperatures near the minimum for growth

The behaviour of cold-adapted, log-phase Escherichia coli in broth cultures incubated at temperatures between 7 and 15 °C was examined by determinations of numbers of colonies recovered on plate count agar (PCA); absorbance at 600 nm (A600); cell lengths from photomicrographs; and cell size distributions by flow cytometry. Cultures incubated between 7 and 10 °C were evaluated for 8 days or until A600 values approached 1.0. Cultures incubated at ≥12 °C were subcultured to maintain them in the log phase for up to 8 days. Numbers of colonies recovered declined when cultures were incubated at 7 °C, but increased when cultures were incubated at higher temperatures. However, A600 values increased during incubation at all temperatures. The mean lengths of cells doubled during incubation at 7 °C for 8 days, but remained constant during incubation at 10 °C for 1.25 days. Forward angle light scatter (FALS) measurements obtained by flow cytometry indicated that the mean length of cells increased at ≤8 °C, but not at 10 °C. A reference value at the 90th percentile of FALS measurements on day 0 was used to determine changes in the distribution of the lengths of cells. About 80% or 17% of the cells were above the reference value after 5 days of incubation at 7 °C or 1.25 days of incubation at 10 °C, respectively. Cultures that were maintained in the log phase at 12 °C became increasingly heterogeneous in cell size after 2 days, but cultures that were maintained at 13 °C remained constant in cell size for 8 days. The observations have implications for the prediction of mesophile proliferation at temperatures that approach their minima for growth.