Behaviours of log phase cultures of eight strains of Escherichia coli incubated at temperatures of 2, 6, 8 and 10 °C

The behaviours of cold-adapted, log-phase cultures of eight strains of Escherichia coli incubated at 2, 6, 8 and 10 °C for 10 days were examined by determining absorbance at 600 nm (A600), viable counts and cell size distribution as indicated by forward angle light scattering (FALS) values, obtained for samples collected each day from each culture. Cell lengths were determined from photomicrographs of samples for which the flow cytometry data indicated the mean cell lengths were maximal or minimal for each culture. At 2 °C, A600 values for all strains and viable counts for some changed little, while viable counts for other strains declined progressively by > 1 log unit. At 6 °C, A600 values for most strains increased at progressively declining rates and then remained constant while viable counts increased to reach maximum values before maximum A600 values were attained, and then declined. At 8 °C, the behaviours of most strains were similar to the behaviour at 6 °C. At 10 °C, seven of the strains grew exponentially, but for most of these the growth rate determined from A600 values differed from that determined from viable count data. Mean FALS values for cultures incubated at 6, 8, or 10 °C showed various patterns of increase and decrease, indicating fluctuations in cell lengths. For all strains, the minimum cell length was < 3 μm, but the maximum cell lengths ranged from < 20 to > 140 μm. The findings suggest that the formation of elongated cells or filaments is usual behaviour for E. coli growing at temperatures approaching or below the minimum for sustained growth.