Role of membrane fluidity in pressure resistance of Escherichia coli NCTC 8164

Abstract

The relationship among growth temperature, membrane fatty acid composition, and pressure resistance was examined in Escherichia coli NCTC 8164. The pressure resistance of exponential-phase cells was maximal in cells grown at 10 degrees C and decreased with increasing growth temperatures up to 45 degrees C. By contrast, the pressure resistance of stationary-phase cells was lowest in cells grown at 10 degrees C and increased with increasing growth temperature, reaching a maximum at 30 to 37 degrees C before decreasing at 45 degrees C. The proportion of unsaturated fatty acids in the membrane lipids decreased with increasing growth temperature in both exponential- and stationary-phase cells and correlated closely with the melting point of the phospholipids extracted from whole cells examined by differential scanning calorimetry. Therefore, in exponential-phase cells, pressure resistance increased with greater membrane fluidity, whereas in stationary-phase cells, there was apparently no simple relationship between membrane fluidity and pressure resistance. When exponential-phase or stationary-phase cells were pressure treated at different temperatures, resistance in both cell types increased with increasing temperatures of pressurization (between 10 and 30 degrees C). Based on the above observations, we propose that membrane fluidity affects the pressure resistance of exponential- and stationary-phase cells in a similar way, but it is the dominant factor in exponential-phase cells whereas in stationary-phase cells, its effects are superimposed on a separate but larger effect of the physiological stationary-phase response that is itself temperature dependent.

FIG. 1.

Effect of growth temperature on pressure resistance of E. coli NCTC 8164. Cells were grown in TSB at 45°C (⋄), 37°C (▵), 30°C (□), 20°C (▴), or 10°C (♦) to exponential phase (a) or stationary phase (b) and treated for 5 min at different pressures. N_i, number of cells after treatment; N₀, initial number of cells. Results are means of at least two observations ± standard deviation (error bars).

FIG. 2.

Effect of growth temperature on pressure resistance of E. coli NCTC 8164. Cells were grown in TSB at different temperatures to exponential (a) or stationary (b) phase and treated at 200 and 400 MPa, respectively. Growth temperatures were as follows: 45°C (⋄), 37°C (▵), 30°C (□), 20°C (▴), and 10°C (♦). N_i, number of cells after treatment; N₀, initial number of cells. Results are means of at least two observations ± standard deviation (error bars).

FIG. 3.

Comparison of the pressure resistance of exponential-phase (□) and stationary-phase (▪) cells grown at different temperatures. Pressure resistance is expressed as the pressure at onset of cell death (±standard error), which was calculated from data shown in Fig. 1 by using the DMFit program (see Materials and Methods).

FIG. 4.

Effect of growth temperature on the membrane FI of exponential-phase (•) and stationary-phase (○) cells.

FIG. 5.

DSC heating profiles for aqueous suspensions of lipids extracted from whole cells of E. coli NCTC 8164 grown in TSB to stationary phase at 20°C (a) and 37°C (b).

FIG. 6.

Relationships between growth temperature and phase transition temperature (T_m) (a) and between T_m and FI (b) of phospholipids extracted from whole stationary-phase cells (error bars indicate standard deviation).

FIG. 7.

Effect of temperature of pressurization on pressure resistance of E. coli NCTC 8164. (a) Exponential-phase cells were grown at 10°C in TSB and pressure treated at 10°C (♦), 20°C (▴), or 30°C (□). (b) Stationary-phase cells were grown at 37°C and pressure treated at 10°C (♦), 22°C (▴), or 29°C (□). N_i, number of cells after pressure treatment; N₀, initial number of cells.