Relationship between membrane damage and cell death in pressure-treated Escherichia coli cells: Differences between exponential- and stationary-phasecells and variation among strains

The relationship between membrane damage and Loss of viability following pr essure treatment was examined in Escherichia coli strains C9490, H1071, and NCTC 8003, These strains showed high, medium, and low resistance to pressu re, respectively, in stationary phase but similar resistance to pressure in exponential phase. Loss of membrane integrity was measured as loss of osmo tic responsiveness or as increased uptake of the fluorescent dye propidium iodide, In exponential-phase cells, loss of viability was correlated with a permanent loss of membrane integrity in all strains, whereas in stationary -phase cells, a more complicated picture emerged in which cell membranes be came leaky during pressure treatment but resealed to a greater or lesser ex tent following decompression, Strain H1071 displayed a very unusual pressur e response in stationary phase in which survival decreased to a minimum at 300 MPa but then increased at 400 to 500 MPa before decreasing again. Membr anes were unable to reseal after treatment at 300 MPa but could do so after treatment at higher pressures, Membrane damage in this strain was thus typ ical of exponential-phase cells under low-pressure conditions but of statio nary-phase cells under higher-pressure conditions, Heat shock treatment of strain H1071 cells increased pressure resistance under low-pressure conditi ons and also allowed membrane damage to reseal, Growth in the presence of I PTG (isopropyl-beta-D-thiogalactopyranoside) increased resistance under hig h-pressure conditions. The mechanisms of inactivation may thus differ at hi gh and low pressures. These studies support the view that membrane damage i s an important event in the inactivation of bacteria by high pressure, but the nature of membrane damage and its relation to cell death may differ bet ween species and phases of growth.