Heat resistance, membrane fluidity and sublethal damage in Staphylococcus aureus cells grown at different temperatures

In this work the influence of growth temperature (10-42 degrees C) on Staphylococcus aureus heat resistance was studied, and its relationship with the ability of cells to repair sublethal damages and with membrane fluidity was evaluated. Non-linear, convex from above survival curves were obtained, and therefore a special case of the Baranyi model was used to fit them. For exponential phase cells, heat resistance did not change with growth temperature in the range between 10 and 37 degrees C, but cells grown at 42 degrees C were significantly more resistant, showing D-58 and shoulder length (sl(58)) values 2.5 and 4 times greater than the others, respectively. For stationary growth phase cells, an increase in growth temperature above 20 degrees C resulted in an increase in D-58 values, and cells grown at 42 degrees C also displayed the highest D-58 and Sl(58) values. The increased heat resistance at 58 degrees C of stationary growth phase cells grown at higher temperatures was coincident with the appearance of a higher proportion of sublethally damaged cells capable of recovery and outgrowth in non-selective medium. Membrane fluidity was measured at treatment temperatures, and it was observed that those cells with more rigid membranes displayed greater heat resistance (Pearson coefficient = 0.969***). Additionally, S. aureus cells whose membrane was fluidized through exposure to benzyl alcohol were notably sensitized against the action of heat, in a concentration-dependent manner. Results obtained in this research indicate that membrane physical state could be an important factor determining the survival capacity of bacterial cells to a heat treatment.