Hyperthermic Sensitivity and Growth Stage in Escherichia coli

Hyperthermic sensitivities of Escherichia coli B/r and ${\rm B}_{{\rm s}-1}$ were determined for lag-, midlog-, and stationary-phase cells at 47, 48, and 49°C. In both strains midlog-phase cells were strikingly more heat sensitive (100-fold greater killing after 4 h at 48°C) than stationary-phase cells, with intermediate sensitivity for lag-phase cells. In contrast to the reported difference in the radiation sensitivity between these two strains, very little difference in heat sensitivity was seen. Patterns of fatty acid composition of both strains were very similar at each phase of growth. From midlog to stationary phase, 16:1 and 18:1 unsaturated fatty acids decrease from 16 and 30% to 0.5 and 3%, respectively, while the C17 and C19 cyclopropane fatty acids increase from 7 and 3% to 22 and 25%, respectively. Concomitant with these changes in fatty acid composition, substantially higher membrane microviscosity values were recorded for stationary-phase cells. Total membrane microviscosity was positively associated with the C17 and C19 cyclopropane fatty acid composition and with cell survival following hyperthermia. In contrast to hyperthermic sensitivity, radiation survival differences between B/r and ${\rm B}_{{\rm s}-1}$ are little affected by growth stage. We propose that these results are consistent with a critical influence of membrane lipids on cellular hyperthermic sensitivity and further that the target sites for radiation and hyperthermia are different in these cells.