Membrane fluidity as a factor in production and stability of bacterial ice nuclei active at high subfreezing temperatures.

Detailed measurements were made of the rate of appearance of bacterial ice nuclei upon cooling of suspensions of Pseudomonas syringae cells and the disappearance of ice nuclei upon warming of the cells before assay for ice nucleation activity. While no substantial change in numbers of ice nuclei active at either -5 or at -9 degrees C was observed in cells that were grown at temperatures lower than 24 degrees C and cooled to 21 degrees C before assay, large increases in -5 but not -9 degrees C ice nuclei were observed in cells grown at temperatures greater than 24 degrees C. Ice nucleation activity of cells subjected to a decrease in temperature before assay increased immediately upon temperature shift, but 8 to 12 min was required before maximum rates of increase in numbers of ice nuclei were observed. The rate of appearance of ice nuclei in cell suspensions incubated at relatively cold temperatures prior to assay was substantially less than those incubated at temperatures approaching 24 degrees C. Cells rapidly lost ice nucleation activity when warmed to above 27 degrees C before assay; the rate of loss of ice nuclei in cells grown at a given temperature increased rapidly as the temperature to which they were warmed before assay increased. Ice nuclei disappeared most rapidly when cells grown at low temperatures were warmed before assay, suggesting that ice nucleus stability was lower in highly fluid membranes. The logarithm of the half-life of ice nuclei in cells was directly related to the concentration of the membrane fluidizing agent, 2-phenethyl alcohol, in which they were suspended.