Cells of Escherichia coli mounted on a hydrophobic filter membrane were dried under various vapor pressures. A mutant defective in deoxyribonucleic acid repair (uvrA recA) was more sensitive to drying at a water activity of 0.53 or below than the parent strain but not at a water activity of 0.75 and above. Sucrose gradient studies showed that single- and double-strand breaks of deoxyribonucleic acid occurred at a water activity of 0.53 or below, but no breaks could be observed at a water activity of 0.75 or above. These results were observed in all cells rehydrated with 0.03 M tris (hydroxymethyl) aminomethane-hydrocholoride buffer solution at 0 or 37 degrees C, in the presence or absence of oxygen, with saturated water vapor or with a hypertonic solution followed by a gradual dilution. Freezable water was detected in the cells only at a water activity above 0.75 by differential scanning calorimetry. Removal of unfreezable water of cells in the drying, therfore, might induce deoxyribonucleic acid strand breaks.
DNA Repair DNA, Bacterial