Studies on the mechanism of sorbed water killing of bacteria

The bacterium Serratia marcescens is particu-larly sensitive to small amounts of sorbed water. It has already been shown (Monk and McCaffrey, 1957) that the logarithmic death rate is negligible for very wet and very dry cells and is maximal for a water concentration of about 33 per cent, which corresponds to only about one-eighth of the amount found inside a normal cell suspended in water. It is the purpose of this paper to report an experimental determination of the temperature dependence of this death rate and various other processes involving this species, and to discuss the possible application of these findings to an under-standing of the process by which sorbed water kills the organisms. MATERIALS AND METHODS S. marcescens strain 8 UK was grown on tryp-tose agar for 18 to 24 hr at 31 C, harvested in distilled water, washed in distilled water, and assayed for viability exactly as already described (Monk and McCaffrey, 1957). The washed cell suspension was stored between experiments for not more than 4 days in a refrigerator. The sam-ples were freeze dried in the apparatus already described, and then were rewetted to about 33 per cent moisture by back lyophilizing 0.020 ml of water. Tubes containing identical moist sam-ples were then stored for 2 to 3 hr at various tem-peratures and the surviving number of cells deter-mined. The water content was determined from the loss in weight suffered by similar samples dur-ing drying at about 100 C in a vacuum oven. RESULTS AND DISCUSSION Since it had already been shown that 33 per cent sorbed water causes the viability to decrease exponentially with time it was considered valid to calculate the death rates in the present experi-ment from the original and final viabilities. Th