- Volume 19, Issue 3, 1958

Summary: The optimum water activity (a w) for survival of dried bacteria has been determined by storing the organisms in water-vapour equilibrium with solutions of known a w value. The position of the optimum differed according to the composition of the suspending fluid, and the presence or absence of air in the storage atmosphere. For Staphylococcus aureus and Pseudomonas fluorescens, dried in papain digest and stored in vacuo, survival at 0·07, 0·11 and 0·16 a w was somewhat better than at 0·00 a w and 0·22 a w. Survival at 0·33–0·53 a w was much less. For Salmonella newport dried in the same medium the optimum, in vacuo was less clearly defined, but was well marked and close to 0·2 when stored in air. S. newport, when stored in vacuo, survived best at 0·22 a w after drying in a salts buffer or in dialysed horse serum, but in whole serum or the dialysable serum fraction survival was better at 0·00 a w. S. newport dried from water survived best at 0·00 a w, in vacuo, but at 0·43 a w in air. In the presence of sucrose the effect of a w, was relatively small. In the presence of glucose or arabinose survival at 0·00 a w, was better than at 0·22 a w, which in turn was much better than at 0·48 a w. In the presence of all three sugars the differences between results in vacauo and in air were small.