The Effect of Residual Water on the Survival of Dried Bacteria During Storage

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Abstract
The optimum water activity (aw) for survival of dried bacteria has been determined by storing the organisms in water-vapour equilibrium with solutions of known aw 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 0-6 aw was somewhat better than at 0-00 aw and 0.22 aw. Survival at 0-33-0-53 a was much less. For Salmonella newport dried in the same medium the optimum aw 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 aw 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 aw S. newport dried from water survived best at 0-00 aw in vacuo, but at 0.43 aw in air. In the presence of sucrose the effect of aw was relatively small. In the presence of glucose or arabinose survival at 0.00 aw was better than at 0-22 aw, which in turn was much better than at 0.43 aw. In the presence of all three sugars the differences between results in vacuo and in air were small.

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