Some factors affecting the viability of dried bacteria during storage in vacuo

The effects of various substances on the viability of freeze-dried cells of Pseudomonas fluorescens and Salmonella newport were studied during storage in vacuo for 5 years. Mixtures of two organisms were dried together and studied in two factorial experiments. The first was a complete factorial using six factors and two levels; the second was a fractional replicate with four factors at two levels and three others at four concentrations. A study of ribose binding by cells of S. newport was made by using [U-(14)C]ribose. Substantial improvements in viability were obtained by drying in the presence of sucrose, glutamate, and semicarbazide. Low concentrations of ribose increased the death rate during drying, but these adverse effects were prevented by equimolar amounts of semicarbazide. Ribose binding increased with storage time and, although its incorporation changed, most of the increase in total ribose occurred after the main decrease in viability. Storage temperature caused larger changes in viability than did level changes of residual water in the cells. Although the results are complicated by a large number of interactions, they confirm and extend the hypothesis that reactions between carbonyl compounds and cellular components are a major cause of the mortality occurring during storage of dried microorganisms. A particular mixture of sucrose, glutamate, and semicarbazide is proposed as a means of reducing death when prolonged storage in the dry state is desired.