Some physical factors that influence the survival of Brucella abortus during freeze-drying.

Since Shackell (1909) described the preservation of biological material by freeze-drying, or drying by sublimation of ice, other workers have improved the process until, at the present time, it is used to preserve a variety of biological products. Flosdorf (1949) has reviewed the course of this development. A few of the workers in the field have published the results of attempts to use this technique for the preservation of maximum numbers of viable, vegetative bacteria. Rogers (1914) made comparisons between numbers of viable cells present in bacterial suspensions before and after preservation. Naylor and Smith (1946) and Stamp (1947) showed that the chemical composition of the fluid used to suspend the cells that were preserved influenced the percentage of cells that remained viable after drying. Insofar as the authors are aware, no one has yet studied the effect of some factors such as time, rate, and extent of drying on the survival of bacteria during freeze-drying. Elser, Thomas, and Steffen (1935), Greaves and Adair (1939); Flosdorf, Hull, and Mudd (1945), Greaves (1946), and others found that such factors as these had a marked influence on the physical properties of preserved biological materials. Consequently, in the work done for this paper the influence of the nature of the drying process on the percentage of viable vegetative bacteria recoverable after drying was determined. The factors studied were the influence of the temperature of subliming ice, the rate of sublimation, and the moisture content of the dried product. The test organism was Brucella abortus (BAI strain 19). Cells to be dried were suspended in fluid similar to that recommended by Naylor and Smith (1946) in their studies with Serratia marcescens. The results of experiments are described in terms of the percentage of cells, viable in the suspension originally, which remained viable after drying.