On the basis of long-term experience with the preservation of the pathogenic bacteria of the Czechoslovak National Collection of Type Cultures, it is concluded that the suspending medium is a factor of primary importance in the preservation of these organisms in the freeze-dried state. The primary index of suspending medium effectiveness is the level of protection it affords against the adverse effects of low (subzero) temperatures, dehydration, long-term storage, and rehydration. A number of simple and compound suspending media were studied for their protective effects on cultures from over 50 genera of predominantly pathogenic bacteria. The results indicate that not only each bacterial genus and species but also frequently every strain of a given serological type must be looked upon as a distinct biological entity, the preservation of the viability, activity, and desirable characters of which must be tested in different protective media in preliminary experiments. Heat testing (at 75 to 100 C) of freeze-dried cultures kept under high vacuum is recommended as a good indicator of adequate protection provided by a suspending medium. With the freeze-drying technique used, the best results for the largest number of species were obtained with a mixture of calf serum or defibrinated sheep blood and lactose solution (to a final concentration of 5 to 10%) and Annear's peptone-containing medium. Individual suspending media were evaluated in relation to individual strains, and recommendations are made as to the preferred medium on the basis of control tests performed at 5-, 10-, and 15-year intervals.


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