Freeze-drying of live oral streptomycin-dependent mutant Shigella vaccines: Survival of streptomycin-dependent Shigella flexneri 2a after freeze-drying

Although the relatively successful freeze-drying of streptomycin-dependent mutant Shigella vaccines has been reported previously, further studies have been undertaken for the following reasons: (1) To improve the survival rate of the vaccine strains after freeze-drying. (2) To see whether monosodium glutamate can be eliminated as a stabilizer as it is reported to produce pharmacological effects in man. Streptomycin-dependent Shigella flexneri 2a was chosen for this study as this strain among the vaccine strains is the most sensitive to the effects of freeze-drying. When 60–90 × 109 organisms are suspended in a solution containing 12.5% sucrose and 15% peptone, and freeze-dried, a survival of 90–100% has been obtained. This is a considerable improvement on previously published methods. During the study a number of streptomycin-independent revertants were isolated, but all were avirulent. There was no evidence that freezing or freeze-drying had any influence on the reversion rate.