SURVIVAL OF BACTERIA UPON REPEATED FREEZING AND THAWING

Abstract

[lond dash]With Lactobacillus fermenti, whether the length of storage between successive freezings is 4 weeks, 1 week, 24 hours, or 1/2 hour, the lethal effect of the 2nd freezing is greater than the lethal effect of the first or of any subsequent freezing. When logarithms of survivors are plotted versus number of successive freezings a sigmoidal curve is obtained, the configuration of which is influenced by the length of the storage interval between successive freezings and by the composition of the menstruum and the initial cell concentration. With Escherichia coli and Serratia marcescens the curve may also be sigmoidal, except that it becomes a straight line when the storage interval is short. At moderately high initial cell concentrations, survival of E. coli to successive freezings is initially greater in aqueous suspensions than in broth suspensions, but after additional successive freezings the converse may be true. Aerated cells of E. coli suspended in water or broth at first survive much better than nonaerated cells, but upon additional freezings the difference becomes less pronounced.