Frozen storage (—20° and —80°C) of soybean Bradyrhizobium cell suspensions

Abstract

Liquid cultures of Bradyrhizobium japonicum were added in a 1:1 ratio to 20% aqueous skim milk, or centrifuged and the cells resuspended in 10% skim milk. The suspensions were stored at —20° or —80°C for 7 months and cell survival assessed. At —20°C, there was a decrease in the viable count of about two logs in liquid culture whilst for cells resuspended in 10% skim milk the decrease was limited to one log. The temperature of —80°C was found to be in itself protective and the surviving rhizobial cells maintained their infectivity and effectiveness. Thus appropriate freezing conditions provide a suitable method to store soybean rhizobia cells prior to preparing the legume inoculant.