quite a number of techniques available like storing at -20C for shorter duration. or at -40 or -80C for longer periods. even you can lyophilize your cultures, which can be stored for years with out affecting the cell activity... so you can choose the most feasible method as per your lab facilities.....

you can presreve it with high concentration of sucrose or keep it in glycerol

you can presreve it with high concentration of sucrose or keep it in glycerol

Thanks to all for your satisfactory answers.

Skim milk is generally used to preserve microorganisms like lactobacilli at -80 C

Store them at -80 degrees C in a 20 % glycerol solution