Effect of cryoprotectant-induced intracellular ice formation and crystallinity on bactria during cryopreservation

Cryopreservation is widely used for the long-term storage of bacteria. Glycerol is one of the traditional cryoprotectants used widely to prevent cryoinjury during the cryopreservation of bacteria,although it may be toxic to the cells. To overcome these issues, synthetic antifreeze polymers are also used as cryoprotectants to inhibit ice formation. In the study, we compared the performance of various antifreeze synthetic polymers including poly(vinyl alcohol) (PVA), poly(vinylpyrrolidone), poly(ethylene glycol), and dextran with glycerol, among which PVA performed best on decreasing the ice growth rate.The impacts of glycerol, trehalose, combined with PVA on the survival of S. thermophilus were also explored. Notably,. S. thermophilus stored in 100 mg/mL trehalose and 1 mg/mL PVA +50 mg/mL trehalose combo showed significantly enhanced survival when compared with those in traditional cryoprotectant (20% [v/v] glycerol), which achieved the survival percentage of only 41.03 ± 0.09%. The effects of the freezing temperature and crystallinity on the survival of S. thermophilus were elucidated.