= Microbial cryoprotectant
A <microbial cryoprotectant> is a substance added to help viable <microorganisms> withstand <freezing> and subsequent recovery during <microbial cryopreservation>. Protection is assessed in the preserved material; the label does not mean that a substance is harmless at every concentration or suitable for every <strain (biology)>.
<Freezing> changes both the amount of liquid <water> and the concentration of dissolved substances. Intracellular <ice> can injure structures, while water movement by <osmosis> can shrink a <cell>. ATCC identifies <glycerol> and <dimethyl sulfoxide> as common protective agents and stresses that useful formulations depend on the material. Protecting a frozen preparation involves more than simply lowering its nominal <temperature>.<Mycology Culture Guide by ATCC>[Preservation: Cryopreservation, Overview and Freeze Medium]{ref}
An original comparison measures recovery rather than assuming protection from composition. Split a benign <yeast> preparation into two matched portions. Suppose each has1000 initial <colony-forming units> per original-volume equivalent, while recovery gives300 units without a candidate additive and700 with it. The recovered fractions are0.30 and0.70: a difference of0.40, or40 percentage points, and a ratio $700/300\approx2.33$. These invented measurements would support improved recovery in that comparison, not a universal survival rate.
Equal original-volume equivalents matter because adding a solution also dilutes the cells. A matched nonfrozen comparison can help separate freezing injury from an additive's effect on the growth readout. Recovery of <colony-forming units>, unchanged <genotype> and preservation of a chosen <phenotype> are separate questions.
The practical motivation is to retain a reproducible biological starting point between experiments. A <microbial cryoprotectant> is one part of that preservation system, alongside the physical freezing history, storage and subsequent authentication.<Bacteriology Culture Guide by ATCC>[Preservation and Bacterial Authentication]{ref}
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