= Microbial cryopreservation
<Microbial cryopreservation> preserves viable <microorganisms> at very low <temperature> for later recovery. It provides stored starting material for future <experiments>, reducing repeated propagation and the associated opportunities for contamination or changes in inherited characteristics. Recovery, purity and identity remain separate properties to verify.<Bacteriology Culture Guide by ATCC>[Preservation, reasons for maintaining frozen stocks; Bacterial Authentication]{ref}
<Freezing> can injure a <cell> even though low <temperature> greatly slows its activity. Formation of <ice> removes liquid <water> from the surrounding solution and concentrates its dissolved substances; intracellular <ice> can damage cellular structures. Water movement by <osmosis> links the external changes to cellular dehydration. <Microbial cryoprotectants>, including <glycerol>, help protect the material, but preservation performance depends on the particular organism and conditions.<Mycology Culture Guide by ATCC>[Preservation: Cryopreservation, Overview and Freeze Medium]{ref}
A constructed recovery measurement makes the accounting explicit. Suppose equal original-volume equivalents of a benign <yeast> suspension give $N_0=1000$ <colony-forming units> before storage and $N_1=700$ after recovery, using the same assay and correcting for any dilution. The estimated recovery fraction is
$$
R=\frac{N_1}{N_0}=0.70.
$$
These are counts of colony-producing units under the assay conditions. Clumps can form single colonies, so the result does not by itself show that exactly seventy percent of individual <cells> survived. Nor does a high recovery fraction establish unchanged <genotype> or every original <phenotype>.
The point of maintaining a preserved reference is reproducibility: separate experiments can begin from material derived from the same stored stock. <Microbial cryopreservation> retains frozen material, whereas <freeze-drying> removes water from frozen material. They are different preservation strategies, whose suitability must be established for the material concerned.<Bacteriology Culture Guide by ATCC>[Preservation, Cryopreservation and Lyophilization]{ref}
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