= Microbial culture preservation
<Microbial culture preservation> maintains recoverable cultures while retaining the biological properties needed for their future use. A successful preservation claim therefore concerns both viability and the identity or relevant characteristics of the recovered <strain (biology)>. It is part of the scientific function of a <culture collection>.<Guidelines for the Establishment and Operation of Collections of Cultures of Microorganisms by the World Federation for Culture Collections>[section 7, preservation; section 8, checking recovered cultures]{ref}
Approaches include <microbial cryopreservation> and <freeze-drying>. Their suitability depends on the material. ATCC's account explains that maintaining reference stocks can reduce the changes associated with repeated growth and passage, including changes caused by genetic instability and selection.<Mycology Culture Guide by ATCC>[printed page 16, Preservation: reference stocks and phenotypic drift during prolonged passage]{ref}
A toy selection model illustrates why repeated growth and preservation are different objectives. Begin with two variants A and B at equal abundance. Suppose each transfer multiplies their abundance ratio by $1.1$ in favour of A. After ten transfers the ratio is $1.1^{10}\approx2.594$, making A's fraction
$$
\frac{1.1^{10}}{1+1.1^{10}}\approx0.722.
$$
Both variants can remain viable while their proportions change from one half to about $72.2\%$ and $27.8\%$. These invented values illustrate cumulative selection, not observed performance of a preservation method.
Recovery and comparison complete the scientific check. Obtaining growth shows that some material remains viable; confirming the intended <genotype> or <phenotype> asks another question. Records of origin, preservation history and characterization connect those observations. Separately maintained duplicates also protect a collection against loss of a single stored resource.<Guidelines for the Establishment and Operation of Collections of Cultures of Microorganisms by the World Federation for Culture Collections>[sections 7.3 and 11, duplicate resources and documentation]{ref}
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