The science behind preservation: a closer look at culture collection methods

The existing global network of culture collections, including CHAP’s National Reference Collection (NRC), developed in partnership with CABI, Fera and Rothamsted Research, act as core resource depositories of biological samples relevant to human health, species conservation and food security.

These facilities not only offer access to a diverse array of high-quality resources, including reference organisms, novel specimens, pathogens at various stages of evolution, and field samples, but also provide essential training materials to support research, industry, and education sectors.

To ensure microorganisms remain in an optimum, stable condition, CHAP’s NRC employs two core methods for long term preservation – cryopreservation and freeze-drying.

What is microbial cryopreservation?

Biological samples are subjected to controlled freezing and are stored at ultra-low temperatures, below -139°C, in the vapour phase above liquid nitrogen. This process ultimately pauses both physical and chemical reactions inside the cells.

The material is slowly frozen at a precise rate of -1°C per minute using a controlled rate cooler and a protective liquid solution called a cryoprotectant. This helps preserve the integrity of the cells by encapsulating microorganisms and preventing the formation of damaging ice crystals.

The ABCs of freeze-drying microbes

The process of freeze-drying, also known as lyophilization, is employed to remove water content from samples. When microorganisms undergo freeze-drying, it involves two distinct stages to eliminate both extracellular water, outside of the cells, and intracellular water, within the cells.

Both stages take place under reduced pressure conditions, which create a vacuum inside the glass ampoules used for storage. In their dried state, microbes can maintain viability for extended periods of time, with some instances exceeding 50 years, while their metabolic activity is temporarily suspended.

Can all organisms be cryopreserved or freeze-dried?

A diverse range of sporulating fungi and bacteria can be conserved using the techniques described above. Nevertheless, for certain obligate, physiologically delicate or non-sporulating microorganisms, alternative preservation methods such as immersion in paraffin oil or other more complex procedures may be required. Alternatively, live cultures can be sustained on agar slopes at 4°C, although, like paraffin oil preservation, this is only recommended for short-term storage.

Regarding communities of organisms, research is underway at CABI, as part of a UKRI project with Rothamsted Research, the John Innes Centre and Scotland’s Rural College (SRUC) to study the cryopreservation of entire soil microbiome communities in tandem, including how community composition can be retained by encapsulation. This ongoing research will continue to ensure that culture collection facilities evolve to support researchers as requirements diversify.

To learn more about CHAP’s NRC facility and browse our public collection you can access them by following the link to our page here. The facility is also open to accept deposits of crop-related microorganisms for in-house preservation and long-term storage.

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