Culture Collections

Freeze-drying bacterial strains in NCTC

3P 1949 V1

 Image: Edwards freeze-drying equipment c.1950


Freeze drying is a water removal process from a frozen suspension. It is the chosen method for bacterial and fungal preservation used in NCTC as it results in a product with proven long viability. Freeze-drying is a multistep, highly manual process that takes place in the CL2 or CL3 laboratory (depending on the hazard group of organism being processed) from start to finish.

The initial step of the process is propagation of a strain, either from a previously freeze-dried ampoule or a new addition to NCTC collection (new accession). All bacterial strains are initially checked for viability and purity. The next step is to increase the biomass of each strain to help it survive the harsh freeze-drying process. The bacterial suspensions are prepared and dispensed into individual sterile ampoules. Ampoules are then subjected to primary and secondary freeze-drying steps.

Primary drying involves centrifugation, freezing at a temperature below -40°C with an added high, controlled vacuum which results in a product with 5-10% humidity.

The primary drying step is followed by secondary drying which also involves freezing at a temperature below -40°C with an added high, controlled vacuum resulting in a product with a final humidity of 1-2%.

The ampoules are removed from the secondary dryer using a torch to allow preserving vacuum within.  The vacuum is crucial for prolonged viability of the final product. By using this method some strains (for example strains from Enterobacteriacae) can survive for 50 years and most anaerobic organisms can survive for approximately 20 years. The general rule is that the more fastidious the organism the shorter viability is usually recorded.  

Freeze-drying bacteria was first developed in 1906. NCTC was established in 1920 and the earliest strain deposited within NCTC dates back to 1915. The Edwards freeze drying equipment was introduced to NCTC in 1950 and this revolutionised the preservation process. The overall method has not changed much since, although constriction and plugging process have been adapted to make the procedure quicker and safer. NCTC supplies nearly 5100 type strains to date and counting.

Written by Dorota Kaushal

September 2020

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