The origin, ultrastructure and microbiology of the sediment accumulating in Dewars used for the storage of cryopreserved samples

CONCLUSION: Ice crystals present in liquid nitrogen storage vessels concentrate biological contamination and provides a mechanism by which cryopreserved samples may become contaminated during storage. The association of the microbial contamination with ice may also provide ways to minimise the problem. A related issue is that storage vessels which have been maintained for several years would be expected to contain a high level of contamination which may be a risk to operators. This aspect has generally been overlooked and should be included in the development of safety policy for IVF laboratories.