Preservation of mycobacterial suspensions at -55 degrees C

Suspensions in dilute Sauton's medium of 14 mycobacterial strains grown under identical conditions were prepared with bacilli harvested at their midlog phase of growth,and were frozen and stored at -55 degrees C. Survivals, estimated by CFU determinations after periodic intervals of storage, showed no adverse effect of freezing and thawing on any of the strains. Effectiveness of preservation, however, varied from strain to strain and no correlation existed between effectiveness and species of mycobacteria. No noticeable changes in the virulence of the H37Rv strain and in the immunizing activity of BCG were detected after prolonged storeage. Results of experimentsperformed with two strains suspended in seven diluents suggest that some are more suitable for long-term preservation, particularly so for the more sensitive one.