Preservation of mycobacteria: 100 percent viability of suspensions stored at -70 C.

Our earlier studies on long-term preservation of mycobacteria have been expanded to include other species in this genus. Mycobacterium kansasii and M. marinum, like mammalian tubercle bacilli and BCG, survive much better when stored at -70 C. By statistical analysis, M. gordonae, M. scrofulaceum, M. xenopi, M. avium, M. intracellulare, M. terrae, M. fortuitum, and M. smegmatis survived equally well at -20 C or -70 C; however, viable counts of all strains stored at -20 C were always lower than those of paired suspensions stored at -70 C, suggesting that the lower temperature is preferable for prolonged storage periods. Advantages of preservation at -70 C in Tween-albumin liquid medium are: (i) 100% viability of bacterial populations for long periods, (ii) highly reproducible inocula for animal experiments, (iii) minimal clonal selection of undesirable mutants, (iv) maintenance of genetic characteristics, and (v) adaptability to a "seed lot" system. On the basis of available information, a discussion of lyophilization versus freezer storage is presented.