Maintenance of Mycobacterium tuberculosis on glass beads

The intent of this study was to estimate the shelf life of Mycobacterium tuberculosis strains, and to observe the loss of viability in some of them from year to year. From 2000 to 2004, 10,015 cultures of M. tuberculosis were preserved by freezing on glass beads at -70 degrees C. With the expectation that the loss of viability might be around 5-10%/yr of storage, 730 strains were analyzed in order to establish the prevalence of recovery within a 5% margin of error. This study shows that 94% of the strains preserved at -70 degrees C on glass beads could be recovered within 30 days. The recovery rates for drug-susceptible and drug-resistant strains showed no significant differences. The growth rates and the number of strains that showed abundant growth before the 30th day of incubation represent important features, since the subculture of a strain preserved for future use ought to quickly produce abundant growth in order to avoid misinterpretation of the tests. Our experience indicates that storage of M. tuberculosis on glass beads at -70 degrees C is a suitable procedure for an active culture collection in a public health laboratory like ours, where maintenance of M. tuberculosis cultures is a complementary activity and must be quick, practical, effective, and economical.