Storage of Mycobacterium tuberculosis culture isolates in MicrobankTM beads at a South African laboratory

Background: Mycobacterium tuberculosis complex (MTBC) isolates are typically stored at −70 °C in cryovials containing 1 mL aliquots of a liquid medium, with or without 50% glycerol. Multiple uses of the culture stock may decrease the strain viability while increasing the risk of culture contamination. Small culture aliquots may be more practical; however, storage capacity remains challenging. MicrobankTM beads (25 beads/vial) for the long-term storage of fungal cultures is well documented, but their use for storing MTBC isolates is uninvestigated.Objective: The study aimed to determine the feasibility of using MicrobankTM beads for long-term storage of MTBC isolates at a laboratory in South Africa.Methods: In February 2020, 20 isolates in liquid culture were stored in MicrobankTM beads, following an in-house developed protocol, at −70 °C. At defined time points (16 months [15 June 2021] and 21 months [18 November 2021]), two beads were retrieved from each storage vial and assessed for viability and level of contamination.Results: Stored liquid isolates demonstrated MTBC growth within an average time-to-detection of 18 days following retrieval, even at 21 months post storage. Contaminating organisms were detected in 2 of 80 (2.5%) culture isolates.Conclusion: MicrobankTM beads will allow for the reculture of up to 25 culture isolates using a reduced culture volume compared to current storage methods. MicrobankTM beads represent a storage solution for the medium-term storage of MTBC isolates.What this study adds: This study evaluated the use of MicrobankTM beads as an alternate method for storing MTBC culture isolates at −70 °C and provided a suitable option for medium-term storage of MTBC.