Purpose: Cycloserine is an effective group C anti-tuberculosis drug. But the reliability and reproducibility of drug susceptibility tests (DST) for this drug cannot be guaranteed and provide poor clinical predictive values. However, DST of cycloserine in practice provides rough estimate of the drug resistance of Mycobacterium  strains, there is practical need to clarify the problem of cycloserine in in vitro DST, and to explore solutions to overcome these limitations.
Methods: The effectiveness of serial cycloserine solutions incubated at 37°C for 1 to 29 days was tested using the Alamar Blue assay, and cycloserine in culture medium was analyzed by UPLC-MS.
Results: The data revealed that cycloserine itself continuously degraded in culture medium. This amount of degradation was sufficient to alter the minimum inhibitory concentration (MIC) value of Mycobacterium  strains and therefore could not be ignored, although it was more stable than in phosphoric acid buffer.
Conclusion: The different test times and the degradation of cycloserine were responsible for the lack of agreements between the cycloserine DST methods and the low reliability of this in vitro test. By adjusting with the incubation time depended degradation ratio of cycloserine, more accurate MIC values may be obtained allowing for improved coincidence between in vitro experiment and clinic use. Furthermore, it can guide clinicians to carry out this anti-tuberculosis treatment more effectively and reliably.
Keywords: Mycobacterium , tuberculosis, drug susceptibility testing, cycloserine, deterioration