Improving antitubercular drug susceptibility testing with liquid media

Area of Science:

• Microbiology
• Infectious Diseases
• Clinical Diagnostics

Background:

• Liquid media, such as the mycobacteria growth indicator tube (MGIT), are standard for antitubercular drug susceptibility testing (DST).
• However, MGIT-based DST can yield discordant or invalid results due to bacterial contamination or mixed mycobacterial colonies.
• These issues necessitate improved diagnostic methods for accurate tuberculosis treatment.

Purpose of the Study:

• To evaluate a modified MGIT drug susceptibility testing (DSTMGIT) method incorporating a Löwenstein-Jensen (LJ) subculture step.
• To assess the modified method's ability to reduce invalid results and improve accuracy compared to standard MGIT DST and LJ solid media DST.

Main Methods:

• A modified DSTMGIT method was developed, involving subculturing positive MGIT results onto LJ media.
• DST was performed on 307 Mycobacterium tuberculosis complex samples using both standard DSTMGIT and the modified DSTMGIT.
• Results were compared against DST performed on LJ solid media (DSTLJ) for isoniazid, rifampin, streptomycin, and ethambutol.

Main Results:

• Of 307 samples, 287 yielded valid DST results. Standard DSTMGIT produced 20 (6.5%) invalid results, 13 of which were resolved by the modified DSTMGIT.
• Both DSTMGIT and modified DSTMGIT showed 41 (14.3%) discordant results compared to DSTLJ, with five samples falsely identified as multidrug-resistant.
• The modified DSTMGIT significantly reduced bacterial contamination and mixed mycobacterial culture rates (P=0.029) while maintaining similar discordance rates (P=0.624).

Conclusions:

• The modified DSTMGIT method effectively reduces invalid results caused by contamination and mixed cultures.
• This approach can enhance the reliability of drug susceptibility testing for tuberculosis, particularly for problematic samples.
• The modified method offers a valuable improvement for accurate antitubercular drug susceptibility testing.