Isoniazid resistance-conferring mutations are associated with highly variable phenotypic resistance.

BACKGROUND

High-dose isoniazid is recommended in the 9-12 months short-course regimen for multidrug-resistant tuberculosis with mutation. However, there is insufficient evidence to support the assumption of genotypic-phenotypic concordance. This study aimed to identify the genetic mutations associated with high-level phenotypic isoniazid resistance.

METHODS

Clinical isolates from patients with drug-resistant tuberculosis were profiled by whole-genome sequencing and subjected to minimum inhibitory concentration (MIC) testing using MGIT based-method. MICs were performed in concentration ranges based on the mutation present: isolates with no isoniazid resistance-conferring mutations and H37Rv, 0.016-0.256 µg/ml; 0.256-4.0 µg/ml, 1.0-16.0 µg/ml; and  +  4.0-64.0 µg/ml. Isolates demonstrating resistance at the upper limit of the concentration range were tested up to the maximum of 64.0 µg/ml. Bootstrap of the mean MICs was performed to increase the robustness of the estimates and an overlap index was used to compare the distributions of the MICs for each mutation profile.

RESULTS

A total of 52 clinical isolates were included in this analysis. Bootstrap MIC means for , and  +  were 33.64 (95% CI, 9.47, 56.90), 6.79 (4.45, 9.70) and 52.34 (42.750, 61.66) µg/ml, respectively. There was high overlap between and  +  mutations (eta = 0.45) but not with and (eta = 0.19). Furthermore, showed poor overlap with  +  mutations (eta = 0.09). Unexpectedly, 4/8 (50.0%) of all mutants demonstrated high-level resistance, while 20/24 (83.3%) of mutants demonstrated moderate-level resistance.

CONCLUSIONS

mutations demonstrated unexpectedly high MICs and showed high overlap with . Contrary to the common belief that mutants are associated with high-level resistance, this mutation primarily showed moderate-level resistance.