Mutations in 406 Are Associated with Low-Level Ethambutol Resistance in Canadian Isolates.

In , molecular predictions of ethambutol resistance rely primarily on the detection of mutations within . However, discordance between 406 mutations and gold standard phenotypic drug sensitivity testing (DST) questions the significance of 406 mutations used in molecular DST. This study tabulates mutations found in Canadian isolates and evaluates the impact of specific mutations on ethambutol resistance. The National Reference Centre for Mycobacteriology culture collection (n = 2796) was screened for isolates with mutations. Phenotypic DST was performed on the BACTEC™ MGIT™ 960 at ethambutol concentrations of 2-5 μg/mL. Whole genome sequencing was used for drug resistance predictions, phylogenomics and single nucleotide polymorphism analysis. Detection of resistance-associated mutations corresponded to a positive predictive value of 64.3%, negative predictive value of 99.2%, 98.7% specificity, and 73.3% sensitivity compared to phenotypic DST. Two 406 mutation subtypes (Gly406Asp, Gly406Ala) were found among 16 isolates, of which 12 were sensitive at 5 µg/mL ethambutol with variable resistance between 2-4 µg/mL. A novel frameshift mutation in regulator (Gln258fs) was found in nine isolates. Mutations in 406 were associated with low-level ethambutol resistance undetectable at the recommended critical concentration (5 μg/mL). These novel mutations may exacerbate variability in ethambutol resistance.