Abstract:
Universal drug susceptibility testing (DST) is an important requirement of the End TB Strategy. The
Sensititre broth micro-dilution assay (BMD) tests multiple drugs quantitatively. We defined interpretive
criteria for this assay and analysed genotypic-phenotypic relationships. 385 Mycobacterium tuberculosis
clinical isolates were processed for BMD and whole genome sequencing. The epidemiological cut-off
value 99% (ECV99) amongst genotypically wild type (gWT) strains defined susceptibility. Minimum
inhibitory concentration distributions of the resistance-associated variants (RAVs) for each drug
were analysed. Susceptibility (μg/mL) criteria were determined as follows: rifampicin (≤0.125),
isoniazid (≤0.25), ethambutol (≤2.0), moxifloxacin (≤0.5), levofloxacin (≤1.0), amikacin (≤2.0),
kanamycin (≤8.0), capreomycin (≤4.0), clofazimine (≤0.25) and linezolid (≤2.0). Most drugs showed
clear separation between gWT and RAV. Isoniazid showed a tri-modal pattern with 14/17 strains at
ECV99 harbouring a fabG1 c. -15C > T RAV. Ethambutol RAVs at embB codons 306, 405 and 497 were
responsible for resistance and showed differential distributions. Moxifloxacin RAVs (gyrA codon 90)
were a dilution or two higher than the ECV99 while gyrB RAVs were uncommon and showed drug
specific resistance propensity. Interpretive criteria established were robust facilitating progress towards
universal DST and individualised precision medicine. This study demonstrates the value of quantitative
DST to accurately interpret mutation data.
