Pretomanid resistance found in 4.5% of tuberculosis isolates in China

A laboratory study of 335 Mycobacterium tuberculosis isolates from China has mapped how the tuberculosis drug pretomanid inhibits the bacterium in broth culture and uncovered a notable baseline level of resistance. The work, published open access in Annals of Clinical Microbiology and Antimicrobials, reports that the 99th percentile minimum inhibitory concentration (MIC) of pretomanid was 0.25 mg/L when tested in Middlebrook 7H9 broth, while 15 isolates, or 4.5 percent of those analyzed, were classified as probably resistant, with MICs ranging from 0.5 mg/L to at least 16 mg/L.

The researchers set out to assess the prevalence and genetic basis of pretomanid resistance and to generate additional MIC data that could support the setting of epidemiological cutoff values, or ECOFFs, and clinical breakpoints for microdilution testing systems.

The isolates were collected from national drug resistance surveillance regions in China and, importantly, came from patients with no prior exposure to pretomanid. All strains were sub-cultured on Löwenstein-Jensen medium and tested against 12 anti-tuberculosis drugs using the Sensititre MYCOTB plate. For pretomanid itself, the drug was serially diluted in 7H9 broth supplemented with 10 percent OADC, covering a wide concentration range from 0.015 mg/L to 16 mg/L.

Genetic analysis of the resistant isolates added a puzzling dimension to the findings. Among the 15 probably resistant isolates, only three carried mutations in genes previously associated with pretomanid resistance: fbiA, fbiC and fgd1. None of the resistant isolates had mutations in fbiB, ddn or fbiD, other genes in the known resistance pathway. The authors conclude that their results suggest the existence of yet-uncharacterized genetic mechanisms underlying pretomanid resistance, meaning current genomic markers may miss a substantial share of resistant strains.

The study population was dominated by lineage 2 strains, which accounted for 77.9 percent of isolates (261 of 335), with the remainder including lineage 4. The authors state that the 99th percentile MIC of 0.25 mg/L applies to lineage 2 and lineage 4 isolates in China. Correlation analysis showed only weak associations between pretomanid MICs and those of the other drugs tested, with correlation coefficients below 0.3, indicating that resistance to pretomanid does not simply track with resistance to other anti-tuberculosis agents.

The 4.5 percent resistance figure is described by the authors as a high baseline resistance rate, which is striking given that none of the source patients had been previously exposed to pretomanid. Because the isolates came from drug resistance surveillance regions, the sampling framework provides a picture of resistance in the circulating bacterial population rather than in patients selected for pretomanid treatment. The genetic findings reinforce the paper’s central message: phenotypic resistance can occur without detectable mutations in the established resistance genes, so molecular assays targeting those genes alone would not capture the full resistance burden.

For diagnostic laboratories, the MIC distribution data offer a reference point for interpreting broth microdilution results. With a 99th percentile MIC of 0.25 mg/L, isolates showing MICs of 0.5 mg/L or higher fall outside the range covered by most wild-type strains in this dataset and were classified as probably resistant in the study.

As with any single-country surveillance study, the findings reflect the isolates and lineages sampled; the predominance of lineage 2 in this collection shapes the distribution, and the authors note the ECOFF-supporting value of the data for isolates in China.

Subject of Research: Pretomanid MIC distribution and resistance mechanisms in Mycobacterium tuberculosis isolates from China

Article Title: Pretomanid MIC distribution and resistance profile in Mycobacterium tuberculosis using middlebrook 7H9 broth

Article References: Zheng, H., Zhao, B., Su, D., Jiao, W., Hou, P., Li, F., Huang, F., Xiao, J., Wang, N., Qi, H., Ren, J., Xia, H., Zhao, Y., Ou, X., & Sun, L. (2026). Pretomanid MIC distribution and resistance profile in Mycobacterium tuberculosis using middlebrook 7H9 broth. Annals of Clinical Microbiology and Antimicrobials. https://doi.org/10.1186/s12941-026-00881-4

Image Credits: AI Generated

DOI: 10.1186/s12941-026-00881-4

Keywords: pretomanid, Mycobacterium tuberculosis, MIC, drug resistance, ECOFF, broth microdilution, lineage 2, fbiA, fgd1, tuberculosis diagnostics, China, 7H9 broth