Mutation in rlmN linked to greater linezolid resistance in TB

Linezolid is an important antibiotic used to treat multidrug-resistant and extensively drug-resistant tuberculosis, infections caused by Mycobacterium tuberculosis (MTB). Clinicians and researchers have long tracked mutations in the 23S rRNA (rrl) and ribosomal protein L3 (rplC) as the main genetic causes of reduced linezolid susceptibility, but those two genes do not explain all observed resistance. In new work led by corresponding author Anna G. Green, scientists searched publicly available data to look for other genetic changes that might be associated with higher linezolid minimum inhibitory concentration (MIC) in MTB. They analyzed strains that had both whole-genome sequencing and linezolid MIC phenotyping available through the Bacterial and Viral Bioinformatics Resource Center (BV-BRC). That approach let the team compare detailed genetic maps to laboratory measurements of how much linezolid is needed to stop bacterial growth. From those paired data, the researchers identified a recurring frameshift mutation in a gene called rlmN, which encodes a methyltransferase, and found that this mutation was significantly associated with increased linezolid MIC in the sampled isolates.

The study combined whole-genome sequencing data with linezolid MIC phenotyping drawn from BV-BRC to find statistical links between specific mutations and antibiotic susceptibility. By comparing the genomes of many clinical isolates with their measured linezolid MICs, the team detected a relatively common frameshift mutation in the methyltransferase rlmN: it appeared in 5.3% of assessed isolates. That statistical association showed that isolates carrying the rlmN frameshift tended to have higher linezolid MICs than those without it. In addition to the population-level statistics, the authors provided evolutionary evidence showing homology to an established linezolid resistance mechanism in Staphylococcus aureus, and they offered structural evidence that the frameshift mutation likely ablates rlmN methyltransferase functionality. The results therefore combine phenotypic testing, genome sequencing, evolutionary comparison, and structural inference to tie rlmN disruption to increased linezolid resistance.

These findings expand the set of genetic changes that may contribute to linezolid resistance in Mycobacterium tuberculosis. Until now, attention has focused mainly on mutations in rrl and rplC, but the identification of a frameshift in rlmN—present in a measurable fraction of isolates—helps explain some of the resistance that those two genes do not account for. The evolutionary link to a known mechanism in Staphylococcus aureus strengthens the case that loss of rlmN methyltransferase activity can influence how the ribosome interacts with linezolid. Structural evidence indicating that the frameshift likely abolishes rlmN function provides a plausible biological explanation for the statistical association with higher MIC. Taken together, the work highlights a previously underappreciated contributor to linezolid resistance and points to rlmN as a genetic marker worth watching in future surveillance and research efforts.