LAUSR

Mycobacterium abscessus complex (MABC) is the most common rapidly growing, nontuberculous mycobacterium (NTM) that causes chronic pulmonary disease and cutaneous infections. Resistance to macrolides, a cornerstone in rapidly growing mycobacterium (RGM) treatment, is a major challenge for effective patient management. Macrolide resistance mechanisms can be acquired/intrinsic (rrl gene, efflux pumps) or inducible (erm(41) gene). M. abscessus treatment is traditionally guided by phenotypic drug susceptibility testing (DST) by the CLSI broth microdilution method for RGM. This method requires incubation for up to 14 days to detect inducible macrolide resistance by the erm(41) gene. Previous work has demonstrated that erm(41) sequencing can be used as a reliable method to detect inducible macrolide resistance. M. abscessus subsp abscessus with a T at nucleotide position 28 in the erm(41) gene are predicted to exhibit inducible macrolide resistance. Those with a T-to-C polymorphism (C28) or deletions at positions 64,65 and 159 (seen in M. abscessus subsp massiliense) lack inducible macrolide resistance. We previously developed a Sanger sequencing assay to detect erm(41) mutations associated with inducible macrolide resistance in MABC. The aim of this study was to verify that the erm(41) sequencing correlates with phenotypic clarithromycin DST in isolates received at our institution. We retrospectively analyzed the results of 635 MABC isolates from September 2023 till June 2024 to determine the erm41 mutation profile and correlate genotypic resistance to the clarithromycin minimum inhibitory concentrations (MIC). In all, 312 (49%) of M. abscessus were reported as predicted for inducible macrolide resistance and 323 (50.8%) isolates were reported as not predicted. Isolates predicted for inducible resistance were of the T28 sequevar. Of these isolates, 99.3% (n = 310) showed inducible resistance to clarithromycin with an MIC of ≥ 8µg/mL. Two isolates were susceptible to clarithromycin even at day 14 and lacked additional polymorphisms that may render the erm(41) non-functional. In isolates not predicted for inducible resistance, phenotypic DST showed that 90% (n = 291) of isolates were clarithromycin susceptible (MIC ≤2 µg/mL), 7% (n = 23) were intermediate (MIC 4 µg/mL), and 2.8% (n = 9) were resistant (MIC ≥8 µg/mL). In clarithromycin susceptible isolates, 121 had an intact erm(41) gene with a C at nucleotide position 28 (C28 sequevar), while 202 isolates had a non-functional erm(41) (deletions at positions 64-65 and 159). Nine isolates not predicted to have inducible resistance were resistant to clarithromycin by phenotypic DST. These clarithromycin resistant isolates emerged by days 4-5 with an MICs ranging from 8µg/mL-16 µg/mL suggesting the role of other acquired or intrinsic methods of macrolide resistance. In conclusion, there is a high positive predictive value for erm(41) T28 sequevar for inducible macrolide resistance. Detection of erm(41) improves time to report inducible macrolide resistance results. However, it is important to correlate erm(41) inducible resistance with clarithromycin DST.