M. abscessus and the erm(41) Question: Does the Macrolide Count?
A woman newly diagnosed with M. abscessus pulmonary disease, whose isolate’s erm(41) genotype suggests a truncated, non-functional gene — the pattern associated with true macrolide susceptibility. The disagreement is whether that genotype result is enough to build the regimen around an actively working macrolide, or whether real-world outcomes in this organism argue for treating it as inert regardless.
Rosa M., a 52-year-old woman, has spent every Saturday morning for the past six years volunteering at her church’s food pantry, sorting donations alongside a small group of regulars she’s grown close to over time — a rhythm she says has done more for her mood than anything her primary care doctor has prescribed in years. She has had bronchiectasis since a bout of severe childhood pertussis left her with permanently damaged airways, managed for most of her adult life with intermittent antibiotic courses for flares and nothing more structured than that. Over the past two months her cough has worsened and she has noticed occasional streaks of blood in her sputum, prompting a workup that grew Mycobacterium abscessus on repeat culture — a rapidly growing nontuberculous mycobacterium with a reputation, even among specialists who treat it regularly, for being difficult to clear. Standard susceptibility testing at day 14 showed a clarithromycin MIC in the susceptible range, and erm(41) gene sequencing, sent given how much that single result can change the treatment plan, returned a truncated gene — the pattern most often seen in M. abscessus subsp. massiliense. Nash and colleagues, who first characterized erm(41) as the gene responsible for inducible macrolide resistance in this organism, found that a truncated copy tracks with genuinely stable, non-inducible macrolide susceptibility rather than the delayed resistance the fuller-length gene produces in other subspecies.
Whether that genotype result is enough to lean on is the actual question dividing the team, not whether the result itself is real. The erm(41) truncation and the low day-14 MIC point the same direction, which is more reassuring than either alone — a single susceptible reading without genotype confirmation has occasionally been overturned by delayed induction in other isolates, but a truncated gene removes the mechanism that delayed induction actually depends on. The outcome data cut in her favor as well, and by a wide margin: pooled series report sputum culture conversion in roughly 79% of massiliense-pattern disease against about 35% for subsp. abscessus, and an individual-patient-data meta-analysis found treatment success in 56.7% versus 33.0% on the same split. Subspecies is, on these numbers, the single strongest prognostic discriminator in M. abscessus lung disease, and she is on its good side. What that figure cannot tell anyone is which side of it she personally lands on — roughly one massiliense patient in four or five still fails, those cohorts are observational rather than randomized, and a number that describes how a subspecies behaves has never yet told a clinician how one woman’s lungs will.
ID clinic, reviewing the susceptibility and genotype results
Build this regimen around an active macrolide. Her erm(41) gene is truncated — the massiliense pattern Nash et al. first linked to stable susceptibility rather than delayed, inducible resistance — and her day-14 MIC came back susceptible. Those two results agree with each other, which is more than a single susceptibility panel alone would give us. Azithromycin plus amikacin and tigecycline gives three genuinely active agents, and adding clofazimine makes four — a real, genotype-supported regimen, not a guess.
The genotype tells us about this specific resistance mechanism. It doesn’t tell us how well this regimen will actually clear her infection. And I’ll give you the number rather than gesture at it, because it doesn’t fully help me: pooled series put conversion near 79% in massiliense against about 35% in subsp. abscessus. Her genotype is worth a great deal. It is also observational data, and it still leaves something like one patient in five failing — the erm(41) call is necessary information, not sufficient reassurance.
I’d build a four-drug regimen — amikacin, tigecycline, clofazimine, and the macrolide — but treat it as adequately covered by the three non-macrolide agents regardless of which way the genotype question turns out.
You’re right that genotype isn’t destiny — I’m not disputing the recurrence data.
But the actual disagreement here isn’t about how many drugs to give. Both of you would build a multidrug regimen either way. It’s about whether the macrolide’s slot counts as a real fourth active agent or an inert placeholder. Confirm with a repeat susceptibility test at three months rather than resolving that argument by assumption now, and treat the regimen as adequately covered by the non-macrolide agents in the meantime regardless of which of you turns out right.
Agreed: a four-drug regimen including azithromycin as a genuinely active agent, with repeat susceptibility retesting at three months to confirm the genotype call holds.
Not agreed: what a persistently positive culture at three months, with a still-susceptible genotype, would actually mean. The pulmonologist reads that outcome as reason to reconsider the macrolide’s active role after all; the infectious disease physician thinks three months is too early to draw that conclusion, given how slowly M. abscessus cultures typically convert even on an adequate regimen.