Essential fatty acids disrupt the mycolic acid-rich cell envelope of clinical Nocardia isolates: a membrane-targeting approach to combat persistent infections

Background Conventional antibiotics against slow-growing, mycolic acid-rich Nocardia often require prolonged therapy and are associated with high relapse rates. Membrane-targeting essential fatty acids may circumvent growth-dependent resistance. Methods Five essential fatty acids were screened against 82 clinical Nocardia isolates by broth microdilution. Docosahexaenoic acid (DHA) was further assessed for bactericidal kinetics, membrane integrity by propidium iodide staining and scanning electron microscopy, transcriptomic profiling, and combined activity with amikacin and tobramycin. Results α-Linolenic acid, γ-linolenic acid, eicosapentaenoic acid and DHA exhibited comparable activity (MIC 50/ MIC 90 : 31.25/62.50 μM), corresponding to mass concentration ranges of 8.7 to 10.3 μg/mL for MIC 50 and 17.4 to 20.6 μg/mL for MIC 90 . However, arachidonic acid was markedly less active. DHA rapidly disrupted the Nocardia cell envelope, causing propidium iodide influx and ultrastructural damage, and achieved concentration-dependent bactericidal killing within 1 h, a feat not observed with imipenem. Transcriptomic analysis of the surviving subpopulation revealed downregulation of the proton-pumping Nuo complex, upregulation of type II NADH dehydrogenase (Ndh), and global induction of translation machinery. These transcriptional changes, together with pre-existing membrane damage, additively enhanced the activity of amikacin against intrinsically resistant N. wallacei (FICI, 0.51-0.56). Conclusions These findings demonstrate that DHA displays rapid membrane-disruptive activity and can enhance amikacin activity against Nocardia under the defined in vitro conditions.