OR17

1:Unitat de Tuberculosi Experimental. Institut d'Investigació en Ciències de la Salut Germans Trias i Pujol (IGTP), Badalona (08916), Catalonia, Spain; 2:Servei de Microbiologia, LCMN, Hospital Universitari Germans Trias i Pujol (HUGTiP), Badalona (08916), Catalonia, Spain; 3:Departament de Genètica i Microbiologia, Universitat Autònoma de Barcelona, Bellaterra (08193), Catalonia, Spain; 4:Centre de Medicina Comparativa i Bioimatge de Catalunya (CMCiB); Badalona (08916), Catalonia, Spain; 5:Centro de Investigación Biomédica en Red de Enfermedades Respiratorias (CIBERES), Madrid, Spain

Mycolicibacterium manresensis, an environmental mycobacterium previously regarded as avirulent in mammalian models, displays unexpected lethality in Drosophila melanogaster, indicating the presence of cryptic virulence. Using an in vitro infection model with S2 haemocyte cells, we observed both high intra- and extracellular bacterial burdens, alongside a marked decline in host cell viability within 48 hours, assessed by flow cytometry. Live confocal microscopy over 24 hours post-infection further revealed the formation of cord-like aggregates—structures reminiscent of the classical cording phenotype associated with virulent Mycobacterium tuberculosis. Preliminary homology searches showed that M. manresensis encodes genes with similarity to known cording-associated factors in M. tuberculosis, suggesting a conserved molecular basis for this phenotype. In vivo, M. manresensis infection elicited a robust activation of the fly’s innate immune system, with significantly elevated expression of both Toll and IMD pathway genes compared to infection with the pathogenic Mycobacterium marinum. This heightened immune response coincided with high bacterial loads and consistent host mortality within seven days post-infection. Thus, despite its lack of pathogenicity in mice, M. manresensis demonstrates virulence traits in flies, suggesting host-specific unmasking of latent pathogenic potential. These findings highlight D. melanogaster as a sensitive model organism capable of revealing context-dependent virulence in environmental mycobacteria. Our work contributes to the understanding of the evolutionary continuum of mycobacterial pathogenicity and underscores the value of alternative host models in uncovering hidden facets of environmental microbes traditionally considered benign.