Mycobacterium chelonae as an Underrecognized Cause of Granulomatous Disease in Fish and Humans: A Molecular One...

Non-tuberculous mycobacteria are recognized as pathogens affecting aquatic animals and humans. Although Mycobacterium marinum is traditionally considered the principal etiological agent of fish mycobacteriosis and fish-tank granuloma in humans, the contribution of Mycobacterium chelonae in fish as well as in humans remains underestimated, particularly in Ziehl-Neelsen (ZN)-negative lesions. In this study, granulomatous lesions of marine fish species and human skin biopsies were investigated using histopathology and molecular diagnostics. Following histological evaluation, all samples were analysed by PCR amplification and sequencing of the hsp65 gene to identify the causative agent. Fish and human tissues showed similar chronic granulomatous lesions, with all cases negative for acid-fast bacilli by ZN staining. Molecular analysis consistently identified M. chelonae in both marine fish organs and human skin lesions exhibiting clinicopathological features traditionally attributed to M. marinum. Phylogenetic analysis revealed high genetic similarity between fish- and human-derived sequences, suggesting shared environmental reservoirs rather than host-specific lineages. These findings identify M. chelonae as an underrecognized cause of granulomatous disease in fish and humans and demonstrate the limitations of histochemical methods in these infections. Incorporating molecular diagnostics into routine investigations of granulomatous disease supports more accurate diagnosis across veterinary and human medicine and strengthens One Health surveillance of aquatic non-tuberculous mycobacterial infections.