Ingrid Van Weerdenburg (Radboudumc)

Summary of presentation:
Mycobacterium marinum is a non-tuberculous mycobacterium (NTM) causing tuberculosis-like infection in fish. In humans, it appears as a nodular granulomatous illness typically infecting skin and soft tissues. Humans can get infected through wounded skin exposed to M. marinum-contaminated environments. This study aimed to compare M. marinum genotypes with clinical data to construct a phylogenetic tree and explore potential genotype-phenotype associations.

Clinical isolates of M. marinum (n= 66) obtained from the Dutch national NTM reference unit were included in a retrospective study (2020-2025). Samples, collected as part of routine care, were sequenced using Illumina short-read technology. Additionally, two M. marinum strains from dolphins were included. A phylogenetic tree was constructed using genomic data from clinical, animal and reference strains (M. marinum Europe, M, E11, MB2, M. liflandii, M. pseudoshottsii) to explore genomic diversity. Detailed clinical characteristics were obtained from consultations and mapped onto the tree to explore its correlation with the genetic data.

Most patients were male (n=58) with a mean age of 57,7 years (22-83 years), and five received immunosuppressive therapy. Eight patients had infections spreading to tendons, bones, or lymph nodes. Treatment was successful in 36 patients, failed in 4, recurred in 4, with 22 missing data. Genomic analysis revealed distinct phylogenetic clusters consistent with known lineages; cluster ‘M’ predominantly contains human strains, while cluster ‘Aronson’ is mostly animal strains. The clinical isolates cluster closely within a single branch within cluster ‘M’. Integration of clinical metadata showed no clear associations with the genotype.

A high degree of genetic similarity was observed among the clinical isolates, without clustering related to clinical metadata. The lack of differentiation could be attributed to the limited host range (i.e. fish species) from which infections are acquired, as most cases are linked to contact with in-house aquaria. Alternatively, these strains may have a virulence advantage.