Seven decades of nontuberculous mycobacteria in Denmark: shifts in species distribution and clinical relevance

The global burden of human disease caused by nontuberculous mycobacteria (NTM) is steadily increasing. Using our unique historical collection of freeze-dried mycobacterial isolates at the Statens Serum Institut, Copenhagen, we investigated long-term trends in NTM clinical relevance and species distribution in Denmark over the past seven decades. A total of 666 historical NTM isolates from human samples collected between 1948 and 1978 were compared with 1,820 contemporary isolates identified during routine diagnostics from 2013 to 2022. Species identification was performed for all isolates using the GenoType CM and AS line-probe assays. Among historical isolates, 16 different species were identified, with the majority classified as Mycobacterium sp. (32.1%, n = 214). In contrast, recent isolates were dominated by Mycobacterium avium (39.6%, n = 721), while Mycobacterium sp. accounted for only 7.8% (n = 142). Furthermore, notable changes in prevalence over time at the single species level were observed for M. avium and Mycobacterium abscessus. Further genomic analyses of historical Mycobacterium sp. isolates revealed several uncommon or previously unrecognized species rarely detected in current diagnostic settings. In conclusion, our findings suggest a significant shift in the spectrum of species responsible for human NTM infections in Denmark over the past seven decades, with possible implications for diagnostic strategies, clinical management, and public health surveillance.IMPORTANCEInfections caused by nontuberculous mycobacteria (NTM) are increasingly recognized worldwide, yet the long-term epidemiology of these human pathogens remains poorly understood. By leveraging a unique historical collection of mycobacterial isolates, this study suggests a shift in the species spectrum responsible for NTM infections in Denmark over the past seven decades, with Mycobacterium avium now predominating. Sequencing of historical isolates further uncovered several rare or previously unrecognized Mycobacterium species, illustrating the dynamic nature of NTM epidemiology over time. These findings underscore the need for sustained surveillance, precise species-level identification, and heightened clinical awareness to guide effective management of NTM infections amid a continually evolving disease landscape.