Significance and Impact of the Study: This study was conducted to investigate the ecological abundance of the most prominent clinical nontuberculous mycobacteria (NTM) in Germany, the Mycobacterium avium ssp. hominissuis (MAH). Examination of soil, water, dust and biofilm samples revealed that MAH in Germany was predominant in soil and dust. No MAH was identified in water and biofilms. Our finding contributes to the identification of the environmental niche of this opportunistic pathogen and proposes soil and dust as sources of MAH infection in Germany. AbstractThe nontuberculous mycobacteria (NTM) are a heterogeneous group of bacteria found in soil, water and dust. The spread of NTM infection depends on the exposure to reservoirs with high proportions of mycobacteria, the virulence of the NTM strains, the enhanced sensitivity to infections such as those of immune-compromised hosts and patient risk factors such as Cystic Fibrosis. Since several decades, NTM lung disease has been increasingly observed in slender postmenopausal women. The most important NTM in Germany is Mycobacterium avium ssp. hominissuis (MAH). The routes of MAH infection are in almost all cases unknown, but water is often suspected as source of infection. We wanted to examine this hypothesis by determining the frequency of MAH in environmental samples of water, biofilms, soil and dust originating from Germany. We found MAH in 33% of the dust samples and 20% of the soil samples. No MAH could be isolated from water and biofilm. Dust and soil clearly presented more abundance of MAH in comparison with water and biofilms. Therefore, more attention should be paid to soil and dust in Germany as an important source of Myco. avium infections. IntroductionNTM are natural inhabitants of soil and water. More than 140 species of environmental mycobacteria are known today and at least 40 of them are associated with lung disorders (Griffith 2010). Mycobacterium avium plays a prominent role among NTM and is a clinically important NTM for being causal agent of pulmonary disease in immunecompromised patients, lymphadenitis in small children and disseminated disease in patients with acquired immunodeficiency syndrome (AIDS) (Reuss et al. 2009;Hoefsloot et al. 2013;Thomson et al. 2013). Over the last decades, slender elderly postmenopausal women have been increasingly diagnosed with NTM lung infection (Chan and Iseman 2010;Epson and Winthrop 2012).A study that analysed the hospitalizations associated with pulmonary nontuberculous mycobacterial infections in Germany from 2005 to 2011 found an average annual age-adjusted rate of 0Á91 hospitalizations per 100 000 of population. The hospitalization rates increased during the study period, and the most pronounced average increase of 6Á4% per year was observed among females (Ringshausen et al. 2013 The source of MAH infection remains unknown for most cases. MAH are opportunistic environmental mycobacteria and their reservoirs include drinking water distribution systems, household plumbing, peat rich soils, boreal soil...