In light of the increasing prevalence of NTM pulmonary disease without a known cause, clinicians have a substantial interest in identifying the source of the NTM infection and the number of species identified in both clinical samples and environments. In this study, NTM was isolated from the showerheads as a potential source of infection for patients with NTM-PD, and a total of 18 species (subspecies, complexes) of NTM were identified. Among them, none of the mycobacteria matched the patients’ infectious mycobacteria, but various NTM species were identified, suggesting that it could be a potential source of infection in susceptible persons.
NTM are ubiquitous in the environment and have been isolated from different environmental sources, particularly water in homes, swimming pools, whirlpool therapy baths, soils, and the workplace21,22. Geographic and behavioral variabilities are also associated with the incidence of NTM infection among patients23. Particularly, water is one of the main vehicles for NTM transmission, as confirmed by isolation of the same species from both water and patients in several studies. For example, Covert et al. reported that 35% of samples from municipal water supplies in 21 states in the United States were found to test positive for NTM24. In one study, strains from the water system identical to those in the patients were found in 7 (41%) of 17 patients sampled12. For another example, Feazel et al. showed that the showerhead environment strongly enriched for microbes that are known to form biofilms in water systems, including Mycobacterium spp., Shpingomonas spp., Methylobacterium spp. and others in United States25. Our results, and those described by various studies, show that NTM are quite frequent in household water. However, in this study, we rarely detected the major causative organisms of NTM disease, including MAC members, such as M. avium and M. intracellulare, or M. abscessus, M. massiliense from the surfaces of the showerheads. In addition, when the isolates from the patients’ showerheads were compared with those from their sputum samples, none had matching pathogens in the present study. Many factors affect the ability of NTM to adhere and form a biofilm on various materials26. The materials used for the showerhead, the water sources, and the type of hot-water supply system may influence the regional presence of NTM spp. on showerheads. In addition to exposure, host factors are influential in NTM infection7,8,27-29. According to the study by Falkinham et al.30, the potential impact of human activities on the ecology of NTM has also been described. Our study examined the household showerhead as a potential source of NTM. Although we did not demonstrate Mycobacteria spp., which was the main pathogen associated with disease in the patients diagnosed with NTM infection, we found that 17 (57%) biofilm (swab) samples collected from the showerheads of patients with NTM pulmonary disease yielded NTM, using 16S rRNA sequences. In this study, many of the isolates were identified as M. lentiflavum 7 (10.9%), M. gordonae 6 (9.4%), M. triplex 4 (6.3%), and M. mucogenicum 3 (4.7%). In our study, as in the results of several studies, M. lentiflavum was also the most frequently identified isolate from the house water system and could be a potential pathogen that can cause pulmonary lung disease14,31,32.
M. gordonae is known to rarely causes pulmonary lung disease due to its low pathogenicity, and when M. gordonae is detected in sputum, it has been considered as a contamination33. However, there are case reports that M. gordonae can cause pulmonary lung disease in immunosuppressed patients such as human immunodeficiency virus infection, steroid treatment, and organ transplant patients34-36. Moreover, M. triplex and M. mucogenicum be known as opportunistic infections in both immunocompromised and immunocompetent humans exposed to environmental sources and may be fatal if the infection is disseminated37-39. Accordingly, our study supports the evidence that a common indoor shower facility can be a potential source of NTM infection, which has clear relevance to public health.
The limitation of this study is that we did not consider some relevant factors, such as household water sources, estimated time since showerhead installation, usage frequency, and cleaning frequency, which may influence the ability of NTM to adhere and form a biofilm. The strength of this study is that we recovered the NTM from environmental samples by establishing methods of sample collection, processing, and cultivation.
In conclusion, NTM are widely distributed in the environment and people are exposed to it frequently. There are several known vehicles of NTM transmission, of which water is one of the most important ones. In the present study, despite not consistent with the patients’ infectious mycobacteria, we showed that the showerhead could be a potential source of NTM infection by identifying NTM through isolation and cultivation. In combination with the earlier studies, these findings will help to elucidate the infectious source and route of NTM-PD.