Lower airway microbiota in patients with clinically suspected Mycobacterium avium complex lung disease

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The relationship between the lower airway microbiota in humans and respiratory illness has gained attention recently. However, the relationship between nontuberculous mycobacterial lung disease (NTM-LD) and the lower airway microbiota is not fully understood yet. We conducted a study to characterize the lower airway microbiota in Mycobacterium avium complex lung disease (MAC-LD), a representative subclass of the NTM-LD. The subject sample included 25 patients clinically suspected of having mild MAC disease whose condition could not be diagnosed using sputum culture. Upon testing MAC antibodies (anti-glycopeptidolipid (GPL)-core IgA antibodies), mycobacterial culture of bronchoalveolar lavage fluid (BALF), and performing BALF 16S rRNA gene sequencing, we divided the subjects into two groups of patients: those in whom MAC was detected in BALF mycobacterial culture (MAC-LD group) and in whom MAC was not detected in BALF mycobacterial culture (non-MAC-LD group), which was then comparatively examined. BALF mycobacterial culture showed that 9 out of 25 patients were positive for NTM; the detected Mycobacterium was MAC in all. No patients were positive for acid-fast bacteria other than MAC. Eighteen patients were positive for MAC antibodies (anti-glycopeptidolipid (GPL)-core IgA antibodies), including nine patients positive for mycobacterial culture. On BALF 16S rRNA gene sequencing, six patients were positive for the genus Mycobacterium and were culture-positive. Among the 16 patients in the non-MAC-LD group, the genus Pseudomonas was detected by 16S rRNA gene sequencing in 7 patients, 4 among whom were positive for MAC antibodies (anti-GPL-core IgA antibodies). Conversely, the genus Pseudomonas was not detected among the nine patients in the MAC-LD group. Other than the genus Pseudomonas, there was no clear difference in the composition of and no significant difference in the diversity of the bacterial flora between the MAC-LD and non-MAC-LD groups. However, we found that the genus Pseudomonas and MAC tended to exist exclusively.