Case Report: First pulmonary infection caused by Mycobacterium colombiense in a non-immunosuppressed host with bronchiectasis: diagnosis facilitated by synergistic mNGS and culture

<i>Mycobacterium colombiense</i>, a rare slow-growing mycobacterium within the <i>Mycobacterium avium complex</i> (MAC), causes disseminated disease almost exclusively in immunocompromised hosts, with no prior reports of localized pulmonary infection in non-immunosuppressed individuals. A 47-year-old non-immunosuppressed male with bronchiectasis presented with progressive cough, night sweats, and fatigue. Computed tomography (CT) revealed bronchiectasis with nodules in the right middle and lower lobes. Empirical β-lactam therapy failed, and conventional bronchoalveolar lavage fluid (BALF) tests (smears, cultures, PCR) yielded no pathogens at 48 h. Although metagenomic next-generation sequencing (mNGS) of BALF detected a low number of <i>M. colombiense</i> sequences (eight reads), definitive confirmation was achieved through extended culture, which is considered the gold standard for the diagnosis of nontuberculous mycobacteria. This culture revealed acid-fast bacilli within 12 days (160 CFU), confirming the presence of viable <i>M. colombiense</i>. Subsequent mNGS of the isolated colonies further confirmed the species identity with high sequence reads (25,787 reads). Guideline-based triple therapy (guided by drug susceptibility testing and guidelines) with clarithromycin, rifampicin, and ethambutol achieved significant radiographic resolution at 24 weeks. This case demonstrates that <i>M. colombiense</i> pulmonary infection is diagnostically elusive and mimics non-specific respiratory syndromes. It defines the clinical features of this pathogen in non-immunosuppressed hosts and highlights the need for heightened surveillance for nontuberculous mycobacteria (NTM) in bronchiectasis patients, given the likelihood of underdiagnosis.