Pseudo-outbreak of Mycobacterium lentiflavum at a general hospital in Japan

is a slow-growing nontuberculous mycobacterium that is widely distributed in soil and water systems, but it is sometimes pathogenic to humans. Although cases of infections are rare, 22 isolates of were identified at a single hospital in Japan. We suspected a nosocomial outbreak; thus, we conducted transmission pattern and genotype analyses. Cases of isolated at Kushiro City General Hospital in Japan between May 2020 and April 2021 were analyzed. The patient samples and environmental culture specimens underwent whole-genome sequencing (WGS). Additionally, we retrospectively collected clinical data from patient medical records. Altogether, 22 isolates of were identified from sputum and bronchoalveolar lavage samples. Clinically, the instances with isolates were considered contaminants. In the WGS analysis, 19 specimens, including 18 patient samples and 1 environmental culture from the hospital's faucet, showed genetic similarity. The frequency of isolation decreased after we prohibited the use of taps where was isolated. WGS analysis identified that the cause of pseudo-outbreak was the water used for patient examinations, including bronchoscopy.