Two Cases of Lung Infection due to Mycobacterium shimoidei, with Special Reference to Bacteriological Investigation

Mycobacterium shimoidei (Tsukamura 1982) is an uncommon but widely distributed pathogen usually isolated from respiratory specimens. We report two cases of lung disease due to M. shimoidei and the associated bacteriological results. A 45-year-old man (Case 1) admitted to National Hospital Organization (NHO) Miyagi Hospital, a 75-year-old man (Case 2) admitted to NHO Higashi-Hiroshima Medical Center were found in initial chest X-ray and thoracic computed tomography (CT) to have a tuberculosis-like cavity in the left apex (Case 1) and the right apex (Case 2). In Case 1, the patient was treated with isoniazid and rifampicin for one month and lesions showed a partial response. In Case 2, the patient responded favorably with rifampicin, ethambutol, streptomycin, and clarithromycin therapy. Mycobacteria were repeatedly detected in smear and culture from sputum specimens in both patients. Isolates were nonphotochromogenic and rough. Isolated colonies developed after two to three weeks on 2% Ogawa egg medium. Organisms grew on 2% Ogawa egg medium at 30, 37, 42, and 45°C, but not 25°C. Both organisms were susceptible t0 500μg of p-nitrobenzoate per mL and 5mg of sodium chloride per mL. Isolates were negative for niacin accumulation, nitrate reduction, semiquantitative catalase, 68°C catalase, 3-day arylsulfatase, iron uptake, and MPB64 antigen production, but positive for Tween 80 hydrolysis (5 and 10 days), acid phosphatase, and pyrazinamidase. Isolates had typical uv-HPLC chromatograms similar to M. shimoidei demonstrating triple-peak clusters with peaks in the early cluster. 16S rRNA gene sequencing showed isolates to be consistent with Mycobacterium shimoidei. Based on composite characterization, isolates were identified as M. shimoidei. This is, to our knowledge, the third case of M. shimoidei infection reported in Japan.