A case of pneumonia caused by Corynebacterium durum mimicking Nocardia pneumonia in a patient with dermatomyositis.

Nocardia can cause severe pneumonia primarily in patients with impaired cell-mediated immunity, such as those taking immunosuppressants. A delayed diagnosis is associated with a poor prognosis. However, early diagnosis is difficult, and conventional identification methods can sometimes misidentify Nocardia as other bacterial pathogens because of its morphological similarity. We report a case of pneumonia caused by Corynebacterium durum primarily misidentified as Nocardia pneumonia in a patient with dermatomyositis who was undergoing intensified immunosuppressive treatment. A 67-year-old woman with dermatomyositis who had received long-term prednisolone and tacrolimus therapy experienced worsening interstitial lung disease and skin and muscle lesions, and the prednisolone dose was increased to 40 mg/day (1 mg/kg/day). During treatment, she developed a productive cough. Nocardia pneumonia was initially suspected on the basis of symptoms, cavitary, infiltrative, and nodular shadows in the dorsal right lobe on computed tomography of the chest, and a sputum smear. Empirical therapy with sulfamethoxazole/trimethoprim 3200 mg/640 mg/day was started, and her symptoms improved quickly. The clinical course and antimicrobial susceptibility test profile were atypical. Therefore, 16S ribosomal ribonucleic acid gene analysis and matrix-assisted laser desorption ionization-time of flight mass spectrometry were performed. Based on this analysis, the pathogen was re-identified as C. durum. Sulfamethoxazole/trimethoprim was continued, leading to improvement of the symptoms and the pulmonary lesions on computed tomography. The findings from this case indicate that C. durum can be misidentified as Nocardia by conventional identification methods and highlight the importance of incorporating more precise identification approaches.