A Case of Granulomatous Nodule Caused by a Previously Undescribed Species of Nontuberculous <i>Mycobacterium</i>

A 70-year-old immunocompetent woman presented to dermatology with a firm, asymptomatic subcutaneous nodule on her right forearm that had persisted for 1 year. The nodule measured approximately 13 mm on examination, was mobile, and showed no epidermal changes. Histopathology revealed numerous well-formed dermal granulomas with lymphocytic infiltrates and abundant acid-fast bacilli on Fite stain. PCR and sequencing of the hsp65 and rpoB genes from two separate tissue samples identified a previously undescribed bacterial species of the Mycobacteriales order, showing only distant similarity to M. leprae , M. ulcerans , and M. marinum . Although nontuberculous mycobacteria (NTM) are common environmental organisms that typically cause cutaneous infections following trauma, surgery, or cosmetic procedures, no recent inciting event was reported during the infectious disease consultation. The patient did note a similar nodule had been excised from a different location decades earlier and described as "tuberculous," though detailed records were unavailable. This history suggests the possibility of an earlier environmental exposure resulting in a latent, subclinical, long-term infection. No further treatment was pursued due to the absence of systemic symptoms and additional cutaneous lesions after excision.