Mycobacterium mucogenicum skin and soft tissue infection of the breast mimicking idiopathic granulomatous mastitis

We present a case of a 28-year-old woman who came to medical attention after noticing a breast mass associated with an overlying eroded plaque of the skin. A core biopsy of the breast mass was negative for malignancy but demonstrated granulomatous inflammatory changes. Acid-fast bacilli and Gomori methenamine-silver stains were negative for microorganisms. The patient was diagnosed with presumptive idiopathic granulomatous mastitis and started on oral steroids. Her symptoms progressed. Tissue culture from a repeat biopsy grew Mycobacterium mucogenicum. The patient responded well to combination oral antimicrobial therapy.