L.S., a 47-year-old white woman with type 1 diabetes, a bleeding diathesis, and iron deficiency anemia, was referred to an infectious diseases physician for assessment of a painful, nonhealing skin lesion. Before the referral, her primary care physician had treated her with several courses of oral antibiotics, including cephalexin, clindamycin, and amoxicillin, without improvement. She denied a history of preceding trauma, recent foreign travel, or exposure to aquatic animals or hot tubs. When questioned about whether the lesion was at the site of insulin injection, she stated this was possible because she had injected insulin at various locations through her clothes since adolescence, as instructed.
On physical examination, the patient appeared well. Her skin examination was significant for a 5-cm erythematous, tender nodule on the anteromedial aspect of her right thigh (Figure 1). No drainage from the lesion or evidence of surrounding cellulitis was found. There was no associated lymphadenopathy, and the remainder of her physical examination was unremarkable.
Routine laboratory tests, including leukocyte count with differential, platelets, electrolytes, creatinine, and liver enzymes, were within normal ranges. Her A1C was elevated at 10.8%.
Histology of the biopsied lesion demonstrated nonspecific inflammation with negative stains for acid-fast bacilli (AFB) and fungi. Cultures, however, were positive for mycobacteria spp. after an extended incubation period of 63 days (compared to the conventional incubation period of 49 days). 16S rRNA sequencing identified the organism as Mycobacterium immunogenum. The antimicrobial agents the organism was susceptible to were limited to clarithromycin and tigecycline, with intermediate sensitivity to amikacin. The organism was resistant to ciprofloxacin, doxycycline, cefoxitin, imipenem, cotrimoxazole, and linezolid.
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