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On hematoxylin-eosin staining, microscopic sections showed deep dermal and subcutaneous inflammation composed of a mixed population of neutrophils, eosinophils, lymphocytes, and macrophages. Focal areas of necrosis and a few plasma cells were also present. Periodic acid–Schiff and gram stains were negative for fungi and bacteria. However, a Ziehl-Neelsen acid-fast stain revealed clusters of bacilli within the abscess.
A chest x-ray film showed no evidence of active pulmonary disease. The patient was treated empirically with doxycycline (100 mg twice a day) for atypical mycobacterial infection while awaiting identification of the organism. No improvement was observed over the next 3 weeks; in fact, new lesions continued to develop. Because at this point cultures of the first biopsy specimen demonstrated no growth, a second biopsy specimen was obtained for culture and drug sensitivity testing. The antibiotic regimen was also changed to ciprofloxacin. After 1 month, the bacilli were finally isolated and identified as M chelonae. Drug sensitivity testing indicated resistance to ciprofloxacin but revealed sensitivity to clarithromycin, amikacin, doxycycline, and cefoxitin. The lesions healed after a 3-month course of clarithromycin. Five months after treatment, there was still no sign of recurrent infection.
Nodules on the Arm of a Diabetic Patient—Diagnosis. Arch Dermatol. 2003;139(1):93-98. doi:10.1001/archderm.139.1.93-a