A 48-year-old male presented to the dermatology clinic for a routine skin screening exam. On total body skin survey, yellow concretions in the shafts of his axillary hair were discovered (Figures 1 and 2). The patient was asymptomatic and unaware of the concretions. Examination with Wood’s lamp revealed blue fluorescence of the hair concretions and coral red fluorescence of the skin of the axilla (Figure 3). Microscopic examination of the hair shafts better delineated the concretions (Figure 4).
Based on the clinical and microscopic exam, the patient was diagnosed with both erythrasma and trichomycosis axillaris, which are both superficial bacterial infections caused by gram-positive bacteria of the genera Corynebacterium. Erythrasma is caused by Corynebacterium minutissimum, and trichomycosis axillaris is caused by Corynebacterium tenuis.1 Both conditions can also less commonly affect the skin and hair of the pubic area. Corynebacterium species colonize intertriginous sites with a warm, humid environment. Factors such as obesity, hyperhidrosis, and poor hygiene favor its development. The characteristic ”coral red” fluorescence of erythrasma is caused by coproporphyrin III produced by Corynebacterium minutissimum.
Diagnosis of these superficial Corynebacterium infections is typically made based on clinical exam including Wood’s lamp. If needed, gram stain of the concretions or culture reveals the coryneform morphology. The differential diagnosis of trichomycosis axillaris includes other concretions of the hair shaft such as hair casts, white piedra, and black piedra. The differential diagnosis of erythrasma includes other causes of axillary erythema such as candidiasis, tinea corporis, and contact dermatitis.