As the summer heat continues into September, I am still seeing a large number of patients presenting with soggy and stinky shoes. This dark, moist microenvironment creates the perfect home for harboring bacteria and fungi.
One of type of bacteria that thrives in this setting is Corynebacterium minutissimum. C. minutissimum is a Gram-positive, non-spore forming aerobic or facultatively aerobic bacillus.1 It is part of the normal skin flora. Given the proper conditions and humidity, these bacteria can multiply within the stratum corneum and lead to a skin infection. Erythrasma is a superficial skin infection cause by C. minutissimum, most commonly affecting the plantar feet and interdigital areas. Erythrasma is the most common cause of interdigital bacterial infection on the feet and is most often visible between the fourth and fifth toes.2
The clinical presentation of erythrasma varies from irregularly shaped, well-defined thin red or brown patches or plaques to white maceration with scaling and fissuring.3 One may easily confuse its interdigital form with tinea pedis. Another presentation of erythrasma is that of pitted keratolysis, in which multiple, 1 to 2 mm punctate crateriform lesions develop. Over time, as the erythrasma infection progresses, the Corynebacterium minutissimum releases proteinases that break down the skin of the stratum corneum, leading to the development of pitted keratolysis.2,3
Interdigital erythrasma is more difficult to diagnose as the web spaces become macerated. Differential diagnosis in this instance includes psoriasis, dermatophytosis, candidiasis and intertrigo.3 Additionally, it is common to see concurrent infections with a bacteria and dermatophyte.3
The classic diagnosis occurs with the appearance of a coral red fluorescence under a Wood’s lamp in the area of suspected erythrasma.2 Corynebacterium minutissimum produces porphyrins, which give it a fluorescent color under the Wood’s light.4 With gram staining, the presence of Gram-positive, purple rods would further confirm the diagnosis while potassium hydroxide preparation can help evaluate for concomitant fungal infection.2 Fortunately for most, erythrasma in the feet is usually asymptomatic.
Treatment typically begins with topical agents. Research has shown that related species of Corynebacterium are sensitive to topical clindamycin. Accordingly, clinicians often utilize clindamycin in the treatment of erythrasma.3 Applying topical clindamycin 1% gel or cream to the affected area two to three times a day for two to four weeks effectively eradicates most infections.3 Alternative topical treatments include erythromycin 2% gel and Whitfield’s ointment.3,4 Whitfield’s ointment is a combination of 12% benzoic acid and 6% salicylic acid, and its success in the treatment of erythrasma derives from its keratolytic effects rather than bacteriostatic effects.3 Recalcitrant cases may require a short course of oral antibiotics.
- Ahmad NM, Ahmad KM. Corynebacterium minutissimum pyelonephritis with associated bacteraemia: a case report and review of literature. J Infect. 2005; 51(5):e299-e303.
- Brice S. Erythrasma. Up To Date. Available at http://www.uptodate.com/contents/erythrasma?source=search_result&search=erythrasma&selectedTitle=1%7E11 .
- Holdiness MR. Management of cutaneous erythrasma. Drugs. 2002; 62(8):1131-1141.
- Schwartz RA, Al Mutairi N. Topical antibiotics in dermatology: an update. Gulf J Dermatol Venerol. 2010; 17(Pt 1):1-19.