Corynebacteria invade the upper third of the stratum corneum; under favorable conditions such as heat and humidity, these organisms proliferate. The stratum corneum is thickened. The organisms that cause erythrasma are seen in the intercellular spaces as well as within cells, dissolving keratin fibrils. The coral-red fluorescence of scales seen under Wood lamp light is secondary to the production of porphyrin by these diphtheroids.

C minutissimum, a member of the normal skin flora, is the causative agent of erythrasma. The bacterium is a lipophilic, gram-positive, non-spore-forming, aerobic, and catalase-positive diphtheroid. C minutissimum ferments glucose, dextrose, sucrose, maltose, and mannitol.

Whole-genome sequencing of C minutissimum has been carried out with the goal of improved understanding of the multiantibiotic resistance that has been observed and its virulence factors, specifically in immunocompromised hosts. This will make it possible to identify the genes contributing to antibiotic resistance and thus to develop better-designed treatment options for these special cases.[4]

Predisposing factors for erythrasma include the following:

• Excessive sweating/hyperhidrosis
• Delicate cutaneous barrier
• Obesity
• Diabetes mellitus
• Warm climate
• Poor hygiene
• Advanced age
• Other immunocompromised states

United States and international statistics

The incidence of erythrasma is reported to be around 4%. This infection is observed all over the world; the widespread form is found more frequently in the subtropical and tropical areas than in other parts of the world. Erythrasma occurs less often in children and tends to be more prevalent among college students in dormitories, soldiers in barracks, and senior adults in nursing facilities.

In a study conducted in Turkey, the rate of erythrasma was found to be 46.7% among 122 patients with interdigital foot lesions.[5]

In a cross-sectional study of 80 patients with confirmed superficial cutaneous intertriginous infections in Tehran, Iran, erythrasma was the second most common infection after dermatophytosis[6] ; it accounted for 35% of the cases. The toe-web spaces were the most common sites, followed by the groin and axillary vaults.

The occurrence of erythrasma on the palm of one patient was described in a report by Rao et al.[7] This appears to be extremely rare, if not unique.

Age-, sex-, and race-related demographics

The incidence of erythrasma increases with age; however, no age group is immune to the disease. Erythrasma has been reported in children as young as 1 year.

Males and females are equally affected by erythrasma; however, the crural form is more common in men. A 2008 study found that interdigital erythrasma was more common in women (83% of 24 patients).[8]  A later study conducted in India confirmed the absence of sex predilection and observed that it was more commonly detected in patients with a body mass index (BMI) higher than 23 kg/m2 (62.5%) and in those with diabetes (50%).[9]

The incidence of erythrasma is higher in Black patients.

The prognosis for erythrasma is excellent; however, the condition tends to recur if the predisposing factors are not eliminated.

Although erythrasma is usually a benign condition, it may become widespread and invasive in predisposed and immunocompromised individuals; this is very rare in immunocompetent hosts. In such individuals, this organism has caused infections other than erythrasma. Reports have cited abscess formation (n= 3),[10] intravascular catheter–related infections (n = 2),[11] primary bacteremia (n = 3), peritoneal catheter–related infections (n = 2),[11, 12] endocarditis (n = 2),[13, 14] pyelonephritis (n = 2),[15, 16] cellulitis (n = 1),[17] endophthalmitis (n = 1),[18] arteriovenous fistula infection (n = 1), cutaneous granuloma (n = 1),[19] and meningitis (n = 1).[20]

In 2014, Shin et al reported the first known case of postoperative intra-abdominal infection caused by C minutissimum in an immunocompetent adult host; the infection was successfully treated with intravenous amoxicillin-sulbactam.[21]

Any patient with erythrasma should be advised to change his or her lifestyle by engaging in exercise and, if relevant, weight loss (because obesity is a major risk factor). In addition, personal hygiene and environment acclimatization should be underscored. Wearing cotton garments rather than synthetic fabrics is yet another consideration for keeping the sites of predilection dry. Finally, eating healthily and limiting intake of sugary foods (especially for people with diabetes) will help minimize the risk for this disease.

The dark discoloration associated with erythrasma is usually limited to body folds that are naturally moist and occluded. Infection commonly is asymptomatic, but it can be pruritic. The duration of erythrasma ranges from months to years. Widespread involvement of trunk and limbs is possible.

Immunosuppressed patients with erythrasma and an increased risk of complications are of special concern. Any possible concomitant infection must be evaluated and treated. In cases of recurrent erythrasma, diabetes should be suspected. Risk factors should be addressed and modified to the extent possible.

The typical appearance of erythrasma is well-demarcated, brown-red macular patches. The skin has a wrinkled appearance with fine scales (see the image below).

Lichenification and hyperpigmentation are common. Skin occasionally appears wrinkled, with scales. KOH test results are negative. Image from Michael Bryan, MD.

Infection commonly is located on the inner thighs, crural region, scrotum, and toe webs. Toe web lesions appear as maceration, with the fourth interdigital space most frequently affected.[8] ​ The axillae, submammary area, periumbilical region, and intergluteal folds are less commonly involved in erythrasma. A few cases of erythrasma of the vulva have been reported.[22]

Given the association of erythrasma with other corynebacterial skin infections (eg, pitted keratolysis and trichomycosis axillaris), it is important that all body folds and feet be screened. Blaise et al investigated this association in 53 patients with pitted keratolysis who had skin signs of other corynebacterial infections.[23] Erythrasma was encountered in two of the 53, whereas trichobacteriosis was noted in four. One patient presented with all three conditions present simultaneously.

A study (N = 30) by Janeczek et al found a high prevalence of erythrasma in patients with inverse psoriasis.[24]

The following complications of erythrasma may develop:

• Fatal bloodstream infection in immunocompromised patients with erythrasma
• Infective endocarditis in valvular heart disease patients with erythrasma
• Postoperative wound infection in erythrasma patients

Wood lamp examination of erythrasma lesions reveals coral-red fluorescence of lesions (see the image below). Results may be negative if the patient bathed before presentation.[31] The cause of this color fluorescence has been attributed to excess coproporphyrin III synthesis by the corynebacteria; the porphyrin accumulates in cutaneous tissue and emits a coral-red fluorescence when exposed to light from a Wood lamp.[32]  If a Wood lamp is not available, a smartphone app that simulates ultraviolet (UV) light may be considered as an alternative.[33]

Under Wood lamp examination, porphyrins produced by bacteria fluoresce with coral pink color. Small focus is visible on photo. If patient has bathed recently, pigment may be washed away. In suspicious cases, repeat examination on following day may be necessary. Image from Michael Bryan, MD.

In culture media composed of 20% fetal bovine serum, 2% agar, 78% tissue culture medium #199, and 0.05% tris, the organisms grow as nonhemolytic 1- to 1.5-mm smooth colonies. Methylene blue stain may be used to highlight both the fungal spores of pityriasis versicolor and the curved or club-shaped bacterial rods of C minutissimum, in case the two organisms are coexisting.[26]

In a study comparing the diagnostic yield of Wood lamp examination with that of Gram staining, it was found that 9% of patients were positive on the former and 15.6% were positive on the latter.[30] Concurrent use of Wood lamp examination and Gram staining resulted in a higher yield of positive findings (22.1%).

The diphtheroid bacteria that cause erythrasma are present in the stratum corneum as rods and filaments.

Erythrasma may be treated with topical agents, oral agents, or both. First-line therapy consists of topical erythromycin or clindamycin or fusidic acid or miconazole cream. In the United States, however, fusidic acid is not available; consequently, topical treatment with the other agents mentioned is the standard of care.

A 2017 report by Greywal et al described the use of mupirocin 2% ointment monotherapy on a series of nine male patients twice daily for 2-4 weeks, with complete resolution of their signs and symptoms.[34]  Another topical treatment that has been tried is ozonated olive oil every 12 hours for 10 days, which yielded a 100% cure rate in a report by Ramírez-Hobak et al.[35]

Oral erythromycin is usually effective and is a good second-line therapy, as is single-dose clarithromycin[36] or amoxicillin-clavulanate, for systemic treatment.[37]

C minutissimum is generally susceptible to penicillins, first-generation cephalosporins, erythromycin, clindamycin, ciprofloxacin, tetracycline, and vancomycin. However, multiresistant strains have been isolated.[38, 39, 40] In a susceptibility study (N = 40) by Turk et al, several antibiotics were tested in patients with erythrasma, including penicillin G, ampicillin, cefaclor, amoxicillin-clavulanate, ampicillin-sulbactam, tetracycline, erythromycin, ofloxacin, fusidic acid, levofloxacin, and azithromycin.[37] Significant resistance to erythromycin, azithromycin, penicillin, and ampicillin was noted. Significant susceptibility to amoxicillin-clavulanate, cefaclor, and fusidic acid was found.

In a large double-blind placebo-controlled randomized trial (N = 151), patients older than 18 years were randomized into five groups and given either erythromycin, single-dose clarithromycin, topical fusidic acid, placebo cream, or placebo tablets.[41] Fusidic acid cream was significantly more effective than other therapies. Additionally, the clarithromycin group did better at 48 hours than  the erythromycin group did; however, there was no statistical difference on day 7 and day 14.

In one report (N = 13), photodynamic therapy using red light (broadband, peak at 635 nm) cleared erythrasma in 23% of cases and improved erythrasma in the remaining cases[42] ; however, such therapy is not the treatment of choice.

The goals of pharmacotherapy for erythrasma are to reduce morbidity, eradicate the infection, and prevent complications.

Class Summary

Antibacterial and/or antifungal agents are used to eradicate C minutissimum and possible concomitant infection.

Erythromycin (E.E.S., E-Mycin, Ery-Tab)

Erythromycin is the drug of choice. It inhibits bacterial growth, possibly by blocking dissociation of peptidyl t-RNA from ribosomes, causing RNA-dependent protein synthesis to arrest. In children, age, weight, and severity of infection determine the proper dosage. When twice-daily dosing is desired, half the total daily dose may be taken every 12 hours. For more severe infections, double the dose.

Erythromycin topical (AkneMycin, Ery)

Erythromycin inhibits protein synthesis in susceptible organisms by reversibly binding to 50 S ribosomal subunits, thereby inhibiting translocation of aminoacyl transfer-RNA and inhibiting polypeptide synthesis.

Clarithromycin (Biaxin)

Clarithromycin inhibits bacterial growth, possibly by blocking dissociation of peptidyl t-RNA from ribosomes, causing RNA-dependent protein synthesis to arrest.

Clindamycin (Cleocin)

Clindamycin has a bacteriostatic effect; it interferes with bacterial protein synthesis similarly to erythromycin and chloramphenicol by binding to the 50S subunit of bacterial ribosomes.

Clindamycin topical (Cleocin T, Clindacin P, ClindaDerm)

Clindamycin is an antibacterial agent that binds to the 50S ribosomal subunits of susceptible bacteria and prevents elongation of peptide chains by interfering with peptidyl transfer, thereby suppressing protein synthesis.

Tetracycline (Achromycin)

Tetracycline inhibits cell growth by inhibiting mRNA translation. It binds to the 16S part of 30S ribosomal subunits and prevents amino-acyl tRNA from binding to the A site of ribosomes. Binding is reversible in nature.

Class Summary

Antibacterial and/or antifungal agents are used to eradicate C minutissimum and possible concomitant infection.

Miconazole topical (Femazole, Lotrimin, Monistat)

Miconazole damages the fungal cell wall membrane by inhibiting the biosynthesis of ergosterol. Membrane permeability is increased, causing nutrients to leak out and resulting in fungal cell death. Lotion is preferred in intertriginous areas. If cream is used, apply sparingly to avoid maceration effects. Use 2% cream.