Topical products used for the treatment of common skin infections

June 2002

Numerous topical antiinfective products are available to patients over-the-counter (OTC) or by prescription. Not only can the active antiinfective agents differ among these products, but their pharmaceutical dosage form (ie, ointment or cream) may as well. The indications for topical antiinfectives, including acne vulgaris, fungal skin infections, or viral skin infections (eg, herpes simplex virus) are too broad to discuss in this column. Discussed in this month’s column will be the treatment of common bacterial skin infections, including impetigo, with topical antiinfectives.

Labeling for the various topical antibiotics includes numerous potential uses, including furunculosis and ecthyma, although their benefit, as proven by controlled clinical trials, is limited. Dermatology references generally recommend topical antibiotics as an option of therapy for impetigo, superficial folliculitis, furunculosis (after incision and drainage), and minor abrasions. Treatment of other pyodermas, such as carbuncles, ecthyma, cellulitis or erysipelas, are best treated with systemic antibiotics.

Pharmaceutical dosage forms

Topical antibiotics are usually available in two pharmaceutical dosage forms, ointments or creams. Ointments are semisolid preparations (water-in-oil) that are generally more occlusive (preventing the escape of moisture) and more difficult to remove from the skin once applied. While there are different ingredients comprising ointments, many contain petrolatum. Petrolatum is an excellent occlusive agent, and thus, functions as an emollient (for water is the most important epidermal plasticizer) and lubricant. Patients may find petrolatum messy, however. Ointments, due to their occlusive effects, are best used for skin disorders with associated dryness, and not for areas with oozing lesions.

Creams are also semisolid preparations (oil-in-water) that are generally not as occlusive as ointments. Creams are generally not as messy as ointments, can be washed off with water, and patients may prefer these dosage forms because of this. As the ratio of oil content increases to the amount of water present, semisolid preparations evolve from creams to ointments.

From a practical standpoint, it is helpful to have an appreciation for the amount of cream or ointment necessary to treat a specific infected area of skin. If not enough medication is prescribed, patients may apply too little or may not obtain more medication by refill. One gram of cream will adequately cover approximately a 10 cm × 10 cm (100 cm²) area of skin; a similar amount of ointment will cover an area 5%-10% larger. A unit of measurement referred to as the fingertip unit (FTU) can be used to estimate how much medication to use. An FTU (adult), the area from the distal skin crease to the index finger tip, is approximately equivalent to 0.5 g. Viewed another way, the area of skin on one adult flat, closed hand would be covered by 0.5 FTU (0.25 g) of ointment.

Impetigo

The most common application of topical antibiotics for active infection is probably for use in the therapy of impetigo. Two forms of impetigo – bullous and nonbullous – require different treatments. Bullous impetigo, while not as common as the nonbullous form, requires the use of systemic antibiotics. The bacterial cause of bullous impetigo is Staphylococcus aureus, which produces an epidermolytic toxin. Bullous impetigo is best treated with a systemic antibiotic that provides activity toward this pathogen, such as dicloxacillin, some cephalosporins (eg, cephalexin or cefuroxime), or clindamycin.

Nonbullous impetigo results from infection with Streptococcus pyogenes, Staphylococcus aureus or both. If nonbullous impetigo is not extensive or involving the mouth area, topical antibiotics can effectively be used. Extensive infection can be treated with oral antibiotics.

While several topical antibiotic preparations can be used, such as bacitracin, triple antibiotic ointment (polymixin B, neomycin, bacitracin), or gentamicin, mupirocin (Bactroban, GlaxoSmithKline) is often recommended. Mupirocin is a unique antibiotic produced from Pseudomonas florescens and is active toward Streptococcus and Staphylococcus, including methicillin-resistant S. aureus (MRSA). Double-blind clinical studies have proven mupirocin to be equally effective as orally administered erythromycin, and superior to simple cleaning of lesions. Mupirocin has not been evaluated by controlled trials when compared with other topical antibiotics or other antistaphyloccal oral antibiotics. Controlled clinical trials have compared mupirocin cream to cephalexin and found equal efficacy. Mupirocin has not been compared with other topical antibacterials for secondarily infected traumatic skin lesions. Bactroban is relatively expensive when compared with the other topical antibacterials discussed here.

Mupirocin is also available in a unique formulation indicated for the eradication of nasal colonization of MRSA in adults to reduce the risk of infection among susceptible individuals (during institutional outbreaks). It has also been recommended in the literature to use mupirocin nasally to eliminate colonization to prevent recurrent impetigo, which may be due to nasal colonization. Clinical trials evaluating this use, however, have not been performed. Bactroban Nasal, available in 1-g single-use tubes, should be applied by administering one half of the tube amount to each nostril twice daily for five days. After application the patient should be instructed to repeatedly press the nostrils together for one minute, as this spreads the ointment within the nostrils.

Appropriate cleansing of minor wounds with antibacterial soaps and application of OTC topical antibacterial products may also be beneficial to prevent recurrent impetigo.

Additional uses of topical antibiotics

Several antibacterial agents are available in OTC or prescription products. OTC products may contain bacitracin, neomycin, polymixin B, or a combination of all three (triple antibiotic products), and can be useful for the treatment of minor abrasions and may possibly prevent the development of recurrent impetigo.

Bacitracin, active toward gram-positive bacteria, is available in ointment formulations and is relatively inexpensive. Neomycin, an aminoglycoside, is active toward many gram-negative pathogens.

A significant disadvantage to the use of neomycin relates to its relatively high propensity to cause allergic contact sensitivity reactions. Risk of such reaction increases with prolonged use. Polymixin B provides activity toward gram-negative pathogens and is available in combination with bacitracin and neomycin. Gentian violet solution is also available OTC for the treatment of minor abrasions. Its use may cause staining of the skin or clothing. Gentamicin is an aminoglycoside available by prescription in ointment and cream formulations. It is infrequently mentioned in the literature as recommended therapy for common skin infections.

Conclusion

In conclusion, topical antibiotic products can effectively treat infected traumatic skin lesions and nonbullous impetigo, allowing patients the choice of an optional therapy where systemic drug adverse effects (such as diarrhea) are eliminated. Mupirocin is equally efficacious as oral erythromycin in the treatment of localized nonbullous impetigo. OTC topical antibiotic products are inexpensive options for treating secondarily infected traumatic skin lesions. Mupirocin, a relatively more expensive option, may offer no benefit for this use.

For more information:

• Hirschmann JV. Topical antibiotics in dermatology. Archives of Dermatology. 1988;124:1691-1700.