logo Anti-infective agents

Skin infections

Anti-infective agents


  • Be able to identify antimicrobial agents from a list of common ingredients of skin preparations.
  • Name three indications for topical antibiotics.

Key points

  • Antiseptics do not result in bacterial resistance.
  • Antiseptics may cause contact irritant or allergic dermatitis.
  • Topical antibiotics are more likely than oral antibiotics to cause sensitisation.
  • Topical antibiotics are more likely than oral antibiotics to cause bacterial resistance.
  • Topical antifungal agents should be continued for at least one to two weeks after clinical signs of infection have cleared.
  • Oral antifungal agents are required for hair shaft infections and for extensive, severe or resistant infections elsewhere.
  • A minimum of a three-month course is necessary to cure onychomycosis.
  • Early use of topical antiviral agents may be effective for recurrent herpes simplex.


Antiseptics are defined as agents that eliminate or reduce the growth of microorganisms and are mainly used to prevent infection. They are also useful as first line treatment for minor skin infections. Antiseptics are frequently incorporated in topical preparations as active ingredients or as preservatives to prevent product deterioration. They do not result in bacterial resistance. However depending on concentration, they may be irritants (resulting in contact irritant dermatitis) or sensitisers (resulting in contact allergic dermatitis in susceptible individuals).

Images of contact dermatitis to antiseptic

Positive formaldehyde patch test

Examples of antiseptics used in cleansers, creams, gels and other skin preparations include:

Topical antibiotics

Antibiotics are able to kill or inactivate bacteria in the body. Topical antibiotics have been widely used to treat wound infections, boils and infected eczema. They are applied to the infected area three times daily for 7-day courses. For a number of years, clindamycin and erythromycin were available over-the-counter to treat acne. They are now prescription medications because induced bacterial resistance affecting oral as well as topical formulations. Topical antibiotics, particularly neomycin, are more likely than oral antibiotics to result in sensitisation.

The role of topical antibiotics is currently undefined but may include:

Localised wound infection suitable for topical antibiotic

Mild inflammatory acne suitable for topical antibiotic and benzoyl peroxide

Topical antibiotics available in New Zealand include:

Oral antibiotics

Antibiotics are used in dermatology to cure acute infections and to suppress chronic inflammatory skin diseases.

Staphylococcus aureus and/or beta haemolytic streptococci cause the majority of acute skin infections. In general penicillins are preferred. Current antibiotic guidelines in use at Waikato Hospital for treating cellulitis empirically recommend at least a five-day course of one of the following:

When the organism has been identified as Streptococcus pyogenes, penicillin-V 500mg 6-hourly is preferred.

A number of chronic skin diseases are routinely treated with antibiotics for 3 to 6-month courses and sometimes much longer. These include:

Antibiotics are used empirically in these conditions; they have been shown to be effective in at least some cases. There are very few published randomised blinded trials to support their use, but benefit is generally self-evident in practice. Mostly the mechanism of action is unknown; probably an anti-inflammatory effect in most conditions but antimicrobial action may be important in others.

Topical antifungals

Topical antifungal medications can often cure fungal infections. Numerous suitable creams can be obtained over the counter without a doctor's prescription. They are applied to the affected area twice daily for two to four weeks, including a margin of several centimetres of normal skin. Continue for one or two weeks after the last visible rash has cleared. Repeated treatment is often necessary.

Most commonly used products include:

Skin diseases caused by malassezia are best treated with topical ketoconazole or ciclopirox. Nystatin continues to be used for candidiasis but imidazoles are more effective.

Mild distal onychomycosis can be treated with antifungal lacquers containing amorolfine or ciclopirox applied once or twice weekly. The medication should be applied to the surface of the cleaned nail plate after it has been roughened using an emery board. Extra lacquer should be applied under the edge of the nail.

Oral antifungals

Oral antifungal medications may be required for a fungal infection if:

Oral antifungals suitable for dermatophytes include:

Oral antifungals suitable for yeasts include ketoconazole, itraconazole and fluconazole. The duration of the course ranges from a single dose to several months, depending on the specific condition and its severity. They may be used long term intermittently for recurrent yeast infections e.g. itraconazole 200mg bd for one day each month.


Topical antiviral agents for recurrent herpes simplex infections:

They are only effective if applied immediately symptoms begin so may not be useful for those patients who do not recognise prodromal tingling. These agents have not been shown to be helpful for other viral infections.

Oral antiviral drugs include acyclovir, valacyclovir and famciclovir. Only acyclovir is currently available in New Zealand. These drugs will stop the herpes simplex virus multiplying once it reaches the skin or mucous membranes but cannot eradicate the virus from its resting stage within the nerve cells. Acyclovir 200mg five times daily for 5 days can shorten attacks if started at the earliest sign of recurrence, and may be useful long term prophylactically at a dose of 400mg bd.

Immune response modulators

Imiquimod is a topically active immune response modulator. It binds to specific receptors on innate and memory immune cells to cause the release of antiviral cytokines including interleukin-2, interferon and tumour necrosis factor. It has been shown to be effective in the treatment of viral warts on mucosal surfaces and molluscum contagiosum. It is less effective in common warts because hyperkeratosis reduces drug penetration into the skin.