Skin Disorders Guide

Bacterial cultures are not often necessary for the diagnosis and treatment of the common skin infections. In fact, they are sometimes more confusing than helpful. The problem arises because cultures taken from moist, eczematous skin lesions very often recover “pathogenic” bacteria that are merely colonizing diseased skin. Antibacterial therapy based on such cultures has little or no effect on the ultimate outcome of the disease. Cultures should be obtained, however, whenever hospital-acquired organisms are suspected, whenever patients are immunosuppressed, and whenever initial therapy leads to treatment failure or rapid recurrence.A particular note of caution should be made regarding the performance of blood cultures in patients with exfoliative erythrodermatitis. Needle punctures through diseased skin, no matter how carefully the skin is prepared, are quite likely to be contaminated. Thus, positive cultures in such patients should be interpreted with extreme care.

Cultures are obtained with swabs provided in the packets that accompany commercially prepared transport media. Crusts should be lifted so that purulent material beneath the crust is sampled. Pustules can be opened sterilely, and the drop of fluid that exudes can be touched with the swab. Care should be taken to avoid contamination with the surrounding skin. Needle aspiration can be carried out when larger vesicular or bullous lesions are present.

Plaques of suspected cellulitis cannot be easily cultured. Some clinicians recommend the injection and subsequent aspiration of sterile saline, but because of low yields I have never found this approach helpful.