Skin Culture Health Article

Definition

A skin culture is a laboratory test used to isolate and identify the microorganism (bacterium, fungus, or virus) causing a skin infection, so the most effective antibiotic or other treatment for the infection can be determined.

Purpose

Skin infections are contagious and, if left untreated, can lead to serious complications. A skin culture helps the physician to diagnose and treat a skin infection.

Precautions

To avoid spreading pathogenic organisms to patients or other individuals, health care professionals should be cautious in the collection and handling of skin culture specimens.

Description

Skin infections may involve the superficial layer (epidermis) only or may involve the deeper dermis, including the sweat glands, oil glands, lymphatics, and hair follicles within. Microorganisms can infect healthy skin, but more often they infect skin already damaged by an injury or an abrasion. The lesion produced by the infection is an early indication of which type of microorganism is causing the infection. For example, pustules are associated with impetigo (pyoderma) the most common bacterial skin infection. Pyoderma is most often caused by group A Streptococcus. Vesicular skin rashes are commonly caused by herpesviruses as in chickenpox. Scaly rashes are most commonly caused by dermatophytes, fungi that infect the keratinized skin (epidermis). Bacterial skin infections are the most common, and can result in ulcers, cellulitis, rashes, boils, abscesses, and other types of lesions.

The following types of microorganisms cause most skin infections and can be isolated by performing a skin culture:

• Bacteria: Aerobic gram positive cocci, Streptococcus pyrogenes, and Staphylococcus aureus are the most common isolates and are responsible for pyoderma. However many other bacteria cause skin infections less frequently. Usually, these are introduced through a wound in the skin caused by a bite, decubitus ulcer, burn, trauma, or puncture. Some notable genera are anaerobic bacteria such as Bacteroides and Clostridium from soils, gram negative rods such as Aeromonas, Plesiomones, and Vibrio from water. Organisms that live in the mouth of dogs and cats such as Pasteurella multocida can infect bite wounds.
• Fungi (molds and yeast): Three genera of fungi commonly cause ringworm of the skin, hair, and nails and are the most common fungi isolated from skin. These are Trichophyton, Epidermophyton, and Microsporum. Candida can colonize the epidermis as part of the normal flora but will infect burned skin and skin folds of newborns. Several other fungi may cause subcutaneous infection.
• Viruses: Rubella (German measles), rubeolla, roseola, and herpes varicella zoster (chickenpox) are common causes of viral rashes in children. Herpes simplex 1 and cytomegalovirus may cause more complex infections in immunosuppressed adults. In addition, skin infections can be caused by enteroviruses, poxviruses and several others.
• Skin infections can also be caused by mycobacteria such as Mycobacterium tuberculosis and M. leprae the cause of leprosy, and skin lesions can be caused by some parasites when the larva enter the skin.

Based on the appearance of the lesion, the physician orders one or more types of skin cultures. Using aseptic technique, the physician, nurse, or other health care professional collects a specimen. For open epidermal infections a sample of the lesion such as skin cells, pus, or fluid can be collected using a swab. For crusted or closed lesions, the surface of the vesicle or pustule should be removed with a scalpel blade in order to expose the infected skin before swabbing. Ringworm should be scraped using a scalpel blade to collect the keratinized skin. Deeper infections should be sampled by aspiration. Swabs for bacterial culture are placed in a sterile container (often containing transport medium such as Stuart or Cary-Blair) before being sent to the laboratory for culture. If anaerobic culture is requested the specimen is immediately placed in prereduced oxygen-free transport medium.