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.
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.
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.
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), rubeola (measles), 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.
Bacterial skin cultures
A Gram stain is prepared by rolling the smear across the center of a glass slide or dropping a liquid specimen onto the center and allowing it to air dry. Gram-positive cells retain the crystal violet stain and appear dark purple, while gram-negative cells do not. Gram-negative bacteria are counterstained by the safranin and appear pink. In addition to classifying the bacteria seen, the Gram stain can identifies yeast, hyphal elements, and organisms that require special culture media. For example, the presence of large gram-positive spore forming rods indicates the possible presence of Clostridium spp. and the need for anaerobic culture.
A routine bacterial skin culture involves inoculating (spreading a portion of the specimen on) several culture plates containing general-use enrichment media and selective media. Commonly used media include sheep blood agar plates, chocolate (heated blood) agar plates, MacConkey agar for isolation of gram-negative rods, and either phenylethyl alcohol (PEA) or colistin-naladixic acid (CNA) blood agar for isolation of gram positive cocci. Plates are incubated in air or 5-10% carbon dioxide and examined for growth daily for at least two days. Bacteria present in the specimen multiply and appear on the plates as visible colonies. These are Gram stained and subcultured (transferred) to other media in order to identify the organism. Complete identification usually requires one to two days following isolation of a pure culture. It is standard practice to perform an antibiotic sensitivity test on any bacterial pathogen isolated from a routine skin culture, except group A Streptococcus, which is susceptible to penicillin and related antibiotics. An antibiotic sensitivity test, also called an antibiotic susceptibility test, grows the bacteria in the presence of different antibiotics to determine which ones will effectively treat the infection by killing the bacteria.
Fungal skin cultures
Physicians request fungal skin cultures less frequently. A group of fungi called dermatophytes cause skin infections such as ringworm and athlete's foot. Yeast infections caused by Candida can thrive on moist skin, such as in diaper areas and in the folds of skin in the groin. Yeast infections can cause significant problems for newborns and patients with AIDS or depressed immune systems. Yeast infections are cultured on sheep blood agar and grow in one to two days. Dermatopohytes are usually identified by a KOH test. In this test, a sample of skin scraped with a scalpel blade and transferred to a slide. After adding KOH, the slide is allowed to stand for five minutes in order to dissolve skin cells, hair, and debris. Lactophenol cotton blue stain can be added to make the fungi easier to see, or if a fluorescent microscope is available, calcofluor white stain may be added to the KOH preparation. This will cause the fungi to become fluorescent making them easier to identify. Dermatophytes are easily recognized under the microscope by their long branch-like tubular structures called hyphae. Fungi causing ringworm infections produce septate (segmented) hyphae. Some show the presence of spores formed directly from the hyphae (arthroconidia). Yeast infections of the skin can also be identified by the KOH test. Yeast cells appear round or oval, and budding forms may be seen.
A culture is requested only when specific identification of the fungus is necessary. For a routine fungal culture, the specimen is spread on a culture plate or tube containing nutrient media designed to grow fungi, incubated for up to four weeks, and observed for growth at regular intervals. Stains and biochemical tests are usually used to identify yeast and other fungi. Dermatophytes may be cultured on a medium called dermatopohyte test medium (DTM). This is an agar slant containing phytone, dextrose, phenol red, cycloheximide, gentamicin, and chlortetracycline. The antibiotics inhibit the growth of normal skin flora. Skin scrapings, hair, or nail samples are added to the medium and allowed to grow at room temperature. Cultures are held for 14 days. Dermatophytes will turn the medium from yellow to red as they grow. Other fungi, called systemic fungi can enter the skin through puncture wounds, abrasions or cuts and cause subcutaneous infection. A common cause of systemic mycosis in the United States among gardeners and farmers is Sporothrix schenckii. Such fungi are cultured from skin aspirates on growth medium for fungi containing antibiotics to inhibit bacterial growth. Most commonly used are Sabouraud dextrose agar with antibiotics and mycosel agar with cycloheximide and chloramphenicol. Cultures are incubated at both 77° and 96.8°F (25° and 36°C). Sporothrix schenckii grows in about four days but other fungi grow more slowly, and plates should be held for 30 days before reporting as negative.
Viral skin cultures
Viruses, such as herpes, can also cause skin infections. A specimen for viral culture is mixed with commercially prepared animal cells usually grown on a coverslip in a shell vial. Characteristic changes to the animal cells caused by the growing virus help to identify the virus. For rapid diagnosis, some laboratories use an electron microscope to identify viruses on the basis of morphology. For example, the herpes virus can be cultured from a vesicle (blister) which has been removed carefully with a scalpel blade. The base of the vesicle is swabbed with a sterile cotton applicator, and the virus taken to the laboratory in a tube of viral transport medium. Herpes can be cultured in several cell lines including human diploid fibroblasts (HDF), HEp2 cells (epithelial cancer cells from the larynx), primary monkey kidney cells (PMK), and rabbit kidney cells (RK). Cell cultures are inoculated and allowed to grow for one to three days at 96.8°F (36°C) in 5-10% carbon dioxide. Usually by the end of the first day of culture the cytopathic effect (CPE), formation of giant cells, can be seen by observing the cells under a microscope.
Before ordering a skin culture, the physician will ask the patient for a complete medical history and perform a physical examination to determine possible causes of the skin infection and whether a skin culture is appropriate. For acute skin infections, immediate treatment is sometimes necessary.
All healthcare professionals who participate in collecting a skin culture specimen should be trained in aseptic technique. Before collecting the specimen, they should scrub their hands thoroughly with an antimicrobial soap and, in some cases, put on sterile gowns, masks, and gloves. Sterile instruments and containers should be assembled near the patient. If appropriate, a sterile drape can be placed around the site of the infection. After cleaning the infected area with alcohol and sterile saline, the physician, nurse, or other healthcare professional uses a sterile blade, swab, needle, syringe, or other instruments to collect a sample of skin cells, pus, or fluid from the lesion. It might be necessary to open the lesion before collecting the specimen.
Collection of the specimen could cause some slight bleeding at the infection site, which might require some attention. Otherwise, no special aftercare is necessary for the patient following a skin culture.
The health care professionals who collect the specimen should ensure that any drapes, gowns, gloves, or instruments used for the collection are placed in the proper containers for disposal or sterilization. Then they should again scrub their hands thoroughly with an antimicrobial soap before leaving the area.
If aseptic technique is not used to collect the specimen, the patient or the healthcare professionals could develop postprocedure infections. The infection could also be transmitted to other individuals by contaminated hands or objects.
Results for bacterial cultures are usually available in one to three days. Cultures for fungi and viruses may take longer—up to three or more weeks.
Many microorganisms that are found on a person's skin are normally considered to be harmless. When these microorganisms grow on a skin culture, they are reported as "normal flora." One of the most common of these microorganisms is Staphylococcus epidermidis. Other bacteria that live in the high salt environment of the skin include Propionibacterium acnes, Corynebacterium xerosis, and some yeasts.
Besides normal flora, any microorganism that grows on a skin culture is considered to be the cause of the infection if it is the only microorganism or the predominant microorganism; if it grows in large numbers; or if it is known to produce infection. Staphylococcus aureus and group A Streptococcus cause most bacterial skin infections. Candida albicans causes most yeast skin infections, and Herpes simplex is the most frequent cause of viral skin infections.
Antimicrobial— A substance or action that kills or inhibits the growth of microorganisms.
Aseptic technique— Practices performed before, during, and after a clinical procedure to prevent or reduce contamination and postprocedural infection.
Pathogen— An organism that causes disease.
Pyoderma— A pus-containing bacterial skin infection.
Selective media— Media designed to enhance the growth of one type of microorganism by inhibiting the growth of other types with antibiotics or other substances.
Sensitivity test— A laboratory test that shows which antibiotics will treat an infection by killing the bacteria.
Health care team roles
The physician determines whether a skin culture is needed to diagnose a skin infection, and orders the test when appropriate. Then the physician, nurse, or other healthcare professional trained in aseptic technique collects the specimen and sends it to the laboratory. The clinical laboratory scientist, CLS(NCA)/medical technologist, MT(ASCP) assumes responsibility for correct handling, culture, identification, and reporting of the results.
Fischbach, Francis. A Manual of Laboratory and Diagnostic Tests. 6th ed. Philadelphia: Lippincott Williams & Wilkins, 1999.
Fitzpatrick, Thomas B., et al. Dermatology in General Medicine. 5th ed. New York: McGraw-Hill, Inc., 1998.
Tierney, Lawrence M., Stephen J. McPhee, and Maxine A. Papadakis. Current Medical Diagnosis and Treatment 2001 (Lange Series). New York: McGraw-Hill Professional Book Group, 2000.
Resnik, Kenneth S., and A. Bernard Ackerman. "On Standard Definitions of Individual Skin Lesions." Archives of Dermatology 134 (May 1998): 636.
American Society of Microbiology. 1752 N Street NW, Washington DC 20036. (202) 737-3600. 〈http://www.asmusa.org〉.
UTMB The University of Texas Medical Branch. 301 University Blvd., Galveston, TX 77555. (409) 772-1011. 〈http://www.utmb.edu〉.