A wound culture is a diagnostic laboratory test in which microorganisms—such as bacteria or fungi from an infected wound, are grown in the laboratory on nutrient-enriched substance called media—then identified. Wound cultures always include aerobic (with oxygen) culture, but direct smear evaluation by Gram stain and anaerobic (without oxygen) culture are not performed on every wound. These tests are performed when indicated or requested by the physician.
The purpose of a wound culture is to isolate and identify bacteria or fungi causing an infection of the wound. Only then can antibiotics that will be effective in destroying the organism can be identified.
A biopsy sample is usually preferred by clinicians, but this is a moderately invasive procedure and may not always be feasible. The health-care professional prepares the patient by cleansing the affected area with a sterile solution, such as saline. Antiseptics such as ethyl alcohol are not recommended, because they kill bacteria and cause the culture results to be negative. The patient is given a local anesthetic and the tissue is removed by the practitioner, who uses a cutting sheath. Afterwards, pressure is applied to the wound to control bleeding.
Needle aspiration is less invasive and is a good technique to use in wounds where there is little loss of skin, such as in the case of puncture wounds. The skin around the wound is cleaned with an antiseptic to kill bacteria on the skin's surface, and a small needle is inserted. To obtain a sample of the fluid to be biopsied, the clinician pulls back on the plunger, then changes the angle of the needle two or three times to remove fluid from different areas of the wound. This procedure may be painful for the patient, so many initial cultures are done with the swab technique. After completion of any of the three procedures, the wound should be cleaned thoroughly and bandaged.
Wounds are injuries to body tissues caused by physical trauma or disease processes that may include surgery, diabetes, burns, punctures, gunshots, lacerations, bites, bed sores, and broken bones. Types of wounds may include:
- Abraded or abrasion: Caused by scraping, such as falling on concrete.
- Contused or contusion: A bruise or bleeding into the tissue.
- Incised or incision: A wound formed by a clean cut, as by a sharp instrument like a knife.
- Lacerated or laceration: A wound caused by heavy pressure, causing tearing of the skin or other tissues.
- Nonpenetrating: An injury caused without disruption of the surface of the body. These wounds are usually in the thorax or abdomen and can also be termed blunt trauma wounds.
- Open: A wound in which tissues are exposed to the air.
- Penetrating: Disruption of the body surface and extension into the underlying tissue.
- Perforating: A wound with an exit and an entry, such as a gunshot wound.
- Puncture: A wound formed when something goes through the skin and into the body tissues. This wound has a very small opening, but can be very deep.
The chance of a wound becoming infected depends on the nature, size, and depth of the wound; its proximity to and involvement of nonsterile areas, such as the skin and gastrointestinal (GI) tract; the opportunity for organisms from the environment to enter the wound; and the immunologic, nutritional, and general health status of the person. In general, acute (sudden onset) wounds are more prone to infection than chronic (long-lasting) wounds. Wounds with a large loss of body surface, such as abrasions, are also easily infected. Puncture wounds can permit the growth of microorganisms because there is a break in the skin with minimal bleeding; they are also difficult to clean. Deep wounds, closed off from oxygen, are an ideal breeding environment for anaerobic infections. Foul-smelling odors, gas, or dead tissue at the infection site are signs of an infection caused by anaerobic bacteria. Surgical wounds can also cause infection by introducing bacteria from one body compartment into another.
Diagnosing infection in a wound may be difficult. One of the chief signs the clinician looks for is slow healing. Within hours of injury, most wounds display a release of fluid, called exudate. This fluid contains compounds that aid in healing, and is normal. It should not be present 48–72 hours after injury. Exudate indicative of infection may be thicker than the initial exudate and may also be purulent (containing pus) and foul smelling. Clinicians will look at color, consistency, and the amount of exudate to monitor early infection. In addition, infected wounds may display skin discoloration, swelling, warmth to touch and an increase in pain.
Wound infection prevents healing, and the bacteria or yeast can spread from wounds to other body parts, including the blood. Infection in the blood is termed septicemia and can be fatal. Symptoms of a systemic infection include a fever and rise in white blood cells (WBCs), along with confusion and mental status changes in the elderly. It is important to treat the infected wound early with a regimen of antibiotics to prevent further complications.
Wound infections often contain multiple organisms, including both aerobic and anaerobic gram-positive cocci and gram-negative bacilli and yeast. The most common pathogens isolated from wounds are Streptococcus group A, Staphylococcus aureus , Escherichia coli , Proteus , Klebsiella , Pseudomonas , Enterobacter , Enterococci, Bacteroides , Clostridium , Candida , Peptostreptococcus , Fusobacterium , and Aeromonas .
The tissue used for the tests is obtained by three different methods: tissue biopsy, needle aspiration, or the swab technique. The biopsy method involves the removal of tissue from the wound using a cutting sheath. The swab technique is most commonly used, but contains the least amount of specimen.
Wound specimens are cultured on both nonselective enriched and selective media. Cultures are examined each day for growth and any colonies are Gram stained and subcultured (i.e., transferred) to appropriate media. The subcultured isolates are tested via appropriate biochemical identification panels to identify the species present. Organisms are also tested for antibiotic susceptibility. The selection of antibiotics for testing depends on the organism isolated.
The initial Gram-stain result is available the same day, or in less than an hour, if requested by the doctor. An early report, known as a preliminary report, is usually available after one day. After that, preliminary reports will be posted whenever an organism is identified. Cultures showing no growth are signed out after two to three days unless a slow-growing mycobacterium or fungus is found. These organisms take several weeks to grow and are held for four to six weeks. The final report includes complete identification, an estimate of the quantity of the microorganisms, and a list of the antibiotics to which each organism is sensitive and resistant.
The physician may choose to start the person on an antibiotic before the specimen is collected for culture. This may alter results, since antibiotics in the person's system may prevent microorganisms present in the wound from growing in culture. In some cases, the patient may begin antibiotic treatment after the specimen is collected. The antibiotic chosen may or may not be appropriate for one or more organisms recovered by culture.
Clinicians must be very careful when finishing a wound culture collection to make ensure that the wound has been cleaned thoroughly and is bandaged properly. It is important to watch for bleeding and further infection from the procedure. In addition, patients may be in pain from the manipulation, so giving pain-killing drugs, such as acetaminophen , may be advised.
Henry, John B. Clinical Diagnosis and Management by Laboratory Methods, 20th ed. Philadelphia: W. B. Saunders Company, 2001
The Wound Healing Society. 13355 Tenth Ave., Suite 108, Minneapolis, MN 55441-5554. [cited April 4, 2003] http://www.woundheal.org/ .
National Institutes of Health. [cited April 5, 2003] http://www.nlm.nih.gov/medlineplus/encyclopedia.html .
Jane E. Phillips, Ph.D. Mark A. Best, M.D.