A wound culture is a laboratory test in which microorganisms from a wound are grown in a special growth medium. It is done to find and identify the microorganism causing an infection in a wound or an abscess. If a microorganism is found, more testing is done to determine how to treat the infection.
Wounds are injuries to body tissues caused by disease processes or events such as burns, punctures, and human or animal bites. Wounds or abscesses also occur within body tissues as a result of surgery or dental procedures. Wounds become infected when microorganisms from the outside environment, or from within the person's body, enter the open wound and multiply. A wound that is red, painful, swollen, and draining pus is probably infected. A fever following surgery indicates an infection at the site of surgery.
To enable healing and prevent the spread of infection to other body tissues, the infecting microorganisms must be killed. A wound culture discovers which type of microorganism is causing the infection and the best antibiotic with which to kill it. This is important as physicians have become less inclined to prescribe antibiotics until certain they are needed because of antibiotic resistance that has developed due to overuse of the drugs.
A sample of material, such as pus or a portion of tissue, is taken from the wound, placed in a sterile container, and sent to the laboratory. In the laboratory, this material is spread over the surface of several different types of culture plates and placed in an incubator at body temperature for one to two days.
A Gram stain is done by staining the slide with purple and red stains, then examining it under a microscope. If many white blood cells and bacteria are seen, it is an early confirmation of infection. The color of stain retained by the bacteria (purple or red), their shape (such as round or rectangular), and their size provides valuable clues as to their identity, and helps the physician predict which antibiotics might work best even before the entire test is completed. Bacteria that stain purple are called gram-positive; those that stain red are called gram-negative.
Bacteria can be grouped into two categories: aerobes and anaerobes. Aerobes are bacteria that need oxygen to live; anaerobes live only where there is no oxygen. Deep wounds, closed-off from oxygen, are an ideal environment for an anaerobic infection to develop. Foul-smelling odor, gas, or gangrene at the infection site are signs of an infection caused by an anaerobic bacteria. Routine cultures typically only look for aerobic bacteria. If the physician tells the laboratory to include a culture for anaerobes, a portion of the wound sample will be put on culture plates, or in a tube of culture broth, and incubated in a special chamber without oxygen.
Bacteria present in the wound sample will multiply and appear as visible colonies on the plates, or as cloudiness in the tube of broth. They are identified by the appearance of their colonies, the results of biochemical tests, and information from Gram staining part of the bacterial colony.
A sensitivity test, also called an antibiotic susceptibility test, is also done. The bacteria are tested against different antibiotics to determine which will treat the infection by killing the bacteria.
If the physician thinks the wound may be infected with a mold or yeast, a fungal culture is also done. The wound sample is spread on special culture plates that are treated to encourage the growth of mold and yeast. Different biochemical tests and stains are used to identify molds and yeast.
Other more unusual microorganisms, such as Mycobacterium leprae, may be the cause of a wound infection. The physician must notify the laboratory to culture specifically for these more unusual microorganisms.
The initial Gram stain result is available the same day, or in less than an hour if requested by the physician. An early report, known as a preliminary report, is usually available after one day. This report will tell if any microorganisms have yet been found, and, if so, their Gram stain appearance. For example, they may have the appearance of a gram-negative rod, or a gram-positive cocci (spherical shape). The final report, usually available in one to three days, includes complete identification, an estimate of the quantity of the microorganisms, plus a list of the antibiotics to which they are sensitive. Cultures for fungi and anaerobic bacteria may take two to three weeks.
Wound culture is also called soft tissue culture, abscess culture, or wound culture and sensitivity.
A piece of the infected tissue is the best specimen. If this is not possible, the next best specimen is pus from the wound. Because many microorganisms normally live on skin and mucous membrane, the specimen must not be allowed to touch the area surrounding the wound.
The physician first cleans the surface of the wound using alcohol. Using a syringe, the physician suctions out (aspirates) as much pus as possible from the wound. Next, this is sent to the laboratory in a sterile container. If it is impossible to aspirate the pus, pus from within the wound can be collected on a swab.
The physician may choose to start the person on an antibiotic before the culture and sensitivity tests are completed. However, the specimen for culture should be collected before antibiotics are begun. Antibiotics in the person's system may prevent microorganisms present in the wound from growing in culture, and thus not be identifiable.
A normal culture may be contaminated by a mixture of microorganisms normally found on a person's skin (normal flora).
It is not uncommon for the microorganism causing a wound infection to not grow in culture. This is particularly true if the specimen was collected with a swab rather than an aspirate or tissue biopsy.
Streptococcus Group A, Escherichia coli, Proteus, Klebsiella, Pseudomonas, Enterobacter, Enterococci, Staphylococcus aureus, Bacterioides, and Clostridium, are common causes of wound infections. More than one microorganism may be the cause of the infection.
"Does Increased Use of Antibiotics Result in Increased Antibiotics Resistance?" Clinical Infectious Diseases July 1, 2004: 18-20.
Wound Healing Society. 1550 South Coast Highway, Suite 201, Laguna Beach, CA 92651. (888) 434-4234. 〈http://wizard.pharm.wayne.edu/woundsoc/WHS.HTM〉.
Aerobe — Bacteria that require oxygen to live.
Anaerobe — Bacteria that live only where there is no oxygen.
Normal flora — The mixture of bacteria normally found at specific body sites.