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The Test Sample
What is being tested?
A bacterial wound culture is a test that detects and identifies bacteria that cause infections (pathogenic) in a wound. Any wound may become infected with a variety of bacteria. A culture helps to determine whether a wound has become infected, which type(s) of bacteria are causing the infection, and which antibiotic would best treat the infection and help heal the wound.
Wounds may be superficial breaks in the skin such as scrapes, cuts and scratches or may involve deeper tissues such as incisions, bites, punctures or burns. (Read the article on Wound and Skin Infections.) A culture is performed by collecting a sample of fluid, cells or tissue from the wound and placing it on or in appropriate nutrient media. The media encourages the growth of bacteria that may be present, allowing for further testing and identification.
Typically, only one kind of pathogenic bacteria is causing the infection in a wound. However, there may be several types of normal skin bacteria present in the culture. Separating the various types of bacteria and identifying the pathogenic bacteria requires one or more days to perform.
A Gram stain is usually performed to help determine the type of bacteria present and provide a rapid result to the healthcare practitioner. The shape and color (morphology and staining characteristics) also help determine what other tests may need to be performed to definitively identify the cause of infection.
Because the results of the stain read under the microscope are not definitive, further tests such as biochemical reactions or mass spectrometry must be performed to identify the bacteria. Mass spectrometry using matrix assisted laser desorption ionization time of flight (MALDI-TOF) can provide an identification to the genus and species level in less than an hour after the bacterial colony is grown on the culture media. This technique significantly decreases the time needed to identify bacteria from traditional biochemical reactions that require overnight incubation.
For many of the pathogens identified in wound cultures, testing is done to determine which antibiotics will be effective in inhibiting the growth of the bacteria (see Susceptibility Testing). The Gram stain of the wound, the culture, and susceptibility testing all contribute to inform the healthcare practitioner which pathogen(s) are present and what antibiotic therapy is likely to inhibit their growth.
How is the sample collected for testing?
A sterile swab may be used to collect cells or pus from a superficial wound site. From deeper wounds, aspirations of fluid into a syringe and/or a tissue biopsy are the optimal specimens to allow for the recovery of aerobic and anaerobic bacteria.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.