The Test Sample
What is being tested?
A bacterial wound culture is a test that is used to detect and identify bacteria that cause infections (pathogenic) in a 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. 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.
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, there is only one kind of bacteria found or one type predominates in a wound. In some cases, however, there may be severaltypes of bacteria present that have different requirements for growth. Some bacteria infecting a wound may require the presence of air (aerobic) while others require a no-oxygen or reduced-oxygen environment (anaerobic or microaerophilic). Care must be taken when handling the samples so that bacterial growth is encouraged and the probability of their detection and identification are optimized.
The next step in the testing process is to identify the different microorganisms present. Identification is a step-by-step process that may involve many tests and evaluations performed on the bacteria found growing in the culture. One such test, the gram stain, involves smearing individual colony types onto glass slides and treating them with a special stain. Under the microscope, the bacteria can be classified into gram-positive and gram-negative organisms and by shape into cocci (spheres) or rods. With this information and additional biochemical tests, the types of bacteria present can be identified.
For many of the pathogens identified in wound cultures, antimicrobial susceptibility testing is required to guide treatment and to determine whether the strain of bacteria present is likely to respond to specific antibiotics. In order to do this, a pure culture (isolate) of the identified bacteria must be available, which may require additional time in the laboratory to separate and identify each bacterial species.
The wound culture, gram stain test, and susceptibility testing all contribute to inform the doctor 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.