Bacterial Wound Culture
At a Glance
Why Get Tested?
To detect a bacterial wound infection, to determine which specific bacteria are present, and to isolate and grow the bacteria for subsequent susceptibility testing
When to Get Tested?
When the doctor suspects that your wound is infected by a pathogenic microorganism; sometimes after treatment of an infection to evaluate its effectiveness
Sample Required?
A sterile swab used to collect cells or pus from the site of the suspected infection; may also include aspirations of fluid from deeper wounds into a syringe and/or a tissue biopsy
Test Preparation Needed?
No test preparation is needed.
The Test Sample
What is being tested?
A bacterial wound culture is a test that is used to detect and identify pathogenic bacteria in a potentially infected 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 which type or types of bacteria are causing an 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 if a person has a wound infection, there will either be a pure culture of the microorganism - only one kind will be found, or one type will predominate. In some cases, for instance with a human or animal bite, there may be several pathogens present. Since wounds may be superficial or involve deep tissue, they may harbour different types of bacteria that have different requirements for growth. Some bacteria infecting a wound may require air for growth (aerobic) while some require a no-oxygen or reduced-oxygen environment (anaerobic or microaerophilic). Care must be taken when handling the samples so that their growth is encouraged and the probability of their detection and identification are optimized.
The next step in the process is to identify the different types of 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 the wound culture, 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.
The Test
How is it used?
A bacterial wound culture is primarily requested to help identify the bacteria causing the infection, and to prepare a sample for susceptibility testing where required.
If a wound culture reveals for the presence of pathogenic bacteria, a susceptibility test is often performed automatically and without an additional test having to be requested by the doctor. This can save time so that antimicrobial therapy can begin as soon as possible.
Gram stains are routinely performed on the original sample that is collected for the wound culture. This is done to give the doctor an initial evaluation of the wound - to look for any bacteria that might be present in the original sample. A Gram stain that does not show the presence of bacteria does not rule out a wound infection.
A wound culture may also sometimes be requested at intervals on a person who has a chronic infection, to help guide further treatment.
If a fungal infection is suspected, then a fungal culture of the wound specimen may be requested along with the bacterial wound culture. Yeast and some fungi may grow on the same media as bacteria, but many fungi are slow-growing. The media used to recover fungi inhibits bacterial growth and supports fungal growth for several weeks.
When is it requested?
This test is primarily requested when a doctor suspects that a wound is infected. It may also occasionally be requested when a person has been treated for a wound infection to evaluate the treatment’s effectiveness and may be requested when indicated to help guide the treatment of a chronic wound infection. Some signs and symptoms of an infected wound may include:
- a wound that is slow to heal;
- heat, redness and swelling at the site;
- tenderness at the site;
- drainage of fluid or pus;
- fever
What does the test result mean?
If pathogenic bacteria are identified in the culture, then it is likely that they are the source of the infection. Often wounds have two or more pathogens (aerobes and/or anaerobes) that may be contributing to the infection. If more than three organisms are present, they may not be identified as individual bacterial species, and the report may refer to them as “mixed bacterial flora.” This may indicate a mixture of normal flora and pathogens from a contaminated sample or from a dirty wound. If there is only normal flora present, then the infection may be due to bacteria normally found on the skin, or the pathogen may have been missed in the sample due to low numbers present, or the infection may be due to another cause.
Very little growth may still be significant, especially when the wound infection is in an area of the body considered to be essentially sterile – such as the eye.
If there are no bacteria recovered in the wound specimen, then there may not be a bacterial infection, or the pathogen was not successfully recovered with the sample and test.
Is there anything else I should know?
Wound culture results may be less typical when the patient has already been treated with antimicrobial drugs and when a patient has a chronic infection.
Typically if an infection has spread from a wound into the blood and/or organs such as the kidneys, then the same microorganism(s) will be detected in blood and/or urine cultures.
Common Questions
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Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
