Bacterial Wound Culture
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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
If a culture reveals that a wound is infected, susceptibility testing is done to determine which antibiotic will inhibit the growth of the bacteria causing the infection. This is often performed automatically as part of the original order on the wound specimen so that antimicrobial susceptibility results can guide appropriate therapy as soon as possible.
A wound culture may also sometimes be ordered for an individual who has undergone treatment for a wound infection to determine whether the treatment was effective. It may also be ordered at intervals for 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 ordered 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 ordered?
- A wound that is slow to heal
- Heat, redness and swelling at the site
- Tenderness at the site
- Drainage of fluid or pus
It may also occasionally be ordered after a person has been treated for a wound infection to evaluate the treatment's effectiveness and may be ordered when indicated to help guide the treatment of a chronic wound infection.
What does the test result mean?
Laboratory reports for cultures typically name the bacteria identified during the test. A negative culture will usually be reported, for example, as "no growth seen in 5 days" or note the presence of normal skin flora only.
If pathogenic bacteria are identified in the culture, then it is likely that they are the source of the infection. An infection is typically caused by a single type of bacteria, but wounds may have two or more pathogens (aerobes and/or anaerobes) that are 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 found on the skin and potential pathogens cultured from a wound site that was not cleansed properly before the specimen was taken or from a "dirty wound" such as a motorcycle accident, where material from the road is contaminating the 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?
With burn wounds, the type and number of bacteria that grow are correlated to the severity of the infection. When burn tissue specimens have a specific bacterial count above a certain number, then removal of dead tissue (debridement) may be indicated.
Wound culture results may be less predictive of what is causing the infection when a person has already been treated with antibiotics or when a person has a chronic infection.
Typically, if an infection has spread from a wound into the blood and other organs, then the same bacteria or yeast may be detected in the person's blood cultures.
Why would my healthcare practitioner collect more than one sample?
Once I have been treated, can my infection return?Most infections will resolve, but if there is still damaged tissue present or a break in the skin’s protection, then there is the potential for another infection to occur. In some cases, it may take extended treatment and/or a change in medication to resolve an infection.
Why might one person get an infection and another person not?
I had a boil on my arm and the healthcare practitioner did not do a culture of the material when he drained it. Why not?
In a healthy person, the clinical presentation may give the healthcare practitioner sufficient information to treat the infection. With an abscess or boil, the most important treatment is incision and drainage, and antibiotics may not be required. However, if your wound does not heal or it worsens after drainage, you may have an infection with an organism that requires antibiotic therapy tailored to the specific pathogen. In this case, a wound culture should be done to identify the pathogen and perform susceptibility testing to guide treatment.
On This Web
Tests: Gram Stain, Susceptibility Testing, Blood Culture, Urine Culture, AFB Testing, Fungal Tests, MRSA Screening
Conditions: Wound and Skin Infections, Staph Wound Infections and Methicillin Resistant Staphylococcus aureus
In the News: 'Flesh-Eating' Bacterial Infections Are Rare but Can Be Life-threatening (2018)
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