Bacterial Wound Culture
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 harbor 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.
Is any test preparation needed to ensure the quality of the sample?No test preparation is needed.
How is it used?A bacterial wound culture is primarily ordered to help determine whether a wound is infected, to identify the bacteria causing the infection, and to prepare a sample for susceptibility testing where required.
If a wound culture reveals the need for a susceptibility test, it is often performed automatically and without an additional test having to be ordered 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 ordered on a patient who has undergone treatment for a wound infection, to determine whether the treatment was effective. It may also be ordered 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 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?This test is primarily ordered when a doctor suspects that a wound is infected. It may also occasionally be ordered when 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. 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;
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?With burn wounds, quantitative culture results may be requested – the number of bacteria that grow are correlated to the number of bacteria in the infected wound. 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 typical when the patient has already been treated with antimicrobial drugs and when a patient has a chronic infection.
Why would my doctor 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 doctor did not do a culture of the material when he drained it. Why not?In a healthy patient, the clinical presentation may give the doctor sufficient information to treat the patient. 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 culture of the wound should be done to identify the pathogen and perform susceptibility testing to predict the best possible antibiotic for clinical response.