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 when a wound has become infected with specific types of bacteria, and which antibiotic would best treat the infection to aid wound healing.

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. Often in a sample from a wound infection there will be either a pure culture of a pathogenic microorganism (only one kind will be found) or one type will predominate within a mixture of organisms. In some cases, such as with a human or animal bite, there may be several pathogens present.

Wounds may harbour different types of organisms that have different requirements for growth. These organisms may be somewhat predictable from the site of infection so specific growth requirements from different specimen types are catered for within the laboratory. Some bacteria infecting a wound may require air for growth (aerobic) while some require a no-oxygen or reduced-oxygen environment (anaerobic or microaerophilic). Moreover, some bacteria may need specialised nutrient factors to grow adequately (fastidious organisms). 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. Conventionally, identification is a step-by-step process that may involve many tests and evaluations performed on the sample before it is cultured or on the bacteria found growing in the culture. One such test is the Gram stain. It involves placing sample or smearing individual colony types onto a glass slide and treating the slide 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 bacilli (rods). With this information and additional biochemical tests, the types of bacteria present can be identified. Nowadays, mass spectrometry is used as a one step process to identify the different bacteria present in a wound sample once it is cultured. 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. 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 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. This may require additional time in the laboratory to separate and identify each clinically significant bacterial species.

The wound culture, Gram stain, and susceptibility testing all contribute to inform the doctor which pathogen(s) is present and what antibiotic therapy is likely to inhibit its growth.

How is it used?

A bacterial wound culture is primarily ordered to help determine when a wound is infected by an identifiable bacteria, and to prepare a sample for susceptibility testing where required.

The doctor will often order microscopy, culture and sensitivity testing (M/C/S) as the initial test for bacterial wound culture. If a wound culture reveals the need for a susceptibility test, this will be performed without an additional test having to be ordered by the doctor. This can save time so that targeted 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, that is, to look for any bacteria or a person’s response to infection (such as neutrophils) 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. Fungal infection does not respond to antibiotics so other antimicrobial agents (antifungals) maybe required to treat the infection.

When is it requested?

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
• fever

What does the 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 then 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 there are several scenarios that your doctor may consider. These include:

• the site is not infected, but normal flora are colonising the wound
• the wound may be infected by bacteria normally found on the skin
• the pathogen may have been missed in the sample due to low numbers present or an organism that is unable to grow on the media
• 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, when a patient has a chronic infection, or when the wound was exposed to an unusual environment (e.g. to seawater, brackish water, or mud). Chronic wounds that are non-responsive to first line antimicrobials may require specialist review and collection of further specimens such as a tissue biopsy and culture for more unusual organisms.

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

• Why would my doctor collect more than one sample?

This may be done to increase the chance of detecting the pathogen or to detect multiple pathogens. It may involve multiple swabs, a combination of swabs, fluid aspiration, and/or tissue biopsy, or distinct aerobic and anaerobic sample collection.

• 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?

Anyone can get a wound infection, but the risk increases with age and with underlying conditions, such as diabetes, that compromise blood flow or the immune system and/or that inhibit healing.

• 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.

More information

Healthdirect Australia: Wound infections