Bacterial Wound Culture

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 the presence of significant pathogenic bacteria, a susceptibility test is often performed routinely without it having to be requested by the doctor. This can save time so that antimicrobial therapy can begin as soon as possible.

Gram stains may be 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 but fungal growth may take several days or weeks. Fungal infection does not respond to antibiotics so other antimicrobial agents (antifungals) maybe required to treat the infection.

This test is primarily requested when a doctor suspects that a wound is infected. It may be repeated if the infection fails to respond to treatment or if there is 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

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 from the sample which may be due to previous antibiotic treatment.

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.

Washing with soap and warm water and keeping wounds clean is important to preventing infection.

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.