Wound cultures are used to identify pathogenic bacteria causing wound infections. Traumatic injury of tissue may be complicated by infection. Infections may be caused by organisms introduced from the external environment, like bite and surgical and traumatic wounds, or by organisms derived from the patients endogenous flora, like peritonitis associated with a ruptured appendix. Wound culture should be considered when a wound shows signs and symptoms typical of infection: swelling, redness, exudate or pus formation, sinus tract formation, pain, swelling, or other.
Special Collection and Transport Instructions
Specimens should be collected from the site of active infection. Adjacent areas may show "sympathetic " � signs of inflammation, but may not yield relevant pathogens.
Wash and decontaminate the collection site, typically using a soap and 70% alcohol.
Collection of infected tissue or aspirate, at least 1 g, is recommended. Collection using swabs is not recommended.
Collection and transport under anaerobic conditions are recommended, especially for closed wounds. Specimens should be transported to the laboratory within 2 hours. Specimens may be held at 4 � �C for a short time if transport is delayed.
Use
Most specimens for bacterial wound cultures should be examined by Gram staining. Specimens from superficial wounds showing a significant numbers of epithelial cells are likely to be contaminated by endogenous flora unrelated to the infection.
Specimens are inoculated onto supportive, enriched, and selective/differential media.
Specimens are inoculated onto supportive and enriched nonselective media, like SBA and chocolate agar media, and selective media, like MacConkey, PEA, and CNA agar. Some laboratories include a broth medium, like thioglycollate broth, to routine cultures. Anaerobic medium is inoculated for appropriate specimens collected and submitted under anaerobic conditions.
Turnaround time: Cultures are incubated for 48 " �72 hours. Additional time is required for isolation, identification, susceptibility testing, and further characterization, as needed.
Interpretation
Expected results: No growth.
Positive results: Cultures of infected wounds often show growth of several types of organisms. Cultures must be interpreted carefully: Mixed cultures may be caused by colonization by endogenous flora or contamination due to poor specimen collection technique. However, mixed cultures may represent synergistic polymicrobial infections, especially when anaerobes are isolated.
Negative results: Negative culture decreases the likelihood of active bacterial infection.
Limitations
Negative cultures may be caused by prior antimicrobial treatment, infection caused by a fastidious pathogen, or collection of the sample from a site other than that of active infection.
Significant pathogens may not be recognized in mixed cultures.
Submission of multiple cultures or cultures of several affected sites may be required for detection, especially in chronic infections. The structure and milieu of abscesses may prevent effective antibiotic treatment. They are avascular spaces, and their outer capsule may prevent entry of antimicrobial agents. In addition, antibiotics may be inactivated by the acidic environment and degradative enzymes present. Adjunctive surgical therapy may be required, especially for large collections of pus.
Common pitfall: Collection of specimens from sites other than the active site of infection, such as sinus tracts, often yields growth of endogenous flora unrelated to the infection.
Other Considerations
Pyogenic infections are usually associated with heavy growth (105 CFU/mL) of the responsible pathogen.
Polymicrobial infections are often treated successfully with surgical and/or empirical antimicrobial therapy. Extensive culture analysis with identification and susceptibility of multiple isolates is usually not clinically indicated.
Certain pathogens are associated with specific types of wound infections, such as P. aeruginosa with penetrating foot wounds through sneakers and P. multocida with cat bites. These infections may require special laboratory techniques for optimal detection; alert the laboratory when suspected.