Bacterial Wound Culture

Bacterial wound cultures, together with clinical examination, are used to determine the presence of infection in wounds.

The reference range for a negative bacterial wound culture result depends on the method, as follows: ^[1]

• Qualitative wound culture: No growth of any pathogenic organism or growth of normal skin flora

• Semiquantitative wound culture: Less than 4+ growth

• Quantitative wound culture: Less than 100,000 organisms per gram (if tissue specimen is used), per milliliter (if fluid collection is used), or per swab (if swab is used). ^[2]

Wound cultures commonly serve as an adjunct to clinical examination, and both should therefore be interpreted concurrently. Interpretation of a qualitative bacterial wound culture requires determination of whether the isolated organism is the cause of the infection or is a contaminant/part of normal skin flora. Quantification by semiquantitative and quantitative methods is used to further determine whether the isolated pathogen is a true infection by standard cutoff values for wound bacterial burden as outlined in Reference Range.

In general, qualitative cultures are useful in identifying pathogenic bacteria. Organisms that are commonly part of normal skin flora and that are not usually pathogenic include the following:

• Staphylococcus epidermidis or other coagulase-negative staphylococci

• Corynebacterium species and other diphtheroids

• Brevibacterium species

• Propionibacterium acnes

• Pityrosporum species

• Alpha or gamma streptococci

• Neisseria species (except Neisseria meningitidis or Neisseria gonorrhoeae)

• Bacillus species (except Bacillus cereus or Bacillus anthracis)

Growth of aforementioned organisms usually represents contamination, ^{[1, 3, 4]} except for coagulase-negative Staphylococcus species, which could represent either contamination or true infection.

The most common pathogenic bacteria isolated in acute and chronic wound infections are as follows: ^[5]

• Staphylococcus aureus

• Pseudomonas aeruginosa

• Enterococci

• Beta-hemolytic streptococci

• Coliform bacteria, including Escherichia coli, Enterobacter species, and Klebsiella pneumoniae

• Coagulase-negative staphylococci

• Pigmented gram-negative anaerobes (Prevotella and Porphyromonas species)

• Nonpigmented gram-negative anaerobes (primarily Bacteroides, Prevotella, and Fusobacterium species)

• Peptostreptococcus species

• Clostridium species

Quantitative cultures obtained via biopsy are considered the criterion standard among wound culture techniques. Growth of more than 100,000 or 10⁵ organisms per gram of tissue or per milliliter of fluid aspirate is considered positive for wound infection. ^[6]

For semiquantitative cultures, blood agar plates are streaked 3 times on one quadrant and then 3 times on each remaining quadrant using a sterile loop for each quadrant. This is thought to create dilutions of the original swab in each quadrant. Growth of more than 30 colonies in a quadrant is read as 4+ growth and is considered positive for infection. Semiquantitative cultures are considered to have good correlation with quantitative cultures for detection of wound sepsis, with a positive predictive value of 100% and a negative predictive value of 93.7%. ^{[2, 7]}

The 3 most common methods of collecting specimen for wound culture include wound tissue biopsy, needle aspiration of fluid, and swab. ^[1]

Wound tissue biopsy

Container: Sterile tube or container

Normal volume: 0.005-0.2 g of involved tissue or 3-4 mm punch biopsy

Procedure: A tissue biopsy is obtained aseptically via either a punch biopsy instrument or excision using a scalpel. ^[1] Other accepted techniques include curettage of superficial devitalized tissue, most commonly used in diabetic foot ulcers, and a dermabrasion procedure for deep tissues without being too invasive. ^[7] For optimum recovery, the specimen should be sent to the laboratory within 60 minutes.

Needle aspiration

Container: Sterile tube or container

Normal volume: 0.5-2 mL

Procedure: This is the best procedure for wounds involving focal fluid collections or abscesses. The area should be prepared aseptically. Multiple aspirations around the wound are performed using a 22-gauge needle attached to a 10-mL syringe. ^[1] The syringe containing the specimen may be capped or the specimen can be transferred onto a sterile container and sent to the laboratory within 60 minutes after collection.

Swab

Normal amount: 1 swab

Procedure: Swabbing, which is probably the most common method of collecting specimen for culture, is usually performed on open wounds. Because surface organisms can easily contaminate wounds, it is essential to clean and irrigate the surface of the wound with saline until it is free of drainage, necrotic debris, eschar, or purulent material. ^[5]

The Z-stroke technique involves rotating a swab between the fingers as the swab is moved in a zigzag fashion across the wound. The wound edges should be avoided. ^[1]

The Levine technique involves rotating the swab over a 1 cm² area with sufficient pressure to express fluid within the wound. As above, wound edges should be avoided. ^{[1, 5]} This technique is found to be superior to the Z-stroke technique for identifying infected wounds, largely because of the technique’s ability to express pus from the wound. ^[8]

Description

Bacterial wound cultures, together with clinical examination, are used to determine the presence of infection in wounds. Additionally, cultures are used to identify the specific organism or organisms and to guide specific antimicrobial therapy. ^{[1, 5]}

Indications/Applications

Bacterial wound culture is indicated for surgical and nonsurgical wounds, both acute and chronic, suspected of being infected. ^[9]

It is also indicated for hospital or local surveillance protocols to monitor drug-resistant microorganisms. ^[10]

A study by Marchand-Senécal et al found that when the Q-score quality metric was used to evaluate wound swab specimens, with those of low quality subsequently rejected for culturing, the rate of antibiotic use for wound infection fell in patients with low-quality specimens. While prior to specimen rejection, the difference in the reflexive antibiotic prescription rate was not significantly different between the low- and high-quality groups, once rejection began, these prescription rates were 4.5% and 9.4%, respectively. ^[11]

Considerations

Tissue biopsy is considered the criterion standard in collecting wound culture specimens. Culture of viable but potentially infected tissue is preferred over necrotic tissue.

Swab cultures may be unreliable because they usually indicate contamination. When appropriate, perform tissue biopsy or aspiration to collect specimens.

Ideally, specimens should be collected before starting antibiotics.

Clinical examination of the wound and how it is progressing should always carry more weight than culture results. Cultures are best used to confirm clinical impression of infecting pathogen(s), to suggest an unanticipated pathogen if the wound is not improving, and to guide alternative antimicrobials (via susceptibility testing), as indicated.

Serial wound cultures to evaluate response to treatment is discouraged.