In “Wound Culturing 101 – How to Correctly Obtain a Wound Culture” at WoundCon Summer, Susie Seaman, NP, MSN, CWCN, CWS, and Dot Weir, RN, CWON, CWS led the audience through a review of pearls and pitfalls associated with this vital information-gathering technique.

Deciding “when” is the first step in a successful wound culture process, explained the speakers. One should culture with the intent to treat, meaning there is likely an infection that requires systemic antibiotics. Accordingly, wounds classified as contaminated or colonized do not typically require culture. But wounds with signs of covert or overt infection often do.¹ Dot Weir shares that wounds with covert or local infection may exhibit the following¹:

• Hypergranulation
• Bleeding or friable tissue
• Pocketing
• Bridging
• Enlargement 
• Delayed healing

Not every wound in this category needs a culture, as systemic antibiotics may not be necessary. In these cases, she advocates for close monitoring, debridement, and topical antimicrobials. An exception, however, is a patient with immunocompromise, where subtle signs may suggest a more serious infection.¹ She explains that overt or spreading infection may exhibit signs such as¹: 

• Erythema 
• Warmth
• Edema
• Purulent or increasing drainage
• Increasing size
• Pain
• Malodor

Topical antibiotics will not be sufficient in these cases. She encourages debridement, culture, empiric antibiotics, and adjustment to antibiotic therapy when the culture and sensitivity return. If the patient is improving on the empiric regimen, adjustment may not be necessary. However, group A Streptococcus is the exception and warrants directed treatment in any instance.¹

What is Bioburden?

Susie Seaman shares that bioburden, or microbial burden, is the number of microorganisms found in a wound, and this number can reflect the pathogenicity and potential virulence.² These microorganisms can be planktonic or part of a biofilm. Planktonic organisms are free-floating and often appear in routine cultures. Antibiotics and other antimicrobials will often address this type of bacteria.³ In contrast, biofilm is a complex community of microbes that adhere to solid surfaces. She adds that these organisms are embedded in an extracellular polymeric substance (EPS). Biofilm organisms do not typically appear on routine culture and can resist systemic and topical antimicrobials.³

What is Biofilm?

Biofilm can include multiple microbe types, including:

• Gram-positive
• Gram-negative 
• Anaerobes 
• Aerobes 
• Fungi 

Mature biofilm is about 5-25% microbes and 75-95% EPS, says Ms. Seaman.^4,5 Up to 90 percent of chronic wounds have biofilm, as opposed to 6% of acute wounds.^4,5 Biofilm is hard to kill and can reach maturity in 48-72 hours, which makes timely intervention vital.^4,5 

Ms. Seaman adds that the challenge in identifying biofilm is that testing is not readily available in most settings and would necessitate equipment such as a scanning electron microscope. She says that minimum inhibitory concentration reports are also not helpful since this only represents planktonic organisms. At present, biofilm identification stems from clinical observation. She stresses that when one notes persistent non-healing of a wound, one should presume biofilm is present, along with instances of slough formation in well-vascularized wounds. 

How To Obtain a Culture From a Wound

Ms. Weir asks the audience, "What is the best way to obtain a culture? Is it a swab or biopsy?” One review looked at 7 studies and found that although biopsy is the gold standard, swab cultures are also acceptable, preferably with the Levine technique.⁶ Another study found 73% concordance between wound swabs and wound biopsy culture results.⁷ However, when indicated and reasonable, she stresses that tissue culture is the gold standard.¹ Such biopsy can reflect pathogens deeper in the wound bed, whereas a swab may only identify surface-level microbes.

Explaining how to obtain a biopsy or tissue culture, she adds that for those with the applicable scope of practice, providers could use a punch biopsy tool, a debridement kit with forceps, a blade, or a sterile curette. The method should begin with proper debridement, followed by wound cleansing. Then, the biopsy site should include tissue from the cleanest appearing areas, with no skin, pus, or necrotic tissue.¹ She points out that knowing the proper labeling and transport methods is vital based on each lab's preference. However, one should not send tissue intended for culture in formalin, she says.

Next, Ms. Weir reviews the Levine technique of swab culture. After appropriate debridement and cleansing, the provider identifies a 1cm-square area of healthy tissue in the wound. They then press the swab into the wound to express fluid, rolling it in that area to collect the fluid.¹

She reminds the audience that it is vital to remember that infection identification is a clinical skill and not solely based on a lab test. Biopsy can provide greater accuracy and specificity and should be the gold standard answer, especially on an academic examination. However, standard of care may dictate choosing a swab culture based on multiple factors, including potential pain, invasiveness, clinician scope, and cost.

Key Components of a Wound Culture

Ms. Seaman points out that there are some standard items providers should order when sending cultures. She says this includes aerobic bacterial culture for most wounds, which will likely pick up most Staphylococcus, Streptococcus, and Gram-negative rods. She recommends adding anaerobic evaluation for wounds with depth or tissue necrosis. Some labs may require a second, different, specialized swab for this, she adds. For wounds unresponsive to treatment, chronic, patients with immunocompromise, or recent travel, she says it may be prudent to add a fungal stain and culture and evaluate for acid-fast bacilli (AFB). Biopsy is best in these cases, but swabs may pick up these organisms. Fungi and AFB may grow at different rates than the more standard microbes, so she recommends checking the lab results regularly.⁸

When providing information on lab requisition, Ms. Seaman advises telling the lab as much history as possible. This information could include the source/location, past cultures if available, current antibiotic therapy, and how long you’d like them to hold the culture. 

Then she moved on to reviewing how to interpret culture results. A Gram stain can be helpful if it becomes available quickly and guides one toward what microbes are present (ie, "many Gram-positive cocci"). "Many" white blood cells on a Gram stain indicate infection, while "few" can be a normal finding. Epithelial cells can be an expected finding, as well. 

Culture Reports: What You Should Know

Preliminary cultures can share information like microbe species and a semi-quantitative, general amount (ie, many, moderate, or few). Suspected methicillin-resistant Staphylococcus aureus (MRSA) is often called in personally to the clinician. Qualitative results will define the species, MIC, sensitivity, or resistance but do not quantify the growth. Quantitative results come from tissue biopsy, with infection designated at greater than 105 colony-forming units (CFU) per gram.⁹ She points out that this is the correct academic answer but advises that if a bacteria is present at 103 CFU/gram or there is a semi-quantitative light growth of MRSA, these still warrant treatment. Beta-hemolytic Streptococcus also always warrants treatment.

Polymerase chain reaction (PCR) results are actually culture-independent. The sample goes into a machine that uses DNA sequencing to analyze the fragments present and numerically quantify the microbes involved.10 This is the most sensitive of the existing reporting methods and more commonly used for blood, respiratory, and gastrointestinal infections. However, she comments that providers may be able to order PCR testing in certain settings.

Conclusion

The faculty stresses that infection diagnosis is a clinical undertaking and may include systemic signs such as fever, chills, nausea, and malaise. They recommend caution with immunocompromised patients, as they may mount a different type of response than others in the infection setting. While awaiting final culture and sensitivity, patients will need empiric antibiotics. They remind the audience that wounds need debridement and cleansing before obtaining culture samples. And, while tissue biopsy is preferred for accuracy, swab culture with the Levine technique can yield a high correlation with biopsy results.

References

1. International Wound Infection Institute (IWII) Wound Infection in Clinical Practice. Wounds International. 2022. Available at: https://woundinfection-institute.com/wp-content/uploads/IWII-CD-2022-we…. Accessed July 17, 2023.
2. Norman G, Shi C, Westby MJ, et al. Bacteria and bioburden and healing in complex wounds: a prognostic systematic review. Wound Repair Regen. 2021;29(3):466-477.
3. Bjarnsholt T, Mastroianni E, Kirketerp-Møller K, et al. The impact of mental models on the treatment and research of chronic infections due to biofilms. APMIS. 2021;129(10):598-606.
4. Phillips PL, Wolcott RD, Fletcher J, Schultz GS. Biofilms made easy. Wounds Int. 2010;1:1‐6.
5. Wolcott RD, Rhoads DD, Bennett ME, et al. Chronic wounds and the medical biofilm paradigm. J Wound Care. 2010;19(2):45-46, 48-50, 52-53.
6. Stallard Y. When and How to Perform Cultures on Chronic Wounds? J Wound Ostomy Continence Nurs. 2018;45(2):179-186
7. Haalboom M, Blokhuis-Arkes MHE, Beuk RJ, et al. Wound swab and wound biopsy yield similar culture results. Wound Repair Regen. 2018;26(2):192-199.
8. Griffith DE, Aksamit T, Brown-Elliott BA, et al. An official ATS/IDSA statement: diagnosis, treatment, and prevention of nontuberculous mycobacterial diseases. Am J Respir Crit Care Med. 2007;175(4):367-416. Erratum in: Am J Respir Crit Care Med. 2007;175(7):744-455.
9. Kallstrom G. Are quantitative bacterial wound cultures useful? J Clin Microbiol. 2014;52(8):2753-6. 
10. Dowd SE, Wolcott RD, Kennedy J, Jones C, Cox SB. Molecular diagnostics and personalized medicine in wound care: assessment of outcomes. J Wound Care. 2011;20(5):232, 234-9.