Wound Cultures: What Is The Best Test?

By Jonathan E. Moore, DPM, MS

When it comes to microbiologic monitoring of a wound, qualitative and quantitative cultures are the most commonly used techniques. In the outpatient wound care environment, you would generally obtain qualitative cultures As we all know, the swab method is easy, inexpensive and is a non-invasive method that gives you specific information about bacterial contamination in the superficial wound. In addition to the class and type of bacterial organism grown from this qualitative culture, the degree of contamination has been reported as very light, light, moderate and heavy growth.
In contrast to the swab culture, you would perform a quantitative culture by taking a biopsy of the wound base and calculate the actual number of organisms in that tissue sample. Using this test enables you to assess the actual number of bacteria per gram of tissue (or colony forming units per gram of tissue) and is generally indicated when you need to differentiate between infection and simple contamination.
Questioning The
Quantitative ‘Gold Standard’
Overall, the bacterial burden of a wound is an integral component when it comes to predicting healing and outcomes. In 1967, Krizek demonstrated that skin grafting would fail if there were greater than 105 bacteria per gram of tissue in the wound bed.1 One year later, Robsen demonstrated a correlation between successful delayed wound closures and a wound bed containing 105 or fewer bacteria per gram of tissue.2
So does 105 bacteria per gram of
tissue represent infection in every wound you see in your office?
Not quite. Whether a patient has infection depends on several variables that can be best demonstrated by this formula:
Infection = dose x virulence/ host resistance
As this formula demonstrates, there is more to the picture than just assessing the number of colonies present in a given wound.3 Although many believe the quantitative tissue biopsy/culture is the “gold standard” when it comes to evaluating the presence of microorganisms within tissue, its results may not give you a definitive determination of infection.
In the case of some pathogens, you’ll find that virulence is always significant no matter how few of the bacteria are present. In a study of surgical wounds, Robson and Heggers demonstrated that Beta hemolytic streptococci at 102 to 103 colony forming units (CFUs) can cause significant host injury.4 However, they also showed that many acute and chronic wounds with greater than 105 colony forming units will heal without
incidence. Other noted virulent pathogens include Mycobacteria, Bacillus antracis, Yersinia pestis and Corynebacteriusm diptheriae.5
Host resistance is one of the most important determinants of wound infection. Local host factors that play an integral role in wound infection include loss of integument, wound duration, depth, wound perfusion and the mechanism of injury. Host systemic factors include vascular disease, diabetes, malnutrition, edema and alcoholism.5
The Debate Over
Culturing Techniques
So when should we culture and what culture technique should we use? It all depends upon the particular clinical scenario. For instance, some have used the quantitative biopsy culture technique to determine the “golden period” after wound injury, in which doing primary closure is acceptable. Others have used the quantitative biopsy in determining the preparedness of the wound bed for skin grafting. Many clinical studies and research modalities use quantitative culture techniques to determine bacterial load response to antibiotics or other wound care modalities.
Assessing the quantity of bacteria in wounds has been studied the most with burns. The septicemia commonly involved with burn wounds prompted the culture techniques that we use to aid in controlling bacterial burden and monitoring treatment. These techniques have resulted in at least a fourfold decrease in mortality rates.6
There has been persistent debate about wound contamination quantity. Should you base the diagnosis of infection on clinical indicators and symptoms (heat, redness, edema)? Then there are those who say you should base it on quantitative cultures. Clearly, there are scenarios (burn wounds) where the number and type of organisms are important.
What About The
Semi-Quantitative Technique?
However, the literature is unclear as to what the exact protocol should be in diagnosing infection for chronic wounds in the diabetic foot. It has been argued that one should be cautious not to apply the 105 rule in every wound. Perhaps the answer to this dilemma may lie in the semi-quantitative swab technique.
Originally described by Levine in 1976, this technique involves performing a thorough cleansing of the wound bed, then rolling and gently rotating a swab over the surface of the wound, avoiding any pockets of pus or necrotic tissue.7 Then you swirl the swab around a single point in the wound bed until you induce a small amount of bleeding. Proceed to break off the swab into the medium and send it for dilution and plating.
The plate is then streaked into four quadrants and inoculated. Be aware that 4+ growth (greater than 30 CFU) has been demonstrated to correlate with 105 or greater organisms per gram of tissue via quantitative biopsy.8
Performing a semi-quantitative line swab culture enables you to count colonies and determine if further treatment (catheters, PICC lines etc.) is necessary. According to the microbiology lab at the University Hospital in San Antonio, when there are more than 15 colonies, you should pursue further treatment. Although this technique has promise for monitoring wounds, it hasn’t been implemented yet into many hospital labs due to the lack of large studies correlating its results with that of a quantitative biopsy.
In Conclusion
Unfortunately, assessing wound infection will never be simple. We need to carefully evaluate clinical signs and symptoms; host factors (both local and systemic); the number of colony-forming units (acute wounds); and the virulence of the particular organism. Assessing these components, along with doing a good exam and providing appropriate wound care, are essential in managing and monitoring the diabetic wound.n

References:

References
1. Krizek T. Robson, M. Kho E., Bacterial growth in skin graft survival. Surg Forum 1967 (18)518.
2. Robson M. Duke W, Krizek T. Rapid bacterial screening in the treatment of civilian wounds. J. Surg Res. 1973 14(420)
3. Altemeier W. and Culbertson W. Surgical Infection. In: Moyer C, et al, eds. Surgery, Principals, Practice 3rd ed. Philadelphia, PA: J.B. Lippincott, Co: 1965
4. Robson M, Heggers J. Surgical infection II: the beta-hemolytic streptococcus. J Surg Res. 1969 (9)289
5. Dow G, Browne A., Sibbald R.G. Infection in chronic wounds: Controversies in diagnosis and treatment Ostomy/Wound Management 1999 45(8) 23-40
6. Edberg S.C., Methods of quantitative microbiological analysis that support the diagnosis, treatment and prognosis of human infection, Critical Review in Microbiology, 1981 September (339).
7. Levine N., Lindberg R. Mason A., et al. The quantitative swab culture and smear: a quick, simple method for determining the number of viable aerobic bacteria open wounds. J Trauma. 1976 16(2)89.
8. Thomson P, Taddonio T, Tait M, et al. Correlation between swab and biopsy for the quantification of burn wound microflora. Proc Int Cong Burn Inj. 1990 (8)413.