How To Assess The Bacterial Burden Of DFUs

Guy Pupp, DPM, FACFAS, and Robert Koivunen, DPM

   Certainly, we are learning that the chronic wound commands special notice and treatment. Acute wounds usually harbor bacteria in the free floating or planktonic form without a complex matrix or biofilm covering. Unfortunately, clinicians commonly utilize a swab culture for diagnosis. It is much more difficult to diagnose biofilms in chronic wounds. A routine swab culture may be adequate for sampling of planktonic forms but might not be adequate in assessing the growth of biofilm, which may go undetected. Lack of identification of a biofilm infection in a chronic wound can lead to amputation and life-threatening sepsis.

   Unfortunately, the patients who develop chronic wounds are often immunocompromised in some way (e.g. poor circulation or hyperglycemia), which facilitates the establishment of bacterial biofilm communities and makes elimination of the biofilm all the more difficult. Wounds in limbs with poor perfusion are among the most difficult to heal.18 Patients with a chronic wound in a diabetic limb with a TcPO2 less than 20 mmHg often must undergo a major limb amputation.

   The healing of wounds, especially in patients with diabetes, can be a life or death situation. Apelqvist and colleagues showed that after a major limb amputation, patients with diabetes underwent contralateral limb amputation in 48 percent of cases within five years.19 Pohjolainen and co-workers demonstrated that the five-year mortality rate for patients with diabetes after a major limb amputation is 80 percent.20

Diagnosing Biofilms In Chronic Wounds

A wound care specialist, who understands that biofilms exist in chronic wounds, needs to seek reliable detection and treatment methods. Unfortunately, satisfactory solutions do not yet exist.

   The current cultivation methods that are in routine use in most diagnostic labs to isolate and identify microbial species do not support the development of biofilms. Unlike their planktonic natural form, biofilms coaggregate, which contributes to a resistance as well as a metabolic advantage, allowing the survival of anaerobic species in the presence of an aerobic environment.20,21 The adherence of biofilm to host tissue, combined with the possibility that biofilm may form in deep tissue, makes the use of swabs on wound surfaces an unreliable method for recovery.

   One should never interpret the presence of slimy material within a wound as the presence of biofilm since the arrangement of cells within that slime is only visible with high power magnification rather than the naked eye. Slough in a wound is not an indicator of biofilm. Although one can easily remove slough by conventional debridement, the clinician cannot remove biofilm in this manner.22

   At present, the detection of biofilm in wounds depends on the examination of biopsy tissue using sophisticated research techniques. Scanning electron microscopy and laser scanning microscopy are appropriate means to observe biofilm, but are usually costly, time-consuming and unavailable in most microbiology labs. Microbial DNA-based studies or molecular diagnostics are very promising tools for biofilm evaluation. Molecular diagnosis is currently in use to personalize topical therapeutic treatment of biofilm infections.23

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