Bacterial protease activity: a prognostic biomarker of early wound infection

High bacterial burden is one of several reasons that wounds fail to heal. At present, clinicians rely primarily on clinical signs and symptoms (CSS) to diagnose infection in hard-to-heal wounds; however, studies have demonstrated that CSS can be unreliable. This is especially true in the early stages of bacterial infection. Bacteria release proteases, virulence factors that promote invasive infection. This clinical trial evaluated the use of bacterial protease activity (BPA) as a biomarker to detect whether a wound was in the period of pathogenicity, prior to overt clinical signs.

Participants were drawn from six US wound centres and had their wounds assessed clinically for infection. In addition, wound fluid swabs were collected and analysed for BPA, inflammatory cytokines (interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumour necrosis factor-α (TNF-α)), and cultured for quantitative microbiology.

A total of 366 patients were recruited. The median BPA level increased with the increasing number of signs of infection. The majority of wounds tested positive for elevated BPA prior to exhibiting at least three CSS of infection, the level at which the criteria for infection are met. BPA tended to increase with the bioburden (colony forming unit (CFU)/ml) although some wounds with high bioburden were negative for BPA, and others with low bioburden were positive for BPA. The mean levels of IL-1β and TNF-α were significantly higher in BPA-positive wounds (p<0.0001 and p=0.0002, respectively).

The results of this clinical trial suggest that measuring BPA can lead to the early detection of pathogenic bacteria in the wound that impede wound healing and may progress to invasive infection. In a large percentage of cases, BPA detected virulent bacteria in the absence of CSS of infection. As a biomarker, BPA has an advantage over measuring bacterial load-hard-to-heal wounds are often colonised with non-pathogenic bacteria that do not inhibit wound healing and, conversely, a low number of highly virulent species could disrupt the healing process.