Bacterial Biofilm in Chronic Wounds and Possible Therapeutic Approaches

Abstract

Wound repair and skin regeneration is a very complex orchestrated process that is generally composed of four phases: hemostasis, inflammation, proliferation, and remodeling. Each phase involves the activation of different cells and the production of various cytokines, chemokines, and other inflammatory mediators affecting the immune response. The microbial skin composition plays an important role in wound healing. Indeed, skin commensals are essential in the maintenance of the epidermal barrier function, regulation of the host immune response, and protection from invading pathogenic microorganisms. Chronic wounds are common and are considered a major public health problem due to their difficult-to-treat features and their frequent association with challenging chronic infections. These infections can be very tough to manage due to the ability of some bacteria to produce multicellular structures encapsulated into a matrix called biofilms. The bacterial species contained in the biofilm are often different, as is their capability to influence the healing of chronic wounds. Biofilms are, in fact, often tolerant and resistant to antibiotics and antiseptics, leading to the failure of treatment. For these reasons, biofilms impede appropriate treatment and, consequently, prolong the wound healing period. Hence, there is an urgent necessity to deepen the knowledge of the pathophysiology of delayed wound healing and to develop more effective therapeutic approaches able to restore tissue damage. This work covers the wound-healing process and the pathogenesis of chronic wounds infected by biofilm-forming pathogens. An overview of the strategies to counteract biofilm formation or to destroy existing biofilms is also provided.

Keywords: anti-biofilm therapeutic approaches; biofilm; chronic wound infections; chronic wounds; skin microbiota; wound healing.

Figure 1

Stages of wound healing. (A) The process starts when platelets become activated by exposed collagen, start to aggregate, and become trapped in fibrin fibers, forming a clot. (B) Afterwards, the inflammatory phase starts with neutrophil recruitment that, by producing ROS, avoids wound infection and promotes fibroblast proliferation and angiogenesis. Subsequently, monocytes are recruited and differentiate into M1 macrophages that produce proinflammatory cytokines and clear neutrophilic debris. (C) During the proliferation phase, macrophages differentiate into M2 macrophages that produce EGF that stimulates keratinocytes to start the re-epithelialization, VEGF that promotes the angiogenesis process, and FGF that stimulates the multiplication of fibroblasts that start to deposit collagen replacing the fibrin clot with granulation tissue. (D) Finally, ECM produced by fibroblasts replaces the granulation tissue, the skin barrier is restored, and thanks to the activity of myofibroblasts, the wound contracts.

Figure 2

Chronic wound. A chronic wound is characterized by stagnation into the inflammatory phase so that excess neutrophils and macrophages are present within the wound with exaggerated ROS production and ECM degradation. This situation promotes bacteria colonization and the formation of biofilms, which further impair healing and a response to treatments. Chronic wounds are characterized by defective angiogenesis that leads to necrosis, impairment in the re-epithelialization process, and the senescence of fibroblasts that causes insufficient ECM production.

Figure 3

The biofilm formation process. Planktonic bacteria adhere to a surface or a wound and form a microcolony in which they start to deposit the extracellular polymeric substance, leading to the maturation of the biofilm. In both the microcolony and mature biofilm, the extracellular polymeric substance is produced thanks to QS signaling pathways that, by regulating gene expression, also regulate metabolism, virulence, and several other cellular functions.

Figure 4

Strategies to reduce biofilms and to promote chronic wound healing. Biofilms can be displaced by mechanical debridement (A) or dismantled either by the use of probiotics (B) or bacteriophages that lead to the lysis of pathogenic bacteria (C).

Figure 5

Strategies to reduce biofilms and to promote chronic wound healing. Biofilms can be dismantled either by enzymatic debridement (A), anti-biofilm antibodies (B), nanoparticles (C), nanoparticles for anti-biofilm agent (orange rectangles) delivery (D), QS inhibitors (E), or by the use of antimicrobial peptides (F).