Emerging Innovations in the Prevention & Treatment of Chronic Wound Infections

The burden of chronic wounds in United States continues to mount for our healthcare system. Recent cost estimates in this country are near $25 billion per year with 6.5 million patients treated annually.¹ Chronic wounds, defined as those that have either not healed completely or have shown limited response to treatment at six weeks, have deviated from normal healing physiology.^1,2 Disrupting factors include ineffective blood supply, immune dysfunction, metabolic disease, and previous tissue injury. The most common disease process explaining the increasing incidence of chronic wounds is diabetes. Improved glycemic control has been proven to significantly improve wound healing, but is a difficult clinical outcome to achieve for many patients. Another common modifiable factor for poor healing is wound infection. Myriad topical preparations and augmented dressings exist for the purpose of preventing and treating infection. Success with this approach has been mixed at best, as highly efficacious antimicrobial agents are often simultaneously toxic to healthy tissue. In addition, topical applications often require frequent and painful wound dressing changes. The aging U.S. population compounds the burden of chronic wound infection. Residents of long-term care facilities are commonly colonized with resistant organisms and infection rates near that of acute care facilities. Frequent transfers between living facilities, acute care facilities, and wound clinics exacerbate selection pressure and contribute to polymicrobial wound infection.³

Within chronic wound infections, biofilms present an additional challenge because of their ability to persist despite antibiotics, antiseptics, and even serial debridement.^4,5 (Biofilm is defined as a collection of bacterial cells secured in an extracellular matrix that is highly resistant to antibiotic treatment.⁶) Wound dressings themselves create another nidus for biofilm formation. Natural inhibitors of biofilms include antimicrobial peptides, metal chelators, and amino acids, and further investigation of these approaches may result in strategies to augment current approaches.⁵ As the burden of chronic wounds continues, infection prevention and treatment remains an area of active and emerging research. Recent efforts marry old and novel approaches to delicately deliver agents that are effective against bacteria but do not interfere with normal wound healing. In some cases, this methodology may even bolster healing. Using this perspective as a backdrop, this article will discuss the current science and future considerations.   

The Silver Standard

With use dating back more than 2,000 years, silver has long been a standard in wound care. It is currently available in a number of topical applications: from creams and ointments to dressings impregnated with the ion. Despite its ubiquity, existing literature on its efficacy is mixed. A 2010 Cochrane review on silver-containing dressings concluded that there was a lack of evidence to support the use of silver-containing dressings to prevent wound infection or to promote wound healing. Furthermore, this review highlighted the risk of silver-associated cytotoxicity.⁷ A 2018 study by Rodriguez-Arguello and colleagues updated the existing literature on silver-impregnated dressings and identified research gaps.² Twenty-seven full-text articles were ultimately reviewed, and the authors concluded that, as with the previous Cochrane review, evidence regarding the efficacy of silver-based therapies was “heterogeneous.” The most compelling conclusion was that the current research gaps make a strong argument for the need for prospective, randomized trials.³

Despite this, silver remains widely employed in wound care due to its biocidal activity against a broad spectrum of bacterial and fungal species.⁸ Its mechanism of action includes oxidative damage to bacterial cell membranes and the inhibition of bacterial replication by denaturing bacterial DNA and RNA.³ Furthermore, bacteria do not readily develop resistance to silver.⁷ Its application, however, has been hindered by its dose-related cytotoxicity. Because of its highly reactive nature, silver is prone to inactivation in the wound environment, which requires either frequent reapplication or high initial concentrations of silver. While frequent wound dressing changes are impractical, the more serious consequences of high concentrations of silver are counterproductive tissue toxicity and impaired wound healing.⁹ The challenge put forth to innovators in wound care has been to harness the antimicrobial benefits of silver while mitigating its cytotoxicity. One promising area of advancement stems from the use of polyelectrolyte multilayer (PEM) thin films. Specific technical preparations vary, but PEM films are made up of alternating layers of oppositely charged polyelectrolytes that can then be impregnated with nanoparticles. This allows for the targeted delivery of these nanoparticles at significantly lower concentrations than with direct application or other current silver applications.⁸

A 2010 study by Agarwal and colleagues provided proof of concept with an in vitro study.⁹ Cover slips coated with PEM films impregnated with various concentrations of silver were applied to a suspension containing murine fibroblasts and Staphylococcus epidermidis to measure both cytotoxicity to healthy cells and antibacterial activity. After incubation for 24 hours, murine cell viability and mean colony-forming units (CFU) were measured. PEMs with as little as 0.4 μg/cm² of silver caused a 99.9% reduction in S. epidermidis. Despite this biocidal activity, the growth of mammalian cells was not inhibited.⁸ This was then extended in a 2012 in vivo study where biologic dressings modified with silver-nanoparticle PEMs significantly reduced CFUs in full-thickness murine wounds.¹⁰ A 2013 study by the same group quantified the transfer of silver from dressing to wound bed with these novel dressings and evaluated the sustained release of silver over a series of days. The transfer of silver from the sheets to the wound bed was greater than 85%. In addition, daily silver release was approximately 1-2 μg/cm². Most significantly, this study found that there was no delay in healing in silver-treated as compared to control mice.⁸ 

Blending Tradition With Novel Agents

Although silver is widely used, it is not effective against biofilm when used in isolation.⁴ As referenced previously, natural inhibitors of biofilm exist. Bowler and Parsons leveraged the features of ionic silver with other inhibitors to improve the antimicrobial action of silver against biofilms.⁴ Using a commercially available silver dressing, they created an augmented dressing that had a silver concentration of 0.16% μg/cm², as well as ethylenediaminetetraacetic acid (chelating substance) and benzethonium chloride (surfactant). When compared to the identical silver dressing alone, the augmented dressing was capable of killing a highly resistant strain of S. aureus. Similarly, the augmented dressing was superior against a multidrug-resistant Pseudomonas aeruginosa biofilm. This suggests that synergy between a chelating substance, surfactant, and silver bolsters the ability of silver to penetrate biofilm without the need to increase the amount of silver contained within the dressing to cytotoxic levels.⁴

A 2013 study investigated the in vitro effect of tryptophan, another natural inhibitor, on P. aeruginosa biofilm formation and motility. When “D” and “L” isoforms were mixed, biofilm formation was inhibited by 93% at 24 hours and 90% at 48 hours. Interestingly, the findings suggested a mechanism of action beyond pure bacterial inhibition and demonstrated that tryptophan increased P. aeruginosa motility, which may help with the dissolution of biofilm.⁵ Another 2015 study with tryptophan found that without intervention, P. aeruginosa formed a biofilm within 48 hours of incubation. Incubation of wound dressings with ≥ 5 mM of D/L tryptophan arrested this biofilm formation but did not reduce cell viability. In addition, wounds that received control biofilms stayed 100% open during the experiment while wounds with dressings including tryptophan remained 65% open.⁴ A similar 2015 study with gallium impregnated in a polyelectrolyte film delivered noncytotoxic concentrations for 20 days and inhibited the formation of P. aeruginosa, as well as dispersing existing biofilms.¹¹ The early promise of these novel agents and approaches herald a new horizon in chronic wound management. As the number of chronic wounds continues to rise, the marriage of technology and synergy between known inhibitors will allow for faster, more effective wound healing that is more tolerable for patients. Similarly, as nanotechnology continues to improve, the cost to produce targeted dressings will continue to drop and the overall cost of caring for chronic wounds can be expected to fall. 

Sarah Waterman Manning is a resident at Mountain Area Health Education Center (MAHEC), Asheville, NC. Michael Schurr is founder of Imbed Biosciences, Fitchburg, WI, which produces MicroLyte^® Ag Matrix. David Humphrey is on staff at Mission Hospital in Asheville.

References 

1. Han G, Ceilley R. Chronic wound healing: a review of current management and treatments. Adv Ther. 2017;34(3):599-610. 

2. Rodriguez-Arguello J, Lienhard K, Patel P, et al. A scoping review of the use of silver-impregnated dressings for the treatment of chronic wounds. OWM. 2018;64(3):14-31. 

3. Nicolle LE. Infection control in long-term care facilities. Clin Infect Dis. 2000;31(3):752-6. 

4. Bowler PG, Parsons D. Combatting wound biofilm and recalcitrance with a novel anti-biofilm hydrofiber wound dressing. Wound Medicine. 2016;14:6-11. 

5. Brandenburg KS, Calderon DF, Kierski PR, et al. Inhibition of pseudomonas aeruginosa biofilm formation on wound dressings. Wound Repair Regen. 2015;23(6):842-54. 

6. Brandenburg KS, Rodriguez KJ, McAnulty JF, et al. Tryptophan inhibits biofilm formation by pseudomonas aeruginosa. Antimicrob Agents Chemother. 2013;57(4):1921-5. 

7. Storm-Versloot MN, Vos CG, Ubbink DT, Vermeulen H. Topical silver for preventing wound infection. Cochrane Database Syst Rev. 2010;3: CD006478. 

8. Guthrie KM, Agarwal A, Teixeira LB, et al. Integration of silver nanoparticle-impregnated polyelectrolyte multilayers into murine-splinted cutaneous wound beds. J Burn Care Res. 2013;34(6):e359-67.

9. Agarwal A, Weis TL, Schurr MJ, et al. Surfaces modified with nanometer-thick silver-impregnated polymeric films that kill bacteria but support growth of mammalian cells. Biomaterials. 2010;31(4):680-90. 

10. Guthrie KM, Agarwal A, Tackes DS, et al. Antibacterial efficacy of silver-impregnated polyelectrolyte multilayers immobilized on a biological dressing in a murine wound infection model. Ann Surg. 2012;256(2):371-7. 

11. Herron M, Schurr MJ, Murphy CJ, McAnulty JF, Czuprynski CJ, Abbott NL. Gallium-loaded dissolvable microfilm constructs that provide sustained release of Ga3+ for management of biofilms. Adv Healthc Mater. 2015;4(18):2849-59.