Introduction: It is commonly accepted that bacteria found within hard-to-heal wounds exist as surface-associated or aggregated communities, often comprising multiple species.1,2 This polymicrobial nature increases virulence and tolerance to antimicrobial agents.3,4 We evaluated antimicrobial dressings containing silver against surfaced-associated microorganisms in a dual-species community grown in a stringent simulated wound model.
Methods: Four silver-containing dressings were assessed: carboxymethylcellulose dressing containing ionic silver, ethylenediaminetetraacetic acid (EDTA) and benzethonium chloride (BEC; ‘CISEB’*); non-adherent polyethylene mesh with polyester core dressing containing silver oxysalts (‘PPSO’†); cellulose ethyl sulphonate fiber dressing containing ionic silver (‘CESIS’‡); and polyacrylate (polyabsorbent) fiber dressing with acrylic core containing silver sulphate (‘PSS’). In brief, challenge microorganisms (Klebsiella pneumoniae and methicillin-resistant Staphylococcus aureus [MRSA]) were grown together on gauze to simulate the surface-associated phenotype and transferred to a simulated wound assembly (SWA). Test dressings were applied and covered with a transparent film dressing, and the SWA was then incubated at 35±3 ºC for up to 72 hours. Enumeration of surviving bacteria was performed in triplicate for each test dressing. A two-sample t-test was used to compare dressings.
Results: CISEB resulted in significantly greater reduction in biofilm than PPSO and PSS at 24 and 48 hours (p< 0.05). At 72 hours, CISEB resulted in significantly greater reduction in microorganisms than all other test dressings (p< 0.05), reducing the dual-species population to below the limit of detection (< 30 colony-forming units/gauze). PPSO demonstrated an initial reduction of ~2 log10 up until 6 hours and CESIS marginally reduced populations by ~2.5 log10 by 72 hours. PSS had little to no impact on the dual-species population with levels remaining similar or greater than that recovered prior to dressing application.
Discussion: This simulated wound model took into account the polymicrobial nature and surface-associated phenotype of microorganisms often found in hard-to-heal wounds. We demonstrate that not all silver-containing dressings are equally effective against this complex phenotype, with CISEB demonstrating significantly greater antimicrobial activity than the other silver-containing dressings. The enhanced antimicrobial activity of CISEB in this study may be attributed to the additional agents (EDTA and BEC) contained within the dressing, which help the silver perform effectively against surface-associated microorganisms.5