058 Silver Dressings: Effective Antimicrobial Therapies?

Many silver‐dressings have been recently developed to prevent infection of burn and chronic wounds. Antimicrobial therapies are usually studied in in‐vitro assays, which evaluate the antimicrobial efficacy against free‐floating bacteria. In a wound environment bacteria attach to tissues and establish bacterial biofilms. Biofilms are communities of colonies of bacteria and other microorganisms encased in a self produced exopolymeric substance. Biofilms help protect the bacteria from the environment and limit the effectiveness of antimicrobials. In this study we evaluated the efficacy of two silver dressings to eradicate biofilm‐associated and planktonic Pseudomonas aeruginosa cells using our biofilm burn wound animal model. Three pigs were used in this study. Second‐degree burn wounds were made on one‐half of the animal’s back and inoculated with a burn wound isolate of P. aeruginosa. These wounds were then covered for 72 hours with a polyurethane dressing to allow biofilm formation. After 72 hours, additional burn wounds were made on the unwounded half of the animal. The new burns were then inoculated with the same P. aeruginosa strain. At this 72 hour point we had established two bacterial groups, one side of the animal had burns with planktonic bacteria and the other half with biofilm bacteria. Both sides of the animal were treated 20 minutes after inoculation of the planktonic group with the following dressings: 1) Nanocrystalline Silver, 2) Hydrocolloid Silver, or 3) untreated. Wounds were cultured from all treatment groups at 24, 48 and 72 hours post treatment. Sites were cultured quantitatively using a novel flush‐scrub technique to obtain both planktonic and biofilm bacterial counts. The baseline biofilm bacterial count was 7.56 LogCFU/ml (prior to treatment). The hydrocolloid dressing significantly reduced planktonic bacteria counts as compared to untreated and nanocrystalline silver dressing at 24, 48 and 72 hours. However, biofilm bacteria counts for both dressings were similar to untreated control at all sample points. Our study demonstrates that both silver dressings showed limited effect against P. aeruginosa biofilm‐associated cells when compared to untreated wounds. Based on our results we question the effectiveness of silver dressings for infected wounds that are colonized with biofilm associated‐cells. We conclude that anti‐biofilm susceptibility models may improve on current antimicrobial sensitivity assays.