Atlanta— In vitro assessment of the efficacy of silver dressings may not adequately reflect their performance in a clinical setting, according to Stephen C. Davis, research associate professor, department of dermatology and cutaneous surgery, University of Miami School of Medicine. Carlos Ricotti, M.D., a wound healing fellow with Prof. Davis, discussed their work on the antimicrobial efficacy of silver dressings at the 14th annual meeting of the Wound Healing Society.
Silver has long been known to have antimicrobial activity. In recent years, silver-containing dressings have replaced topical silver sulfadiazine as the preferred silver treatment for chronic wounds or burn wounds.
"The problem with topical silver is that the silver is too readily released and may be rapidly inactivated due to proteins found within the wound exudates," Prof. Davis says.In silver dressings made of hydrocolloid material, the silver ions are released slowly in proportion to the absorption of exudates by the material. This mechanism is thought to give the dressings a two-fold advantage over topical silver compounds. Absorption of exudates creates a wound environment that promotes healing. In addition, sustained release of silver ions results in the presence of silver in the wound environment over a longer time period.
Silver dressings have been shown to be effective in preventing pathogenic infection in chronic wounds or burn wounds, but their efficacy in combating pathogenic colonization and infection has not been adequately studied in vivo, according to Prof. Davis. All published reports evaluating the antimicrobial activity of silver dressings have been performed in vitro on the planktonic bacterial sub-population, not against biofilm bacteria.
In vitro assays not all-encompassing
"Results obtained using in vitro assays should be carefully interpreted," Prof. Davis says. "In vitro assays are good for determining initial efficacy of the silver dressing against specific pathogens, but fail to take into account other factors that may influence treatment efficacy, such as wound fluid, growth factors and proteinases."
In an unsponsored study, Prof. Davis and co-workers at the University of Miami compared the antimicrobial activities of a hydrocolloid dressing and a silver-containing dressing in an animal model. Each of three young, female pigs was given a second-degree burn wound on half of the back. The wounds were inoculated with a clinical isolate of P. aeruginosa from a burn wound. To encourage biofilm formation, the wounds were covered with polyurethane dressing for 72 hours.
After 72 hours, burn wounds were also created on the uninjured half of each animal's back, and the wounds were inoculated with P. aeruginosa. The bacteria in the older wounds had formed biofilms by this time, whereas the bacteria in the freshly inoculated wounds were planktonic. Before silver dressings were applied, cultures were taken from the older wounds to determine baseline biofilm bacterial counts.
Approximately 20 minutes after the fresh wounds were inoculated, the wounds on both sides of the animal were treated with a non-crystalline silver dressing (Acticoat, Smith & Nephew), a hydrocolloid silver dressing (Contreet, Coloplast Corp.) or no dressing. A modified scrub technique was used to obtain cultures from the wounds in all groups at 24, 48 and 72 hours. Serial dilutions of the cultures were plated and counted using standard microbiological techniques.
Planktonic bacterial counts were significantly reduced at 24, 48 and 72 hours in wounds treated with the hydrocolloid dressing, compared to wounds treated with the nanocrystalline silver dressing or the untreated controls. However, neither dressing differed significantly from the untreated control in biofilm bacterial counts at any point.
"It is important to consider both in vitro and in vivo studies when developing antimicrobial dressings or agents," says Prof. Davis.
Disclosure: Prof. Davis reports no conflicts of interest.