Antimicrobial efficacy of a novel silver hydrogel dressing compared to two common silver burn wound dressings: Acticoat™ and PolyMem Silver®
Introduction
The increase of multi-resistant bacterial strains and fungal infections in burn wounds requires the development of new burn wound medications. Various silver products have become efficient alternative agents in burn care instead of antibiotics [1] and there are a number of studies reporting their efficacy against a broad range of micro-organisms [2], [3], [4]. There are several mechanisms by which silver ions and their salts have antimicrobial effects. Silver is believed to interact with thiol groups of key respiratory enzymes [5] as well as interrupting the hydrogen bonding of microbial DNA [6] and it has been shown to damage bacterial cell membranes [7]. The powerful antimicrobial actions of silver nano particles (SNPs) have been previously reported [8], [9]. The decrease in size and increase in surface area of SNPs increases their antimicrobial efficacy, but the mechanism of action is unclear due to the difficulty in detection of the unstable silver nano form. Silver treatments used in burn care have also been reported to be toxic to human cells [10], [11]. The balance between the cytotoxicity and antimicrobial properties of each dressing should always be considered for each wound environment.
Previously, our research group has developed a novel silver hydrogel dressing which is composed of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) sodium salt and silver nano particles. Preliminary in vitro cytotoxic studies in mammalian cells indicate the novel dressing has less cytotoxicity compared to some commercial silver agents used in burn care.
This study compares the antimicrobial efficacy of the novel silver hydrogel dressing to two common dressings (Acticoat™ and PolyMem Silver®) used in our burn centre. The same silver hydrogel dressing that had been kept at room temperature for one year was also tested for efficacy. Three different antimicrobial assays were used, including; the disc diffusion method, broth culture and the Live/Dead® Baclight™ bacterial viability assay (Molecular Probes, Life Technologies, Australia).
Section snippets
Burn wound dressings
Novel burn wound dressings based on a 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS-Na+) hydrogel were created in our laboratories. Silver nitrate was added to a concentration of 5 mM in 40% AMPS sodium salt aqueous solution (pH 7.0 ± 0.1) in the presence of N, N′-methylenebis(acrylamide) (MBA) crosslinker. The mixture (pH 7.0 ± 0.1) was loaded into a nylon bag and exposed to gamma irradiation at 25 kGy to form silver nanoparticle-infused hydrogel. Neat hydrogel was similarly produced
Disc diffusion method
When assessed by the disc diffusion method, Acticoat™ had the most effective antibacterial activity, with an inhibition zone length of 13.9–18.4 mm (Table 3, p < 0.001 for all microbes). Its most effective inhibitory action was against C. albicans with an inhibition zone of 18.4 mm. Acticoat™ was the only treatment to show an inhibition zone (13.9 mm) for MRSA (Table 3). PolyMem Silver® had a minimal zone of inhibition for only MSSA (p < 0.001) and C. albicans (p < 0.001). Silver hydrogel and one year
Discussion
In this study, three different antimicrobial activity assays were used, each with their own advantages and disadvantages. The disc diffusion method is considered the easiest and cheapest method, however it requires 24 h of incubation to observe microbial colonies and differences in the diffusion and absorption properties of each silver dressing may lead to bias in the assay [14]. The broth culture method is considered to be the standard to measure antimicrobial activity [14] and it allows
Conclusion
In this study, all three antimicrobial activity assays determined that Acticoat™ had the most antimicrobial efficacy. PolyMem Silver® was found to be the poorest antimicrobial dressing and was found to promote the growth of VRE after 24 h. The novel silver hydrogel is an effective antimicrobial dressing and these results support the possibility of using the novel silver hydrogel as a burn wound dressing.
Conflict of interest
The authors Benjawan Boonkaew and Pitt Supaphol are involved in a patent application for the novel hydrogel. The other authors have no conflict of interest regarding the novel dressing.
Acknowledgements
The authors would like to acknowledge Ms. Kay A. Ramsay for supply of bacteria subcultures. The facilities for the microbiology work were provided by Queensland Paediatric Infectious Diseases Laboratory, Department of Infectious Diseases, Royal Children's Hospital. This work was supported by grants from Queensland Children's Medical Research Institute and the Children's Health Foundation Queensland, Brisbane, Australia.
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