Antimicrobial Activity of Silver-Containing Dressings is Influenced by Dressing Conformability with a Wound Surface
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The SCH dressing is made of the hydrocolloid polymer carboxymethylcellulose to which silver ions are attached.14 The dressing fibers absorb wound fluid, swelling to form a soft cohesive gel that covers the wound surface.15 This moist gel mass is kept in direct contact with the wound bed leaving little or no space between the dressing and the wound.16 The ionic silver in the dressing has been shown to provide broad-spectrum antimicrobial activity for up to 14 days.11
In contrast, the NSC dressing consists of 1 layer of a rayon/polyester inner core sandwiched between 2 layers of polyethylene net.17 This net is coated with nanocrystalline silver that is released when water contacts the dressing.18 Studies have shown that silver release from NSC dressings is faster and is delivered more rapidly than from either silver nitrate or silver sulfadiazine dressings.19 The NSC dressing provides an effective antimicrobial barrier for up to 7 days.17
This study used in-vitro models to assess the conformability of SCH and NSC dressings to unevenly contoured wound-like surfaces and to examine the degree to which this correlated with antimicrobial effect against 2 common wound pathogens, Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus.
Dressing contact with wound tissue and simulated dry eschar. Small sections of wound tissue obtained from electively amputated lower limbs and simulated dry eschar (human dermis, approximately 10 mm x 4 mm, dried overnight at 37oC) were fixed in a vertical plane on a freshly cleaned microscope slide using a cyanoacrylate adhesive (LOCTITE® 4062 instant adhesive, Loctite, Hertfordshire, UK). The Ethics Committee of North East Wales Trust, Wrexham Maelor Hospital, UK, granted ethical approval to allow human skin to be used from lower limb amputations.
A dry piece of each silver-containing dressing (approximately 10 mm x 4 mm) was then placed carefully onto the upper tissue surface. Glass inserts were placed at either end of the microscope slide to allow a second slide to be placed on top of the tissue and dressing to create a sandwich effect. The ends of the combined microscope slides were then clamped together using bulldog clips, allowing the slide to be placed horizontally onto a microscope stage (Wild Heerbrug, Germany). Images were captured using an attached digital camera (Polaroid DCM 1e, Polaroid Corporation, Waltham, Mass, USA).
Once the dry images had been captured, water was added to each dressing to ensure hydration to saturation point without causing a visible fluid leakage. The volumes used were up to a maximum of 100 µL for the NSC dressing and up to 400 µL for the SCH dressing. Water was added through the gap between the slides to allow observation of the dressing/tissue interaction in the hydrated state.
Dressing contact with an inoculated indented agar surface. Standard reference strains of methicillin-resistant Staphylococcus aureus (MRSA) (NCTC 12232) and antibiotic-resistant Pseudomonas aeruginosa (NCTC 8506) were used to investigate the effect of dressing conformability on antimicrobial efficacy for a SCH dressing and a NSC dressing.
An agar plate model was developed to investigate dressing conformability, ie, the degree to which each dressing maintained intimate contact with a surface-inoculated agar plate during a specified time period. Nonwoven, fabric-folded 4 cm x 4 cm swabs (Topper 8, Johnson & Johnson, Somerville, NJ, USA) were aseptically transferred to the center of standard, pre-dried tryptone soy agar plates (TSA, Lab M) and pressed onto the agar to enable direct contact without damage to the agar surface. A 15-mL volume of molten TSA (precooled to approximately 40oC) was then poured over each swab-impregnated plate to create a second 2–3 mm layer of agar.
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