Assessment of silver-containing gelling fiber dressings

Introduction

Dressings play a key role in wound care by providing the optimal conditions for healing through bioburden control and exudate management. Wound dressings containing a source of ionic silver—a broad-spectrum antiseptic—are commonly used to manage wound bioburden. However, in hard-to-heal wounds microorganisms largely exist in biofilm form, making them more difficult to manage than those in planktonic form.¹ Biofilm has long been implicated as a barrier to healing of hard-to-heal wounds. Microorganisms in biofilm are encased in a matrix of extracellular polymeric substances and have slower replication and metabolic rates, thus providing strong defense against the host immune system as well as antibiotics and antiseptics. There are several dressings available containing fibers that gel on absorption of wound fluid. Gelling of such dressings enables absorption of large amounts of exudate and micro-contouring to the wound bed, reducing dead spaces where microorganisms can grow. While maintaining a moist wound environment is considered best practice, establishing the right balance is crucial as excessive moisture can also result in damage (eg maceration of peri-wound skin).

There are a number silver-containing gelling fiber dressings available that provide both management of exudate and bioburden control. The gelling properties, as well as the silver content and form, can differ substantially in such dressings. Several in vitro studies have been performed to assess the antibiofilm activity of various wound dressings; however, the methods used vary in robustness and validity, making it difficult to differentiate between the dressings. Given the challenge of biofilm combined with the emergence of antibiotic-resistant pathogens and increasing concerns around antimicrobial stewardship, there is a need for stringent and more realistic models to differentiate the ability of antimicrobial dressings to reduce biofilm.

The aim of this in vitro study was to evaluate the effectiveness of a range of silver-containing gelling fiber wound dressings against antibiotic-resistant biofilm bacteria in a stringent, robust biofilm model.

Materials and Methods

Test Dressings

Four dressings were evaluated to compare their in vitro antibiofilm activity:

• Carboxymethylcellulose (CMC) dressing containing ionic silver, ethylenediaminetetraacetic acid (EDTA) and benzethonium chloride (BEC) (“CISEB”; Aquacel^® Ag+ Extra™, Convatec Ltd., Deeside, UK)
• CMC dressing containing silver oxysalts (Ag³⁺) (“CSO”; KerraCel^® Ag, 3M, Bracknell, UK)
• Polyacrylate (polyabsorbent) fiber dressing with an acrylic core and silver sulphate (‘PSS’; UrgoClean Ag, Urgo Medical Ltd., Loughborough, UK)
• Non-woven polyvinyl alcohol fiber dressing containing silver sulphate (‘PVASS’; Exufiber^® Ag+, Mölnlycke Health Care Ltd., Milton Keynes, UK)

Preparation of Biofilm Model

Separate suspensions of each challenge organism, extended spectrum beta lactamase-producing antibiotic-resistant Pseudomonas aeruginosa (RPA; NCTC 13437, National Collection of Type Cultures, Salisbury, UK) and community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA; USA300; HPA Reference: H045260142, Health Protection Agency, Salisbury, UK), were prepared in maximum recovery diluent (MRD; Neogen Corporation, Lansing, MI, USA) and adjusted to yield a concentration of approximately 1×10⁸ colony forming units (CFU)/mL. An inoculation medium was prepared by diluting 0.1 mL of the bacterial suspension in 9.9 mL of 50:50 v/v tryptic soy broth (Neogen Corporation, Lansing, MI, USA) and fetal bovine serum (FBS, Biowest, Nuaillé, France) in sterile 100 mL Duran bottles.

Samples of a sterile knitted viscose gauze (N-A^® Gauze, 3M, Bracknell, UK) were aseptically prepared (40 mm diameter) and transferred into the Duran bottles containing the inoculation medium. The bottles were then incubated at 35°C (± 3°C) for 48 hours in a shaking incubator set at 150 rpm. Following incubation, the biofilm-gauze samples were washed in 0.85% saline (2×100 mL volumes) and cut to a uniform size (35 mm diameter) using a biopsy punch. A total viable count (TVC) was then performed on biofilm-gauzes to confirm the level of biofilm (T_0hr count).

A series of tryptic soy agar (TSA) contact plates were inserted into the center of separate simulated wound assemblies (Figure 1). Biofilm-gauzes were then individually transferred onto each TSA contact plate to simulate a biofilm-colonized wound bed (n=3 per dressing for each time point).

Figure 1 Simulated wound biofilm model with CISEB and secondary transparent film dressing application within the wound assembly (A) and following removal of dressing for enumeration of surviving biofilm on the gauze (B). Abbreviations: CISEB,…