ASSESSMENT OF A CADEXOMER IODINE-CONTAINING DRESSING AGAINST BIOFILM USING AN IN VITRO SIMULATED CHRONIC WOUND BIOFILM MODEL
Ruth Scully, Section Leader/Senior BMS, Electron Microscopy Unit, Cellular Pathology, Cardiff & Vale UHB, University Hospital of Wales; Dr Jan Hobot, Electron Microscopy Facility, Institute for Translation, Innovation, Methodology & Engagement (TIME), School of Medicine, Cardiff University; Mike Walker PhD, Independent Wound and Skin Care Consultant, Holywell, UK.
Planktonic microorganisms are known to be susceptible to treatment with antibiotics and antiseptics, but it is now well recognised that one of the leading causes of delayed wound healing and infection is due to the presence of biofilm (Metcalf & Bowler, 2013). Production of an extracellular matrix provides the microorganisms protection from external hostile influences such as antimicrobials and host defence mechanisms (Høiby et al, 2010; Leid, 2009). It has recently been suggested that cadexomer iodine may be an effective antimicrobial agent against biofilm using an in vitro model (Philips et al, 2015), and the aim of this study was to further assess this dressing technology in a challenging in vitro simulated chronic wound biofilm model.
A PET track-etched membrane cell culture insert in tissue culture wells was used as the biofilm substrate, as this has been previously shown to produce biofilm growth within 6 hours, which subsequently becomes established within a 24-48 hour period, as demonstrated by a variety of microscopic methods including light (LM), scanning (SEM) and transmission electron microscopy (TEM) (Scully et al, 2014). Each insert contained a thin slice cut from a piece of sterile pork belly, and a 100 µl volume of a mixed bacterial culture (a 50 µl volume of a 3.4x107 cfu/ml Pseudomonas aeruginosa NCIMB 8626 culture and 50 µl of a 3.4x107 cfu/ml Staphylococcus aureus NCIMB 9518 culture). Following application of the mixed culture, a cadexomer iodine dressing (Iodoflex™; Smith & Nephew) was added to each tissue culture well (Fig 1). Multiple plates were incubated with agitation at 37ºC for 3, 6, 12, 24, 48 and 72 hours to observe the effectiveness of the dressing in the prevention of biofilm development. At 24 hour intervals the external wells were flushed 1.5 ml of Tryptone Soya Broth to simulate wound exudate. Representative biofilm models and dressings at each time point were then examined by LM, SEM and TEM.
3 hours: Cadexomer iodine beads were observed by SEM on the simulated chronic wound surface (Fig 2A), releasing iodine in form of crystals (Fig 2B).
6 hours: It was observed under LM that some of the cadexomer beads were now detached from the dressing (Fig 3A) and appeared to providing a substrate for bacterial growth (Fig 3B). Similar results were observed with SEM, with less iodine now visible (Fig 4). It was observed that within a matter of hours the dressing appeared white and opaque, the disappearance of the orange colour suggesting iodine depletion, as has been observed clinically (Jenny Hurlow GNP CWOCN, personal communication).
24 hours: With SEM, biofilm was clearly visible, growing not only on individual beads, but also on small groups of beads with no iodine visible (Fig 5A-C). TEM provided observable evidence of bacteria between cadexomer beads (Fig 6A), with evidence of extracellular polymeric substances visible (Fig 6B).
48-72 hours: Biofilm continued to develop beneath the cadexomer iodine dressing, on the cadexomer beads, in the apparent absence of iodine (Fig 7).
Iodine has a long history of safe use as an antimicrobial agent in wound care. Iodine solutions and formulated dressings, such as povidone iodine and cadexomer iodine, are commonly used. However, their use in wounds at risk of infection due to the presence of biofilm should be carefully considered, as illustrated in recent clinical case studies where their use was previously reported to be ineffective (Walker et al, 2015; Metcalf et al, 2016). In this study, using a combination of microscopic techniques to examine biofilm development beneath a cadexomer iodine dressing in a complex, fluid-containing, simulated chronic wound biofilm model, the dressing was observed to lose effectiveness rapidly. The results presented suggest that the use of iodine-based products may not always be appropriate for long-term use if being used as part of a protocol of care to help reduce wound biofilm.