Microbicidal Properties of a Silver-Containing Hydrofiber Dressing Against a Variety of Burn Wound Pathogens

Microbicidal Properties of a Silver-Containing Hydrofiber Dressing Against a Variety of Burn Wound Pathogens P. G. Bowler, MPhil, S. A. Jones, BSc, M. Walker, PhD, D. Parsons, PhD Partial-thickness burns are often characterized by microbial contamination and copious exudate produced during the early postburn period. Consequently, topical wound management often relies on the use of antimicrobial agents and absorbent dressings, and an AQUACEL Hydrofiber Dressing containing ionic silver has been designed to meet such needs. To assess the antimicrobial properties of the AQUACEL Hydrofiber dressing, samples were challenged with a wide variety of recognized burn wound pathogens in a simulated wound fluid model. Dressing samples were inoculated with the challenge organisms at time zero and then reinoculated on days 4 and 9 to mimic the worst-case clinical scenario. The dressing was shown to be microbicidal against aerobic and anaerobic bacteria (including antibiotic-resistant strains), yeasts, and filamentous fungi during a 14-day test period. Based on our results, the silver-containing dressing is likely to provide a barrier to infection, in addition to providing proven fluid-handling benefits of the AQUACEL Hydrofiber dressing, in the management of partial-thickness burns. (J Burn Care Rehabil 2004;25:192 196) Skin provides a physical barrier between the host and the external environment and is critical to the regulation of heat and moisture and to the exclusion of microorganisms from sterile tissues within the body. Consequently, the loss of significant areas of skin after thermal injury dramatically affects the physiologic status of the host, allowing microbial contamination of moist devitalized tissue. Wound infection and sepsis are major causes of mortality in the burned patient, and these are associated primarily with the loss of epidermal barrier function that facilitates colonization of normally sterile tissues by endogenous microorganisms. A correlation among anatomic site, burn wound microflora, and local endogenous flora has been observed previously. 1 The microbiology of burns is known to be complex, and a wide variety of microorganisms have been implicated in wound infection. Aerobic bacteria, From ConvaTec Global Development Centre, Deeside, United Kingdom. The authors confirm that they have an institutional financial interest in the dressing (medical device) described in this article. Address correspondence to P. G. Bowler, ConvaTec Global Development Centre, First Avenue, Deeside Industrial Park, Deeside, Flintshire, United Kingdom CH5 2NU. Copyright 2004 by the American Burn Association. 0273-8481/2004 DOI: 10.1097/01.BCR.0000112331.72232.1B such as Pseudomonas aeruginosa and Staphylococcus aureus, have long been recognized as major burn wound pathogens, 2,3 and emerging resistant strains of several types of bacteria are exacerbating the threat posed by these organisms. Anaerobic bacteria are also prevalent in burn wounds 3,4 and Bacteroides spp. has been shown to play a significant role in burn wound sepsis. 3 Yeasts (predominantly Candida spp.) and filamentous fungi (predominantly Aspergillus spp.) are also potentially pathogenic in burn wounds, 3,5 7 and in studies by Mousa, 3,5 C. krusei and A. niger were the most prevalent species of these organisms in infected burn wounds. Infections with these organisms have been observed with the use of porous (gauzetype) dressings, 3,6 and in view of this, the use of barrier dressings that prevent the penetration of environmental and endogenous pathogens is important from an infection-control perspective. To minimize the opportunity for wound infection (and subsequent secondary deepening) during the early postburn period, topical antimicrobial agents are widely used. The discovery of silver nitrate as an antiseptic agent coincided with the advances in bacteriology towards the end of the 19th century 8 and has since played an important role in the treatment of burns. During the 1960s renewed interest in the use of silver in burns led to the development of other topical antimicrobial agents and, in particular, silver 192

Volume 25, Number 2 Bowler et al 193 sulfadiazine (SSD) has maintained a lead role in minimizing the risk of burn wound infection. SSD cream 1% is normally applied to burned tissue once or twice daily until spontaneous re-epithelialization has occurred or until surgical intervention (excision and/or grafting) is performed. Because SSD treatment is frequent, the trauma to the patient associated with daily removal of partially dried cream (and gauze dressings) and the labor for the burn care practitioner are significant. To ensure that the early management of infected or at-risk partial-thickness wounds is minimally traumatic and minimally burdensome to both the patient and the practitioner, interference with and topical treatment of the wound need to be kept to a minimum. Consequently, a key requirement in burn wound management is for a topical formulation that can provide an antimicrobial environment for periods of up to 14 days, thereby eliminating the need for frequent dressing changes. This requirement has been met by an ionic silver-containing Hydrofiber dressing (SHD; AQUACEL Hydrofiber dressing, ConvaTec, a Bristol-Myers Squibb Company, Princeton, NJ) that is also able to effectively manage exudate, which is particularly important during the early stages of wound repair. To determine the likely clinical performance of the SHD in burn wound management, the efficacy and spectrum of activity of the dressing was investigated during a 14-day period (simulating maximum clinical use) against a variety of burn wound pathogens (including antibiotic-resistant bacteria) in a simulated wound fluid model. METHODS A variety of recognized burn wound pathogens were used as follows: Methicillin-resistant S. aureus (clinical isolate), vancomycin-resistant Enterococcus (clinical isolate), Pseudomonas aeruginosa (NCTC 8506, multiresistant strain), Serratia marcescens (multiresistant clinical isolate), B. fragilis (NCTC 9343), C. krusei (NCPF 3876), and A. niger (NCPF 2275). Before use, all organisms were stored in cryovials (Microbank; Pro-Lab Diagnostics, Ontario, Canada) at 70 C. Porous beads from each cryovial were transferred to appropriate agar culture media, and the plates were then spread in standard fashion. Plates were incubated under appropriate conditions (bacteria at 35 3 C, [B. fragilis was also incubated under anaerobic conditions], yeasts and fungi at 20 25 C) until adequate growth was observed. Representative colonies of each organism were then harvested in Maximal Recovery Diluent (MRD, Lab M, Bury, United Kingdom) and then diluted out to a concentration of approximately 1 10 7 colony-forming units (cfu)/ml using a Jenway 6051 Colorimeter (Essex, United Kingdom). The growth of each microorganism was measured in the presence of a SHD (AQUACEL Hydrofiber ) dressing and a control dressing that did not contain silver (HD; AQUACEL Hydrofiber dressing) during a period of 14 days. To best determine how effectively the antimicrobial dressing is likely to perform clinically, a simulated wound fluid model was prepared that enabled the measurement of activity during the maximum time period that a dressing is likely to be in situ. Because components of wound fluid (eg, proteins, ions) are likely to interfere with antimicrobial agents such as silver, the simulated wound fluid consisted of fetal calf serum (First Link (UK) Limited, West Midlands, United Kingdom) and MRD at a 50/50 vol/vol ratio concentration. A 1-ml volume of each challenge organism (prepared in MRD) was inoculated into separate 9-ml volumes of simulated wound fluid to achieve a final population density of approximately 1 10 6 cfu/ml. Total viable counts were performed in duplicate on each microbial suspension to confirm inoculum levels. A sample of SHD (0.25 g) was then aseptically placed into each 10-ml inoculated simulated wound fluid sample. A negative control dressing for each microorganism (HD) was also included. Each test sample was then incubated under appropriate, as described previously, and total viable counts were performed at regular time points throughout the test period. From each test sample (which had previously been gently shaken without causing aeration), 100 l of simulated wound fluid was removed and transferred to 9.9 ml of MRD-containing sodium thioglycollate 0.1% (to neutralize residual silver activity). Serial 10-fold dilutions were made with the same diluent, and using appropriate dilutions, 1-ml volumes were prepared for culture using a pour-plate technique (for aerobic bacteria, Tryptone Soy Agar, Lab M, was used; for yeasts and fungi, Sabouraud Dextrose Agar, Lab M, was used). Each dish containing molten agar was gently swirled before solidifying and then incubated under appropriate conditions for 72 hours. Duplicate total viable counts were performed on each test suspension on days 1 (4 and 24 hours), 2, 3, 4, 7, 9, 11, and 14. Because of the fact that a fastidious anaerobe agar containing blood was required for the growth of B. fragilis, a surface inoculation procedure was used in preference to a pour-plate procedure. Surface inoculation was also used for A. niger because growth in-

194 Bowler et al March/April 2004 volves the spreading of hyphae and spore formation on the agar surface. After incubation, cfus on each plate were counted, results were averaged, and a population density was calculated for each dressing/microorganism/time point combination. At the day 4 and day 9 time points, each test sample was reinoculated with approximately 1 10 6 cfu of the original microorganism to measure the sustained antimicrobial efficacy of the dressing against heavy microbial populations, as is likely to occur in wounds. The test (SHD) and control (HD) dressings were tested on five separate occasions against each microorganism, and results were averaged. RESULTS SHD sustained microbicidal activity against a wide range of wound pathogens during a period of at least 14 days in the simulated wound fluid model (Figures 1 4). The populations of P. aeruginosa, S. marcescens, and B. fragilis declined to less than 0.01% of their initial inoculum concentration within the first 24 hours. A. niger was the least susceptible microorganism, although a 90% reduction in population was still observed within the first 24 hours. Sustained microbicidal activity was observed after reinoculation of challenge organisms on days 4 and 9. These data demonstrate that a single piece of the SHD was capable of maintaining a sufficient level of ionic silver in the simulated wound fluid for 14 days to kill and prevent regrowth of a variety of wound pathogens. DISCUSSION Topical antimicrobial agents can play an important role in the management of burn wounds by minimizing microbial growth on and within devitalized tissue (eschar) and hence preventing subsequent invasion, infection, and secondary deepening in underlying tissue. Because of the fact that burn wounds are often polymicrobial, often involving a diversity of potential pathogens, 1 3 it is important that broad-spectrum topical antimicrobial agents are used to minimize the risk of infection. Silver is one such agent that is effective against a variety of burn wound pathogens and consequently has played a prominent role in burn care during the last 50 years. Historically, silver has been presented as metallic (silver foil), solution (eg, silver nitrate) and cream (eg, SSD) formulations, but more recently silver-containing dressings have also become available. Silver is a noble, nonreactive metal that demonstrates antimicrobial activity only when it converts, by oxidation, to the ionic form (ie, silver atoms that carry a positive electrical charge, Ag ). Although the form of antimicrobial silver (ie, Ag ) is the same irrespective of the topical (carrier) formulation, the amount and availability of Ag is critical to the efficacy of the formulation, and the delivery vehicle largely dictates this. For example, silver nitrate solu- Figure 1. The antimicrobial efficacy of silver-containing Hydrofiber dressing against Serratia marcescens and Pseudomonas aeruginosa. S. marcescens reinoculated at 1.2 10 6 cfu (R1) and 1.9 10 6 cfu (R2). P. aeruginosa reinoculated at 8.3 10 5 cfu (R1) and 1.9 10 6 cfu (R2).

Volume 25, Number 2 Bowler et al 195 Figure 2. The antimicrobial efficacy of silver-containing Hydrofiber dressing against Methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus. Methicillin-resistant Staphylococcus aureus reinoculated at 6.4 10 5 cfu (R1) and 2.1 10 6 cfu (R2). Vancomycin-resistant Enterococcus re-inoculated at 6.5 10 5 cfu (R1) and 1.8 10 7 cfu (R2). Figure 3. The antimicrobial efficacy of silver-containing Hydrofiber dressing against Bacteroides fragilis. Bacteroides fragilis reinoculated at approximately 6.4 10 5 cfu (R1) and 5.2 10 5 cfu (R2). tion 0.5% delivers a large dose of silver (3,175 g/ ml 9 ) immediately upon application, and there is no controlled delivery. Also, a water-soluble mafenide acetate formulation (one of the first-developed agents of this kind) is considered to be more effective than SSD cream in penetrating eschar and is therefore more effective in dealing with organisms thriving in such tissue. 7 Consequently, the selection of an appropriate delivery vehicle is very important with respect to optimal availability of the antimicrobial agent. In our in vitro studies, the SHD dressing was shown to be microbicidal against a wide range of

196 Bowler et al March/April 2004 Figure 4. The antimicrobial efficacy of silver-containing Hydrofiber dressing against Candida krusei and Aspergillus niger. Candida krusei reinoculated at approximately 1.1 10 5 cfu (R1) and 1.3 10 5 cfu (R2) Aspergillus niger reinoculated at approximately 1.9 10 5 cfu (R1) and 2.7 10 5 cfu (R2). recognized burn wound pathogens during a period of at least 14 days. This indicates that the dressing is able to provide a sustained low-but-efficacious amount of ionic silver in the dressing while in situ (ie, no immediate dose-dumping effect from the dressing vehicle and hence no requirement for frequent dressing changes to maintain the antimicrobial effect). Taking into account both the sustained antimicrobial and exudate-management properties of this dressing, the need for frequent and regular dressing changes is unlikely, thereby providing the best opportunity for healing to progress undisturbed while minimizing pain and trauma for the patient and treatment time and costs for the caregiver. CONCLUSIONS The silver-containing AQUACEL Hydrofiber dressing has been shown to provide excellent antimicrobial activity against a wide variety of burn wound pathogens (including antibiotic-resistant bacteria) in a simulated wound fluid model, reducing high and repeated inoculations of each microorganism to nondetectable levels during a 14-day test period. The combination of ionic silver with the unique physical and chemical characteristics of AQUACEL Hydrofiber dressing is likely to provide both a barrier to infection and fluid-handling benefits in the management of partial-thickness burns. REFERENCES 1. Brook I, Randolph JG. Aerobic and anaerobic bacterial flora of burns in children. J Trauma 1981;21:313 8. 2. Vindenes H, Bjerknes R. Microbial colonization of large wounds. Burns 1995;21:575 9. 3. Mousa HA-L. Aerobic, anaerobic and fungal burn wound infections. J Hosp Infect 1997;37:317 23. 4. Murray PM, Finegold SM. Anaerobes in burn wound infections. Rev Infect Dis 1984;6:S184 6. 5. Mousa HA-L, Al-Bader SM. Yeast infections of burns. Mycoses 2001;44:147 9. 6. Mousa HA. Fungal infection of burn wounds in patients with open and occlusive treatment methods. East Med Health J 1999;5:333 6. 7. Howard PA, Cancio LC, McManus AT, Goodwin CW, Kim SH, Pruitt BA. What s new in burn-associated infections? Curr Surg 1999;56:397 405. 8. Klasen HJ. Historical review of the use of silver in the treatment of burns. I. Early uses. Burns 2000;26:117 30. 9. Yin HQ, Langford R, Burrell RE. Comparative evaluation of the antimicrobial activity of ACTICOAT antimicrobial barrier dressing. J Burn Care Rehabil 1999;20: 195 200.