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In-vitro Comparison of Antimicrobial Efficacy of Various Wound Dressing Materials
Abstract:
Many new antimicrobial dressings have been used for the treatment of or protection against wound infection since the early 1980s. There are many different wound dressings such as silver, povidone iodine, and chlorhexidine impregnated materials on the market today. Various manufacturers assert that their dressings are the most effective and therefore should be preferentially employed. However, it is difficult to find a study that clearly identifies the most effective antimicrobial wound dressing. Methods. Eight different commercially available wound dressing materials were compared in terms of their antimicrobial effectiveness on 18 different microorganisms via disk diffusion test (Kirby-Bauer Method) on Mueller-Hinton (MH) agar. Among the 16 bacterial and 2 yeast species grown on MH agar plates, Contreet was the most effective antimicrobial dressing tested (P < 0.001). Results. A statistical difference was not found concerning efficacy against gram-positive and -negative bacteria among any of the materials except for Aquacel Ag and Inadine, the activity of which were found to be higher on gram-positive bacteria compared to gram-negative bacteria (P = 0.029, P = 0.030). In-vitro data suggest that Contreet is the most effective for topical treatment of contaminated wounds. Conclusion. Further methods of assessment, including the use of infected animal models and clinical studies, will be necessary to better understand the antimicrobial efficacy of these dressings.
Exposed subcutaneous tissue provides a favorable medium for microorganisms to contaminate and colonize. The conditions become optimal for microbial growth if the involved tissue is devitalized and/or the host immunity is compromised. This situation presents a considerable problem for both the patient and caregiver. Oral and systemic antibiotics are often used to control infection. While systemic antibiotheraphy is mandatory for advanced skin infections, from a basic wound management perspective, the three methods of eliminating a local wound infection are debridement, wound cleansing, and topical antimicrobial application.1,2 Many different topical antimicrobial agents are available such as silver, povidone iodine, chlorhexidine, and honey.
Silver has been used as an antimicrobial for thousands of years either as liquid, cream, or more recently, silver-coated wound dressings. Its antimicrobial activity is attributed to its ability to block the transmembranous energy metabolism in bacteria.3 Chlorhexidine is a persistent antimicrobial agent that is active against a wide range of gram-positive and gram-negative bacteria. It has been widely used for surgical hand and skin antisepsis. Povidone iodine is another commonly used antimicrobial agent, which has been demonstrated to be effective in killing a broad range of the microorganisms typically associated with wound infection.4 The antimicrobial properties of honey in relation to wound management date back to ancient times. Recently, studies have been published regarding the antimicrobial activity of honey against methicillin-resistant Staphylococcus aureus (MRSA) and Psuedomonas aeruginosa.5,6
Advancements in science and technology have spawned significant improvements in wound dressings. Some newer dressings incorporate antimicrobials and overcome the disadvantages associated with topical antimicrobial application. Currently, various different antimicrobial wound dressings are available on the market. However, studies that clearly identify which materials are superior do not exist. The aim of the present study was to compare the antimicrobial properties of different commercially available wound dressing materials.
Materials and Methods
Organisms. Eighteen microorganisms were evaluated in this study and all clinical isolates plus one standard strain S aureus ATCC 25923 were collected from routine clinical species submitted to the Mersin University Hospital (Mersin, Turkey). All isolates were identified morphologically and biochemically by a standard laboratory procedure. The following species were used in the experiments: extended spectrum beta-lactamase (ESBL), producing and non-producing Escherichia coli, Acinetobacter baumannii (carbapenem resistant), Stenotrophomanas maltophilia, ESBL producing and non-producing Klebsiella pneumonia, Pseudomonas aeruginosa, Enterobacter cloacae, methicillin-sensitive Staphylococcus aureus (MSSA), methicillin-resistant Staphylococcus aureus, (MRSA), methicillin-sensitive coagulase-negative staphylococci (MSCNS), methicillin-resistant coagulase-negative staphylococci (MRCNS), penicillin susceptible enterococci, vancomycin resistant enterococci (VRE), and two strains of a Candida albicans.
Dressing materials. The wound dressings tested were: Bactigras® (Smith and Nephew, Hull, UK), Inadine® (Systagenix, North Yorkshire, UK), MelDra® (Dermagenics BV, The Netherlands), Textus® (Biocell, Engelskirchen, Germany), Acticoat® (Smith & Nephew, St. Petersburg, FL), Aquacel Ag® (ConvaTec, Skillman, NJ), Contreet® (Coloplast, Humlebaek, Denmark), Ultec™ (Covidien, Mansfield, MA). Ultec was used as a control (Table 1). All dressing materials were cut into 4-mm diameter discs under sterile conditions.
Methods. The antimicrobial efficacy of the wound dressings was evaluated against various microorganisms using the zone of inhibition test. The tested organisms were sub-cultured on agar plates with 5% blood over an 18-hour period after which a suspension representing approximately 106 to 107 CFU/mL (0.5 McFarland) was prepared. The dressing discs were placed on the inoculated Mueller-Hinton (MH) agar plates and observed for the zone of inhibition around the dressings (Figure 1). Plates were incubated for 24 hours at 37˚C. A metric ruler was used to measure the diameter of the zone of inhibition for each disc used (Kirby-Bauer method).
Statistical Analysis
SPSS for Windows (SPSS, Inc., Chicago, IL) software was used for data management and statistical analysis. The zone of inhibition diameter from each dressing disc was compared using a paired sample test. Each dressing material was analyzed using a chi-squared test in regards to efficacy against the different microorganisms. The level of significance was set at 0.05 for all statistical tests.
Results
The present study included 9 gram-negative, 7 gram-positive bacteria, and 2 yeast isolates. An inhibition zone was not detected for Textus and Ultec (control dressing). Therefore, these materials were excluded from the statistical evaluation and regarded as most ineffective against microorganisms. The results for the size of the zones of clearance around the discs for all microorganisms on MH agar are shown in Table 2. Comparisons of the zones of inhibition are shown in Table 3.
Among the 16 bacterial and 2 yeast species grown on MH agar plates, Contreet was the most effective antimicrobial dressing tested (P < 0.001). Textus and Ultec were found to be ineffective. The bactericidal activities of Acticoat, Aquacel Ag, and Bactigras were similar and more effective than Inadine and MelDra (P < 0.05). MelDra had the most variable bactericidal activity. It was interesting that the bactericidal activity of MelDra was more rapid and pronounced against MRSA than it was against MSSA.
A statistical difference was not found among any of the dressings concerning efficacy against gram-positive and gram-negative bacteria, except for Aquacel Ag and Inadine. Both Aquacel Ag and Inadine were effective against both gram-positive and gram-negative microorganisms, yet their antibacterial activity was found to be higher on gram-positive bacteria compared to gram-negative bacteria (P = 0.029, P = 0.030; [Figures 2 and 3]).
Discussion
Intense bacterial colonization is commonly associated with acute and chronic wounds. Several factors determine whether wounds colonized with microorganisms will become infected. These factors include the amount of bacteria per gram of tissue, species of microorganism, and the host immune response. Colonization can be defined as the presence of replicating microorganisms in the wound that does not damage tissue. Colonization alone does not delay the wound healing process.
Wound infection is characterized by an invasion of microorganisms into the tissue (more than 105 organisms per gram of tissue) and has clinical signs and symptoms such as hyperemia, fever, pain, and cellulitis. An intermediate stage, critical colonization, is increasingly being recognized as part of the wound infection process. This is a relatively new description and refers to a transitional state between colonization and invasive wound infection. In this situation, granulation tissue is locally infected without any clinical signs of infection. Both wound infection and critical colonization can interfere with the healing process. Therefore, preventing the progression from colonization to infection is a major concern in wound care.7
The primary treatment of a wound infection is copious irrigation with normal saline, drainage, debridement, and appropriate antibiotic therapy. Topical antimicrobial agents such as silver, povidone iodine, and chlorhexidine have been widely used in the presence of local infection (critical colonization) after meticulous debridement of all non-viable tissue. Bacterial biofilm formation is especially problematic since it is associated with resistance to host defenses and renders microorganisms more resistant to topical antimicrobials. Surgical removal of the biofilm layer is essential to facilitate the effects of topical antimicrobials and to eliminate local infection. More recently, several wound dressing materials with varying antimicrobial properties have become available for clinical use. The considerable variations in the structure, composition, and antimicrobial content of these contemporary dressings make the selection of an appropriate dressing more difficult. Numerous studies have evaluated the advantages of using these modern dressings.8–10 However, a comprehensive study has yet to clearly identify which antimicrobial dressing materials are superior.
In the present study, inhibition zone sizes were used to compare the antimicrobial efficacy of various dressing preparations. Inhibition zone sizes in this in-vitro test are dependent upon two factors: the ability of the antimicrobial agent to diffuse into the agar, and the antimicrobial activity of the agent against the microorganism. The diffusion kinetics of the tested antimicrobial agents is not the same in the agar gel—this might be regarded as a limitation of the study design. However, the diffusion kinetics are also not equal in vivo on actual wounds among these agents. To standardize the study design, all of the dressings were tested in the same agar plate.
Contreet is a newer-generation silver-based dressing that contains 13 µg silver per cm2 that provides sustained release of ionic silver to exert an antibacterial effect within the wound.11 Ip et al12 demonstrated that Contreet provided maximal bactericidal activity for Enterococcus faecalis, Enterobacter cloacae, Proteus vulgaris, P aeruginosa, and Acinetobacter baumanii. Castellano et al13 reported similar findings in 2007 when they compared eight silver containing dressings to determine their in-vitro antimicrobial efficacy. Their findings suggest that Contreet was the most effective dressing.
In the present study when Contreet was compared with other dressing materials, it displayed a superior antimicrobial activity on all microorganisms tested (P < 0.001). Significant differences were not detected concerning the bactericidal activities of Contreet against gram-negative and gram-positive bacteria.
Acticoat is another silver-containing wound dressing that utilizes nanotechnology to release nanocrystalline silver crystals.12 Many studies have reported that this material had strong bacterial inhibitive properties and reduced wound infection. Heggers et al3 compared the efficacy of Acticoat, Silvasorb™ (Medline, Mundelein, IL), and Silverlon® (Argentum Medical, Chicago, IL) against S aureus and P aeruginosa infections. Heggers et al found that these silver dressings were equally effective in reducing the microbial colonization in wounds.3
In the present study, Acticoat had the second most obvious inhibition zone. However, when it was compared with the Aquacel Ag and Bactigras, a statistically significant difference was not found (P > 0.05).
Aquacel Ag is a silver-impregnated dressing with hydrofiber (sodium carboxymethylcellulose) and contains 21 µg ionic silver per cm2.11 This dressing material had a mean inhibition zone of 9.61 mm ± 4.15 mm among the tested microorganisms in the present study. Although Aquacel Ag was proven to have a moderate antibacterial activity against the tested microorganisms, this activity was not the strongest among the tested dressing materials. Antibacterial activity of Aquacel Ag was higher for gram-positive bacteria compared to gram-negative bacteria.
The in-vitro antimicrobial efficacy of Textus silver dressing was found to be inefficient. Textus did not have a remarkable inhibition zone for any of the microorganisms tested.
Bactigras is a dressing made of cotton impregnated with 0.5 % chlorhexidine acetate. Chlorhexidine is known as a persistent antimicrobial agent that is active against a wide range of gram-positive and gram-negative bacteria. In the present study it was observed that Bactigras was as effective as some of the silver-based dressings (Acticoat and Aquacel Ag). A significant difference was not found in terms of its effectiveness against gram-negative or gram-positive bacteria.
Inadine uses povidone-iodine as its antimicrobial agent. Povidone-iodine has been demonstrated to be effective at killing a wide spectrum of microorganisms generally associated with wound infection. In the present study, although it demonstrated potential to reduce microorganism growth, it was unsatisfactory compared to the silver impregnated dressings.
MelDra contains medical honey and had the most variable bactericidal activity. Interestingly, its killing effect was more evident against MRSA than MSSA.
Conclusion
In-vitro data suggest that Contreet is the most effective dressing for the topical treatment of contaminated wounds. In the present study, a diffusion technique was used because it is known that antimicrobial agents within dressings exert their effect through direct contact with the microorganisms via diffusion. The diffusion kinetics of the tested antimicrobial agents is not the same in the agar gel, which might be regarded as a limitation of the study design. However, the diffusion kinetics among these agents is also not equal to in vivo clinical wounds. To standardize the study design, all of the dressings were tested in the same agar plate. However, further methods of assessment, including the use of infected animal models and clinical studies, will be necessary to better understand the antimicrobial efficacies of these dressings.
References
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