Abstract
Wound dressings containing silver as antimicrobial agents are available in various forms and formulations; however, little is understood concerning their comparative efficacy as antimicrobial agents. Eight commercially available silver‐containing dressings, Acticoat® 7, Acticoat® Moisture Control, Acticoat® Absorbent, Silvercel™, Aquacel® Ag, Contreet® F, Urgotol® SSD and Actisorb®, were tested to determine their comparative antimicrobial effectiveness in vitro and compared against three commercially available topical antimicrobial creams, a non treatment control, and a topical silver‐containing antimicrobial gel, Silvasorb®. Zone of inhibition and quantitative testing was performed by standard methods using Escherichia coli, Pseudomonas aeruginosa, Streptococcus faecalis and Staphylococcus aureus. Results showed all silver dressings and topical antimicrobials displayed antimicrobial activity. Silver‐containing dressings with the highest concentrations of silver exhibited the strongest bacterial inhibitive properties. Concreet® F and the Acticoat® dressings tended to have greater antimicrobial activity than did the others. Topical antimicrobial creams, including silver sulfadiazine, Sulfamylon and gentamicin sulfate, and the topical antimicrobial gel Silvasorb® exhibited superior bacterial inhibition and bactericidal properties, essentially eliminating all bacterial growth at 24 hours. Silver‐containing dressings are likely to provide a barrier to and treatment for infection; however, their bactericidal and bacteriostatic properties are inferior to commonly used topical antimicrobial agents.
Keywords: Antimicrobial agents, Quantitative culture, Silver dressings, Wound treatment
Introduction
The use of silver in the treatment of burns and chronic wounds has been used for centuries. The antimicrobial properties of silver made water potable as early as 1000 bc (1). Used in its solid form, silver nitrate is known in several languages by different terms. ‘Lunar caustic’, an English term, was derived in the Middle Ages from silver’s association with the moon. ‘Lapis infernalis’ in Latin and ‘pierre infernale’ in French are also descriptive terms for silver nitrate (2). There are different suggestions regarding the first use of silver nitrate, with most referring to the Middle Ages as the period of its beginning, while others refer to the discovery of silver nitrate in the 15th century by Basilius Valentinus (2). John Woodall published the use of Lapis infernalis in ‘The Surgeon‘s Mate’ in 1617, with uncertainty that his description was that of silver nitrate because he considered it the same as the ‘hard causticke stone’ used for opening abscesses (2).
In the 1700s, venereal diseases were treated with silver nitrate, as well as fistulae from the salivary glands, from bone and from perianal abscesses 2, 3. Although its astringent properties were recognised, silver nitrate was also considered an alternative to the cauterising iron (2). In the 19th century, silver nitrate was used to remove granulation tissue therefore allowing epithelization, and to promote crust formation on the surface of the wound for large surface wounds that healed by epithelization from the edges and contraction (2). Varying concentrations of silver nitrate were also used in the 19th century for the treatment of fresh burns. Ophthalmia neonatorum was prevented with the introduction of silver nitrate eye drops by Carl S.F. Credé in 1881 2, 3. Halsted reported the use of silver wire for suturing, covered by silver foil in a description of a hernia operation 2, 4. He referred to the inhibitory effect of silver foil on Staphylococcus aureus as well 2, 4. C.S.F. Credé’s son, B. Credé, created dressings with silver impregnated in them, the ‘white silver dressing’ which was wide‐mesh cotton in which silver foil was mounted, and the ‘grey silver dressing’ which was sterilised cotton with metallic silver powder, both of which were applied to skin grafts (2).
In 1967, Moyer revived the interest in silver nitrate as a treatment of burns with his introduction of continual irrigation of 0·5% silver nitrate solution 5, 6, 7. He reported that 0·5% silver nitrate aqueous solution does not interfere with epidermal proliferation that occurs with a 1% aqueous solution, yet affords the bacteriostatic properties against S. aureus, β‐hemolytic streptococci, Pseudomonas aeruginosa and Escherichia coli (4). The hypotonic silver nitrate solution can cause electrolyte disturbances, which Moyer describes as the sodium sink 5, 8, 9. Mafenide acetate, Sulfamylon®, was introduced by Lindberg, Moncrief and Mason modified from the mafenide hydrochloride which had been used widely during World War II in Germany as topical treatment of wounds 7, 8, 10. Although it has a broad antibacterial spectrum and an ability to penetrate the eschar, mafenide acetate was found to cause pain on application of partial thickness burns 6, 9. Because it is a carbonic anhydrase inhibitor, hyperpnea and hyperchloremic acidosis can develop (9). Despite these adverse effects, the 0·5% silver nitrate solution and Sulfamylon® became the mainstay of burn therapy until the introduction of silver sulfadiazine. Silver sulfadiazine, Silvadene®, was found effective as a topical antimicrobial treatment against Enterobacter, Klebsiella, E. coli, Proteus, Staphlococcus, Streptococcus and Pseudomonas, with some antifungal and antiviral properties, painless on application, and non irritating to tissues 6, 10.
The antimicrobial effects of silver products appear to be directly related to the amount and rate of silver released (3). Chemically, although silver in its metallic state is inert, when it interacts with moisture from the skin and with fluid from a wound, silver is ionised leading to antimicrobial effects (3). The ionised silver moity is highly reactive, binding to tissue proteins and causing structural changes in the bacterial cell wall and intracellular and nuclear membranes, ultimately leading to cellular distortion and loss of viability 3, 11. Additionally, the silver ion further exhibits its bacteriostatic properties by binding to and denaturing bacterial DNA and RNA, and thereby inhibiting bacterial replication (3).
Silver’s mechanism of action is attributed to its strong interaction with thiol groups present in cell respiratory enzymes in the bacterial cell. The mechanism of action of silver sulfadiazine is believed to be by binding to cell membranes and to the bacterial cell wall rather than by interacting with cellular DNA (8). Products such as Acticoat® release both silver ions and silver radicals, causing impaired electron transport, bacterial DNA inactivation and other cell membrane damage (3). Silver radicals are gaining favour as a mechanism of action because of their high potency for reactivity due to the presence of unpaired electrons in the outer orbits (3).
Infection is a leading cause of morbidity and mortality from extensive burn injury, traumatic injuries and surgical procedures 12, 13, 14, 15. Therefore, silver has drawn much attention as a treatment and prophylaxis for burn and wound care. Topical silver creams and solutions, and other topical antimicrobial preparations have a broad spectrum of antimicrobial activity, low development of resistance, few adverse reactions and a low risk of systemic toxicity, but require frequent application, are care intensive to apply and remove and are sometimes painful. In contrast to these topical silver agents, wound dressings containing silver as antimicrobial agents have been introduced in various designs by the wound care industry in recent years, however. The dressings are designed to control the release of silver to the wound allowing the dressings to be changed with less frequency. The purpose of this experiment is to comparatively evaluate eight silver dressings: Urgotul® SSD, Acticoat® 7, Acticoat® Moisture Control, Acticoat® Absorbent, Silvercel™, Contreet® F, Aquacel® Ag, Actisorb®, two silver agents: Silver Sulfadiazine and SilvaSorb® Gel, Sulfamylon® and Gentamicin Sulfate cream to determine their antimicrobial effectiveness against S. aureus, Streptococcus faecalis, P. aeuruginosa and E. coli.
Materials and methods
The eight silver‐impregnated dressings and topical silver antimicrobial agents used are described below.
Silver‐impregnated dressings
Acticoat® Absorbent (Smith and Nephew)
Acticoat® Absorbent uses the Silcryst™ Nanocrystals and consists of a silver‐coated calcium alginate fabric. The manufacturer states that there is sustained release of ionic silver activity, as well as the ability to absorb excess exudates via the alginate to form a gel maintaining a moist environment.
Acticoat® 7 (Smith and Nephew)
Acticoat® 7, like all Acticoat® products, contains Silcryst™ Nanocrystals. This dressing consists of two rayon/polyester non woven inner cores laminated between three layers of silver‐coated high density polyethylene mesh, designed to be the barriers against the bacterial invasion.
Acticoat® Moisture Control (Smith and Nephew)
Acticoat® Moisture Control also uses Silcryst™ Nanocrystals. This dressing consists of an absorbent three layer dressings of a nanocrystalline silver‐coated polyurethane layer, a white polyurethane foam layer and a blue waterproof polyurethane film layer. It also maintains a moist environment in the presence of exudate.
Aquacel® Ag (Squibb and Son, UK; ConvaTec)
The dressing contains 1·2% ionic silver in a Hydrofiber®, a fibrous non woven felt of sodium carboxymethylcullulose fibres. It absorbs moisture to form a gel, binding sodium ions and releasing silver ions (16). In addition, the manufacturer states that by utilising the Hydrofiber® technology, the dry Aquacel® Ag fibers gel on contact with wound fluid, locking exudates that contains bacteria away from the wound, as well as forming a cohesive gel that reduces dead space.
Urgotul® SSD (Laboratory Urgo, France)
This gauze dressing is coated in a Vaseline paste containing hydrocolloid and silver sulfadiazine particles (15).
ACTISORB® (Johnson and Johnson)
The ACTISORB dressing consists of an activated charcoal cloth impregnated with silver within a spun‐bonded nylon envelope. The manufacturer states that there is 220 mg silver per 100 g activated charcoal cloth, approximating 33 μg silver per square centimetre cloth. In addition, it is claimed by the manufacturer that the porous charcoal layer binds toxins and odour‐causing molecules.
Contreet® foam (Coloplast Corporation)
The Contreet® foam dressing is silver‐impregnated based on Biatain foam technology. The manufacturer states that it combines both sustained silver release and manages exudates, stating that the more exudate the wound produces the more silver released by the dressing.
Silvercel™ (Johnson and Johnson)
The Silvercel™ dressing is a non woven felt composed of calcium alginate carboxymethylcellulose fibres. It is blended with a metallic silver‐coated nylon fibre (15).
Topical silver antimicrobial agents
SilvaSorb® Gel (Medline)
SilvaSorb® is a microlattice synthetic hydrogel matix that contains silver chloride (15). It is claimed to maintain a moist environment and hold a significant amount of exudate due to its hydrogel matrix.
Silvadene® Cream 1% (King Pharmaceuticals)
Silvadene®, silver sulfadiazine, is a white, non staining anti‐infective cream.
Topical antimicrobial agent
Sulfamylon® (Mylan‐UDL Laboratories, Inc.)
Sulfamylon® cream is a white, non staining, anti‐infective cream containing mafenide acetate (10%).
Gentamicin Sulfate Cream 0·1% (Fougera)
Gentamicin sulfate is a white cream base 0·1% by weight.
Zone of inhibition determination
The disc diffusion test was used to determine the zone of inhibition and was performed in triplicate. Six‐millimetre diameter samples of each of the eight silver dressings and each of the topical antimicrobial silver agents were prepared. Each inoculation was adjusted to 0·5 McFarland turbidity standards to ensure reproducibility and prepared on unsupplemented Mueller–Hinton agar plates within 15 minutes of preparation of the inoculum in accordance with National Committee for Clinical Laboratory Standards recommendations (16). The accuracy of the adjusted density of the inoculum was verified using a spectrophotometer. Multiple 6‐mm diameter circular samples of each silver dressing and each topical agent were placed evenly at least 24 mm apart on the plates by using sterilised forceps within 15 minutes of inoculation of the agar plates. The prepared plates were incubated at 37°C in a humidified atmosphere of 5% carbon dioxide for 24 hours. The zone of inhibition was calculated by measuring the diameter to the nearest whole millimetre using sliding callipers of the inhibited growth around the dressing disk. Because the unsupplemented Mueller–Hinton agar was used, the measuring device was held to the inverted illuminated petri dish. Average zones of inhibition were calculated between the triplicate runs which showed close approximation.
Quantitative analysis of antimicrobial activity
Quantitative analyses were performed in triplicate by the following method: Bacterial suspensions were generated by inoculating 3 ml Tryptic Soy broth with 5 × 108 of the following four bacterial species: S. aureus (ATCC 29213), S. faecalis (ATCC 29212), P. aeruginosa (ATCC 27853) and E. coli (ATCC 25922). Turbidity equivalent was adjusted to a 0·5 McFarland standard using a spectrophotometer. One square centimetre sample of each of the eight silver dressings or one millilitre of the two topical antimicrobial silver agents or the two other (non silver) topical antimicrobial agents was added to the bacterial suspensions and allowed to inoculate at 37°C in a humidified atmosphere of 5% carbon dioxide. After the initial 24‐hour incubation, the suspensions were quantitatively cultured on blood agar plates and MacConkey agar plates and incubated for 48 hours at 37°C in a humidified atmosphere of 5% carbon dioxide. Colony counts were manually performed and expressed as CFU/ml. The bacterial suspension with the silver dressings or topical agents continued to incubate for a total of 48 and 72 hours, with the previously described procedure performed to determine their respective colony counts.
Results
The zone of inhibition test (1, 2, 3, 4) showed considerable variation in the ability to inhibit the growth of the four bacterial organisms: S. aureus (ATCC 29213), S. faecalis (ATCC 29212), P. aeruginosa (ATCC 27853) and E. coli (ATCC 25922). All silver dressings and topical antimicrobial non‐dressing agents showed inhibition of bacterial populations to some extent (1, 2, 3, 4). Contreet® F and the various Acticoat® dressings showed the largest zones of inhibition.
Figure 1.
Average zone of inhibition of eight silver dressings and antimicrobial creams Escherichia coli.
Figure 2.
Average zone of inhibition of eight silver dressings and antimicrobial creams Psuedomonas aeruginosa.
Figure 3.
Average zone of inhibition of eight silver dressings and antimicrobial creams Staphylococcus aureus.
Figure 4.
Average zone of inhibition of eight silver dressings and antimicrobial creams Streptococcus faecalis.
Bacterial death after 72 hours of incubation showed the susceptibility of the bacteria to the specific agent, be it a silver‐impregnated dressing or topical silver agents. Quantitative analysis, shown in Table 1, E. coli was 100% susceptible to five silver‐impregnated dressings at 72 hours: Acticoat® 7, Acticoat® Moisture Control, Acticoat® Absorbent, Contreet® F and Aquacel® Ag. As depicted in Table 1, S. faecalis was shown to be 100% susceptible to four silver‐impregnated dressings at 72 hours: Urgotul® SSD, Acticoat® 7, Acticoat® Moisture Control and Acticoat® Absorbent. Table 1 also shows 100% susceptibility of S. aureus to two silver‐impregnated dressings at 72 hours: Acticoat® Absorbent and Contreet® F. Finally, P. aeruginosa was 100% susceptible to two dressings at 72 hours: Acticoat® 7 and Acticoat® Absorbent. All four bacteria showed greatest susceptibility to the Acticoat® Absorbent dressing by quantitative analysis. Bacteria appear to show the least susceptibility to dressings with the lowest silver concentration (3). Topical antimicrobials, including silver sulfadiazine, Sulfamylon®, SilvaSorb® and gentamicin sulfate, showed superior bacterial inhibition and bactericidal properties in vitro (Table 1, Figures 1–4[1, 2, 3, 4]), demonstrating complete inhibition of growth in quantitative cultures at 24, 48 and 72 hours.
Table 1.
Comparison of quantitative culture results
| Staphylococcus aureus | Streptococcus faecalis | Escherichia coli | Psuedomonas aeruginosa | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 24 hours | 48 hours | 72 hours | 24 hours | 48 hours | 72 hours | 24 hours | 48 hours | 72 hours | 24 hours | 48 hours | 72 hours | |
| Control | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 |
| Acticoat Absorbent | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG |
| Acticoat Moisture Control | 7·0 × 104 | 4·0 × 103 | 1·1 × 104 | NG | NG | NG | NG | NG | NG | NG | 5·6 × 104 | >1 × 105 |
| Acticoat 7 | 3·5 × 104 | 1·9 × 104 | 4·0 × 103 | NG | NG | NG | 1 × 105 | NG | NG | >1 × 105 | 1·0 × 105 | NG |
| Contreet F | >1 × 105 | >1 × 105 | NG | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | NG | NG | >1 × 105 | >1 × 105 | 3·0 × 103 |
| Urgotul SSD | >1 × 105 | >1 × 105 | 3·0 × 103 | >1 × 105 | 2·0 × 103 | NG | 6·9 × 104 | >1 × 105 | 2·6 × 104 | NG | >1 × 105 | >1 × 105 |
| Aquacel Ag | 1 × 104 | >1 × 105 | 2·0 × 104 | >1 × 105 | >1 × 105 | 4·8 × 104 | 2·4 × 104 | NG | NG | 6·2 × 104 | >1 × 105 | >1 × 105 |
| Silvercel | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 |
| Actisorb | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | 5·0 × 105 | 4·0 × 103 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 | >1 × 105 |
| Silver Sulfadiazine | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG |
| Sulfamylon | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG |
| SilvaSorb | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG |
| Gentamicin Sulfate | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG | NG |
NG = No growth.
Discussion
Infection is a significant cause of delayed or prolonged wound healing, and high bacterial levels interferes with the progression of wound healing 12, 13, 14. Silver’s antibacterial properties have made it an attractive and practical choice for creating silver‐based topical creams and dressings to comprise a class of antimicrobial topical treatments that have been used for wound care 17, 18. This study compares topical silver compounds, mafenide acetate, gentamicin sulfate cream and eight commercially available silver‐impregnated dressings, with respect to their antimicrobial properties against S. aureus, S. faecalis, P. aeruginosa and E. coli. The results of this study show the differences and importance in choosing the appropriate dressing or cream for an infected wound.
The quantitative analysis of the antimicrobial activities concluded that of the silver‐impregnated dressings, Acticoat® dressings and its derivatives, specifically Acticoat® Absorbent, showed greater antimicrobial activity. Of the silver‐impregnated dressings, Acticoat® Absorbent showed superior effects to gram‐positive organisms, with Acticoat® 7 and Acticoat® Moisture Control close behind in susceptibility. Acticoat® Absorbent also showed the greatest susceptibility of the silver‐impregnated dressings to gram‐negative organisms. It has been shown in other studies that Acticoat® is an effective antimicrobial agent against various gram‐negative and gram‐positive organisms (1). Tredget et al. (11) showed a decrease in the frequency of the development of burn wound infection in the Acticoat®‐treated wounds than in the silver nitrate‐treated control sites. Our study suggests that topical antimicrobial silver agents as well as non‐silver agents performed superiorly to the silver‐impregnated dressings, demonstrating superior gram‐positive and gram‐negative bacterial susceptibility, making thema more logical choice for wounds containing bacterial loads of >1 × 105 CFU, to rapidly reduce bacterial counts, based on this in vitro data.
The reason for reporting both zones of inhibition and quantitative analyses over time was because both tests may have some disadvantages when comparing both dressings and creams or gels (19). Gallant‐Behm et al. suggested that the results from the two tests did not correlate at all. However, in our study, both tests suggested that in general, higher silver concentrations correlate to higher bactericidal antibacterial properties. This corroborates what was reported by Lansdown (3). However, Parsons et al. (15) showed that a greater amount of silver released by a dressing does not lead to a greater rate or degree of antimicrobial activity. It is suggested by some that the active form of silver released from a commercially available dressing is different than the active form of silver released by silver nitrate or silver sulfadiazine (11). Whether the active form of silver differs between the dressings and whether the concentration is independent of effectiveness are unknown, and would be interesting aspects for further investigation.
Overall, silver‐containing cream or gel compounds as well as non‐silver antimicrobials Sulfamylon® and gentamicin sulfate show superior bactericidal properties than commercially used silver‐containing dressings. The other issue regarding topical antimicrobial agents and dressings is their potential for cytotoxicity. Fleming (20) has stated that anything that is bactericidal may well be tissue cidal. The relative cellular toxicities of the dressings, creams and gel reported here have not been addressed in this study. Further investigation is necessary to determine the relative safety of these products on the healing wound. Once that is done, the relative value of the products can be determined by balancing their antimicrobial and cytoxicity characteristics.
Acknowledgments
Dr Payne has received research funding from Johnson and Johnson Wound Management for past projects (however, not for this project).
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