Wound healing failure often represents the interaction of a complex series of abnormalities in the wound bed and the host's responses to tissue injury. These may be considered the pathways to wound healing failure and are the focus of wound assessment and treatment. Factors that contribute to chronic wound healing failure include infection, any condition that produces abnormal blood flow and hypoxia, cellular failure, and trauma. Correcting one of these common pathways frequently improves the other pathways and significantly aids in wound healing.
Although bacteria are present on intact skin, infection is rarely a problem because of the mechanisms that are in place to control bacteria. The outer layer of the skin, for example, provides a physical barrier to invasion. The skin's surface is not normally conducive to bacterial growth because it has a slightly acidic pH and it secretes fatty acids and other antibacterial polypeptides. The normal flora on the skin's surface helps prevent potentially pathogenic bacteria from becoming established.
However, once this protective barrier is lost and the skin has an open wound, the subcutaneous and deeper tissues are at risk from the potential adverse effects of bacteria and other microorganisms. The presence of a wound, in essence, creates a portal of entry for these organisms.
An increased bacterial burden on the surface of a wound and in wounded tissue increases the metabolic requirements of the wound and of the host's response to that heavy bacterial load. Bacteria produce endotoxins, exotoxins, proteases, and local tissue injury. In some cases, as in gas gangrene caused by Clostridium species, bacteria produce systemic effects that are ultimately responsible for the death of the host. The presence of a bacterial burden in a wound stimulates a proinflammatory environment; the presence of bacteria induces the in-migration of monocytes, macrophages, and leukocytes-all of which initially act in an appropriate fashion but later produce a response that is exaggerated and deleterious. This is evidenced by the fact that wounds associated with a heavy bacterial burden often show healing failure.
An increased bacterial burden in a wound can also affect tissue oxygen availability. Leukocytes are needed in the wound bed to kill phagocytic bacteria-by mechanisms that involve an oxydated burst and the consumption of significant amounts of molecular oxygen. In severely malperfused wounds, this increased oxygen consumption by inflammatory cells can act as a sump, "stealing" oxygen required for basic wound metabolism. In addition, the white blood cells' inflammatory response-needed to kill bacteria-increases the release of damaging oxygen free radicals. The increased production of enzymes and the release of toxins can also facilitate an induced cellular failure.
Bioburden may be defined as the metabolic load imposed by bacteria in the wound bed. Bacteria will compete with normal cells for available oxygen and nutrients. In addition, bacteria and bacterial products, such as endotoxins and metalloproteinases, can cause disturbances in all phases of wound healing.
SPECTRUM OF INFECTION
Bacteria in wounds represent a continuum, or a spectrum of presence, moving from contamination to colonization (Figure 1). Contamination is the presence of nonreplicating microorganisms within a wound; colonization is the presence of replicating microorganisms adherent to the wound in the absence of injury to the host. All wounds are contaminated to some degree. Even those defined as clean surgical wounds have bacterial contamination. This is not necessarily problematic. It is well accepted that chronic wounds are contaminated and colonized. However, a fine balance, or equilibrium, exists. In clinical practice, it may be difficult to identify when the bacterial load is acceptable and when it has reached a level that is causing impaired wound healing.
It is widely believed that the wound moves from contamination to colonization when the bacteria on the wound's surface begin to replicate and increase their metabolic activity. This does not become pathologic unless invasive bacterial growth occurs within the tissue of the wound, causing damage to the tissue and the host. The infection may be local and reasonably contained, but also may advance and spread, causing more widespread injury or a systemic infection.
For approximately 30 years, bacterial bioburden has been described in terms that attempt to identify the number of organisms associated with infection or pathologic change. Greater than 105 bacterial cells per gram of tissue is widely perceived to be a key determinant in infection and delayed wound healing. That evidence is based on studies of traumatic wounds, burn wounds, skin graft preparation, and other surgical wounds. It has been well documented that bacterial levels greater than 105 cause a negative effect on healing. This has been seen in both the acute and the chronic wound.1 However, it is possible that this number may not be as valid for chronic wounds because these wounds almost always occur in an impaired host.
Significant controversy surrounds the best methods to quantify bacterial levels in a wound. At this point, one can assume that bioburden assessment is a combination of the assessment of host resistance, wound characteristics, and wound culturing. A critical factor in wound bed preparation is the consideration of the nature and extent of the bacterial bioburden.
HOST CHARACTERISTICS
When assessing a chronic wound for infection, it is important to consider not only the number of organisms, but also the condition of the host. A number of local and systemic factors increase a patient's risk of infection (Table 1). These are similar to the factors associated with wound healing failure, which means that many patients with chronic wounds will have 1 or more local or systemic factors that impair their response to any degree of bacterial contamination. Impairment in either the local wound response or systemic host immune response increases the risk of infection.2 In addition, malperfusion of the wound bed increases the risk of transition from a contaminated or colonized wound to one that is infected.
Chronic nonhealing wounds are frequently hypoxic and infected. Although this is not a direct cause-and-effect relationship, the association is a reasonable one. Hypoxia induces tissue necrosis; tissue necrosis creates a fertile metabolic environment that favors bacterial growth. Killing of phagocytized bacteria by polymorphonuclear leukocytes is significantly impaired when the transcutaneous oxygen level is below 30 mm Hg. Environments with low oxidation-reduction potential facilitate the growth of anaerobic bacteria.2
Some diseases increase the risk of infection, such as diabetes. Nearly half of all people with diabetes have at least 1 hospital or physician claim for an infectious disease yearly. There is evidence of compromised neutrophil function in diabetes, impaired chemotaxis and mobility, impaired adherence, and impaired phagocytosis. Diabetes also depresses antioxidant systems and, to some extent, humoral immunity, making a patient with diabetes who has any degree of skin contamination more prone to developing a wound infection.
In addition, patients with diabetes are more likely to have other comorbidities, such as cardiovascular disease or renal impairment. These comorbidities also increase the likelihood-by decreasing the host immune response-that bacterial contamination will lead to infection. Diabetes confers an independent, increased risk of developing and dying from an infectious disease.3
See Table 2 for infections by chronic wound type.
PATHOGEN CHARACTERISTICS
When identifying and evaluating an infection, the number of organisms alone is not always as significant as the specific pathogen present. Virulence is determined by bacterial characteristics and the production of endotoxins and exotoxins that cause local and systemic effects. In 1918, Wright et al4 reported that any amount of Streptococcus pyogenes in a wound creates an infection that prevents surgical closure (delayed secondary closure). Robson and Heggers5 identified beta-hemolytic Streptococcus as the only bacterium capable of causing infection at levels significantly below 105 colony-forming units (CFU) per gram of tissue.
Most wounds are polymicrobial, and infections in acute and chronic wounds generally involve mixed populations of aerobic and anaerobic bacteria. However, anaerobic bacteria may be present in numbers significantly greater than appreciated. An overall average of results from multiple studies suggests that anaerobic bacteria are present in 38% of noninfected and 48% of infected wounds.2Staphylococcus aureus is present in 24% to 29% of pure cultures of cutaneous abscesses. Trengove et al6 suggest that no single pathogen is more detrimental to wound healing, but a lower probability of healing is seen when 4 or more pathogens are present, based on their synergistic relationships. Microbial synergy may increase the net effect of the pathogens and the severity of infection by increasing local oxygen consumption, producing local hypoxia, and lowering oxidation reduction potential; providing specific nutrients required for more fastidious cohabiting bacteria; and impairing local cell-mediated host defenses against infection.2
CONTAMINATION TO INFECTION
Considering that bacteria are present in all chronic wounds, there is a bacterial balance between host resistance and the quantity and virulence of bacteria. This balance must be maintained for wound healing to occur.
A number of factors can tip the scale in either direction. Host resistance is an important variable in determining the risk for infection in chronic wounds. Local and systemic factors can also impair healing. Malperfusion, for example, is associated with the pathophysiology of chronic wounds and an increased risk of wound infection. In addition, factors such as immunosuppression, diabetes, and medications can influence whether the bacteria present will lead to impaired healing. Virulence factors that may tip the scale in favor of the bacteria include adhesins, which mediate cell adherence to the host; cell capsules, which prevent phagocytosis; biofilms, which are an extracellular polysaccharide matrix in which the bacteria become embedded for protection; and antibiotic resistance.7 Prolific use of antibiotics contributes to antimicrobial-resistant strains of bacteria.
Most chronic wounds are not homogenous environments. Often, various microbiologic ecosystems exist. Organisms found in slough and exudate may not be the same as those found within necrotic tissue, which may not be the same as the organisms growing in the hematoma in the wound bed, which may not relate to the organisms in the sinus tract, abscess, or exposed bone. Therefore, practitioners must pay attention to where cultures are performed and where tissue is sampled to determine a diagnosis. The practitioner should look for various causes of impaired host immune response when conducting a clinical assessment.
Other important considerations in wound infection include specific characteristics of the organisms present, or virulence, and the relationships between multiple organisms when present together in the wound. Typically, wound microbial flora changes over time and becomes progressively more pathogenic. In wounds with multiple compartments, biofilm development may be an issue and may make these organisms somewhat resistant to traditional methods for cleansing the wound and for penetration and action of topically applied agents. These wounds almost always occur in impaired hosts, with infection causing impaired wound healing.
SIGNS AND SYMPTOMS OF INFECTION
Chronic wounds do not always behave like acute wounds in the presence of an increased bacterial burden. Historically, signs and symptoms of infection in acute wounds-identified by rubor, dolor, and calor-include advancing erythema, fever, warmth in the zone of erythema, local edema and swelling, pain, and purulence. However, these findings are not typically present to the same degree in chronic wounds.
In 2001, Gardner et al8 examined the validity of clinical signs and symptoms associated with chronic localized infection. Thirty-six chronic wounds were assessed for signs and symptoms of wound infection, including those considered to be classic and those described as secondary, such as serous exudate, delayed healing, discoloration of granulation tissue, friable granulation tissue, pocketing at the base of the wound, foul odor, and wound breakdown. Quantitative tissue cultures were also obtained. The results of clinical observations were compared with the results of tissue biopsy. The only findings that had a positive correlation with the presence of culture-demonstrated infection were friable granulation tissue, an increase in odor or abnormal odor, increased pain at the wound site, and wound breakdown.8
Traditional signs and symptoms, therefore, need not be present for a chronic wound to have a local infection. In the study by Gardner et al,8 secondary signs and symptoms occurred more often than classic signs and symptoms in chronic wound infections, as demonstrated by quantitative wound biopsy. No single sign or symptom is 100% sensitive, suggesting that none should be considered crucial or necessary to identify a chronic wound infection. Increasing pain and wound breakdown are probably the most useful clinical indicators.8
DIAGNOSIS
Definitive diagnosis of infection is reached by identifying organisms from a culture taken from tissue in the wound, or from a blood culture in cases with systemic involvement. The bulk of the literature suggests and supports that a biopsy of tissue is the best practical clinical approach to the diagnosis of infection. However, biopsy of living tissue in the wound bed for culture is not available in all settings. Table 3 outlines proper surface culture techniques for identifying wound infection. When infection is suspected, an appropriate culture should be taken and immediately sent to a laboratory. The culture should then be evaluated in the light of the clinical findings and the patient's condition (Table 4).
A study that examined current culturing practices in wound care centers in the United States found that the most common method for determining wound infection was examining the wound's physical and clinical characteristics.9 The investigators found that 11% of practitioners never cultured wounds; 20% cultured wounds only after treatment failure.9 In addition, 69.7% of treated wounds were never cultured, 54% of wounds were cultured by swab only, and 42% were cultured by swab and biopsy.9 The most common swab culture method was the 10-point diagonal method followed by the 1-point rotational method. Saline was the solution most often used to cleanse and prepare the wound site for culture.
Tests of biopsy material include histopathology to demonstrate necrosis and inflammatory cells in tissue and Gram's stain, acid-fast bacilli (AFB) stain, and fungal stain to demonstrate microorganisms in tissue. Cultures determine the presence of routine aerobic and anaerobic bacteria, fungal infections, and AFB. Heggers et al10 and Levine et al11 reported that a rapid Gram's stain technique predicts the presence of bacteria in amounts greater than 105 CFU/g of tissue if a single bacterium is seen. In addition, semiquantitative surface swab in burn wounds correlated to quantitative culture by biopsy.12
Limitations of a single swab biopsy site have been reported by Bowler et al2 and Sapico et al.13 They suggest that many chronic wounds are not homogeneous in the quantity and types of bacteria within wound tissue. Sapico et al13 reported a 63% quantitative concordance between peripheral and central biopsy culture from pressure ulcers. Multiple site biopsies, therefore, may be needed when taking a culture from a wound.
WOUND BED PREPARATION
Taking steps to manage infection is one of the key principles in the new model of wound bed preparation. The others include removal of nonviable tissue, maintaining adequate moisture, and managing the wound edge to facilitate reepithelialization and closure.
Wound bed preparation should focus on interventions to reduce the bacterial burden. Necrotic tissue harbors bacteria and serves as a physical barrier to wound healing. The single most important intervention for reducing the level of bacteria in a chronic wound is to remove all of the devitalized tissue. In most cases, saline is sufficient for cleansing the wound surface. Commercial wound cleansers can be used to enhance the efficacy of wound cleansing. This is accomplished through the use of surface-active agents (surfactants), which improve removal of wound contaminants. The strength of the chemical reactivity of these agents is directly proportional to their cleansing capacity and toxicity to cells. Skin cleansers are not intended for use in wounds. The key to wound cleansing is to remove debris and eschar from the surface of the wound while maintaining an adequate wound bed moisture balance.
Antiseptic agents are not recommended for routine use in wounds.14 Antiseptics do not have a selective antibacterial mechanism of action and damage all cells on contact. However, in some circumstances, such as when control of the bacterial bioburden takes priority, slow-release antiseptics are appropriate. For example, slow-release iodine can be contained within a cadexomer polysaccharide matrix that allows for absorption of wound exudate and slow release of 0.9% iodine. This system provides an antimicrobial effect without the harmful actions associated with iodine.
Good topical cleansing is usually sufficient in wounds that do not have secondary signs of infection and that do not need a bioengineered tissue graft or surgical closure. However, when a wound progresses from simple colonization to the preinfection stage of critical colonization or local infection, it is important to intervene with aggressive debridement, as appropriate, and the use of topical antimicrobials.
When considering a topical agent, be careful about using agents that are also used systemically (Table 5). Inappropriate use may encourage the development of resistant organisms if the topical agent does not reach killing concentrations on the wound's surface and eradicate bacteria. Simply exposing bacteria to an agent below killing concentrations provides the opportunity for the bacteria to develop resistance and transfer that resistance to other organisms.
Systemic antibiotics may be of little use in trying to reestablish the surface bacterial balance in heavily contaminated or infected wounds. Systemic antibiotics also may not reach adequate tissue levels in chronic granulation tissue to have an effect on bacterial levels.1
In addition, care must be taken when using topical agents in which the antimicrobial agent or the delivery vehicle is a known allergen; the patient may become sensitized to the topical agent. This is frequently encountered with neomycin: About 50% of patients become sensitized to it in a matter of weeks.14
Local or topical therapy is often adequate in wounds that present with strictly local signs of infection. In wounds that show signs of systemic infection, however, systemic therapy is indicated. When treating a patient with systemic antibiotics, it may be wise to continue topical therapy because there is no vascular supply for delivery of the systemic antibiotics to necrotic tissue. In addition, systemic antibiotics may not be well delivered to a wound on an ischemic lower extremity.
THE ROLE OF SILVER
Silver-based wound dressings are often used to prepare the wound for healing. Silver is a broad-spectrum antimicrobial agent that controls yeast, mold, and bacteria, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE), when it is provided at an appropriate concentration. It kills microbes on contact through multiple mechanisms of action, such as inhibiting cellular respiration, denaturing nucleic acids, and altering cellular membrane permeability. Silver has low mammalian cell toxicity. It is now known to have potent anti-inflammatory properties, which are dependent on the delivery system.15-17 It may be used to maintain a microbe-free, moist wound healing environment.
Silver does not induce bacterial resistance if used at adequate concentrations. This is evident when the literature is reviewed. Very few cases of resistance were reported in the past when silver nitrate (0.5% or 3176 mg/L) and silver sulfadiazine (1% or 3025 mg/L) were the primary source of silver. However, these low levels of resistance may no longer be the norm, as many of the new silver-containing dressings release silver at concentrations below the minimum inhibitory concentrations for many organisms.
The difficulties with many current topical silver antimicrobials lie in their low silver release levels, the limited number of silver species released, the lack of penetration, the rapid consumption of silver ions, and the presence of nitrate or cream bases that are proinflammatory. Other issues include staining, electrolyte imbalance, and patient comfort. Various silver-based dressings have been introduced in the past few years to address these issues.
The balance of this article will focus on the science behind silver technology. This is important: It is easier to design an effective treatment plan based on what is known about the science of silver technology than it is to apply the products and make a series of trial-and-error observations. When practitioners make evidence-based decisions regarding treatment of wounds, they will be using technology optimally.
SILVER DELIVERY OPTIONS
Different silver delivery systems exist, including those that deliver silver from ionic compounds, such as silver calcium phosphate and silver chloride, and those that deliver silver from metallic compounds, such as nanocrystalline silver.18 Silver nitrate and silver sulfadiazine are the gold standards, both having been used for the last 40 years. See Table 6 for additional information on silver delivery systems, including silver concentrations and silver exposures of various ionic and metallic compounds.
Ionic silver is highly reactive. It will react with chlorides and other halides, inorganic compounds, organic acids, protein, DNA, and RNA. Many of these are found in a wound; therefore, silver released into a wound can be rapidly consumed. Metallic silver is less reactive, although some surface sulfur compounds react with it. Other substances that react with metallic silver include oxygen, which is a surface effect, and acids, such as nitric and sulfuric acid. However, these are not commonly found in wounds.
Problems with reactivity and its impact on efficacy raise the question of how efficacy should be tested. Several methods exist to test efficacy, including zone of inhibition (ZOI), minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and log-reduction testing. Of these, the ZOI test is not recommended for silver because zone sizes are not proportional to silver release.19-21
The MIC test indicates susceptibility of organisms to silver. It does not yield information about the most effective treatment dose. In general, however, the treatment dose should be greater than the mutant prevention concentration (MPC), which is greater than the MIC. This avoids the problem of merely exposing organisms to an active agent, which may enable them to develop resistance.
The MBC and log-reduction tests both assess killing activity. To achieve a broad-spectrum bactericidal effect, or a 3-log reduction, data in the literature support the use of silver concentrations (Ag+) of at least 30 to 40 mg/L in a complex fluid containing organic matter and chloride.22-24
Log-reduction assays are used to determine microbicidal activities of antimicrobial agents. In sterilization assays, a 1-log reduction is a kill of 90% of the population in the time allotted. Bactericidal activity is indicated by a minimum 3-log reduction in the microbial population.
It is difficult to compare the results of log-reduction testing reported in the literature because investigators have used different incubation times, media, and organisms. To make such a comparison, one of the authors of this paper (RB) examined data from 1974 to 2001. The data were generated by various investigators who had reported on microbial log reductions in complex media at exposure times ranging from a half hour to 48 hours.23-28 In summarizing their data, it was found that when silver concentrations were less than 36 mg/L, 16% of the data showed greater than 3-log reductions. When silver concentrations were greater than 36 mg/L, 67.9% of the test points showed greater than 3-log reductions. These clearly showed that as the concentration increased, the effect of the silver also increased.
Log-reduction tests are very sensitive for differentiating the activity of silver delivery systems. They clearly distinguish between materials that give similar results in qualitative tests, such as ZOI assays. The ZOI for various silver dressings is often about the same size, but log-reduction assays produce dramatically different results.23,29
SILVER RESISTANCE
Resistance may be an issue with newer silver dressings that release silver ions in the high parts per billion (ppb) to low parts per million (ppm) range; the literature suggests that silver must be delivered at concentrations greater than 30 to 40 mg/L to be effective. Resistance will develop if the concentration of silver is not high enough to generate a lethal effect. Of even greater concern is the fact that the development of resistance may be optimized if the concentration of silver is not above the MPC.22
Li et al30 generated silver resistance in a clinical isolate of Escherichia coli using a multistep exposure procedure. Exposures started at half the MIC value (2 to 4 mg/L for silver nitrate and silver sulfadiazine, respectively) and increased with each generation. The initial MICs, 4 and 8 mg/L of silver nitrate and silver sulfadiazine, or about 2.5 mg/L of Ag+, were quickly overcome as the isolate developed resistance to a silver level greater than 1024 mg/L. Interestingly, the investigators were unable to develop mutants in a one-step procedure (XZ Li, personal communication).
This was understandable given the antimicrobial nature of silver. It is known to have multiple mechanisms of action, which limits the ability of bacteria to develop resistance in a single step. That is, if silver has 5 mechanisms of action that occur at different concentrations, then 5 mutations would need to occur simultaneously for resistance to develop, if the concentration of applied silver is higher than that required to kill the organism through the least sensitive mechanism of action. The probability of that occurring would be in excess of 1 in 1 x 1040 [(1 x 108)5] cell divisions if the probability of any single mutation were 1 in 108.
If, however, the silver is applied at a concentration that is at or near the MIC (the MIC is the minimum concentration required to inhibit growth through the most sensitive mechanism of action), then the probability of developing resistance to that concentration of silver is about 1 in 1 x 108 cell divisions. If the concentrations are increased with each generation, it would be theoretically possible to have a fully resistant organism in 5 x 108 cell divisions. This clearly highlights the problems and risks of exposing microorganisms to silver concentrations at or near the MIC.
Following the general procedure of Li et al29, one of the authors of this paper (RB) tested the development of silver resistance in Pseudomonas aeruginosa. Exposures started at 2.25 mg/L of silver. The MIC after the first exposure was no longer 4.5 mg/L; it was somewhere between 4.5 mg/L and 9 mg/L. After a second exposure, at 3.8 mg/L, the MIC was again between 4.5 mg/L and 9 mg/L. After a third exposure, at 4.5 mg/L, which was the initial MIC, the new MIC was greater than 13.5 mg/L. This was an increase of at least 300% in the MIC value after only 3 exposures.
Silver resistance, therefore, is a very real possibility. The fact that more silver resistance has not been seen in the past is most likely related to its multiple mechanisms of action and the large amounts of silver used in silver nitrate and silver sulfadiazine treatments. For example, assuming 1 mL per square inch of dressing 12 times per day, patient exposure per square inch of wound after 14 days of treatment with a 0.5% silver nitrate solution that contains approximately 3176 mg/L of silver is 533,568 mcg Ag, or 38,112 mcg Ag/day/in2 of silver.18 After 14 days of treatment with 1% silver sulfadiazine (3025 mg Ag/L twice per day), patient exposure per square inch of wound is 220,000 mcg Ag, or 15,730 mcg Ag/day/in2 of wound.
At these concentrations and exposures, multiple simultaneous mutations would have to occur, one for each mechanism of action, before the organism developed resistance to the antimicrobial agent. If treatment is started at a concentration at or just below the MIC for the organism, resistance can easily develop because fewer simultaneous mutations are needed at that point. This highlights the importance of multiple mechanisms of action and a high treatment concentration.
Maintaining an adequate concentration of silver in a dressing over time has been a challenge. Metallic-coated dressings release silver over a long period but provide a low concentration of silver.18 Silver nitrate has a high concentration of silver but no residual activity, which can be solved by applying it 12 times a day. Silver sulfadiazine provides an adequate concentration of silver but has limited residual activity. However, it is a significant improvement over silver nitrate because it needs to be applied only twice a day. Galvanic action has a long duration of release but a low concentration of silver. Silver carbomethylcellulose releases a low concentration of silver and has no residual activity.18 Silver calcium phosphate and silver chloride release silver over a long period but not at high enough concentrations. There is no point in having a long duration of activity if the concentration may result in the development of resistance.
A newer approach to delivering silver to a wound is nanocrystalline silver, which has a large surface area, small grains, and a different mechanism of dissolution than bulk metallic silver surfaces.18 Nanocrystalline silver (available in various Acticoat dressings; Smith & Nephew, Largo, FL) starts out as a nanocrystal and results in the release of both Ag+ and silver zero (Ag0) in solution.31
ANTIMICROBIAL PROPERTIES
The delivery vehicle, the available silver concentration, and the duration of release are interrelated and affect the antimicrobial properties of silver. The delivery vehicle, or the mechanism with which the silver is released, can make a difference in preventing resistance. The same amount of silver can be contained in different dressings, but the antimicrobial activity can be vastly different, based on how quickly the silver is released and how much silver is available.
In 1999, Yin et al23 examined the differences between a nanocrystalline silver dressing, silver sulfadiazine, and silver nitrate. One hour after inoculation of a nanocrystalline silver dressing with 107 CFU of S. aureus, fewer than 100 organisms were recovered. Silver nitrate and silver sulfadiazine took 4 and 6 hours, respectively, to achieve the same level of kill. Mafenide acetate generated a 1-log reduction of the population in 6 hours. This compound is an antimicrobial agent that does not contain silver and was included for comparative purposes.
Antibiotic-resistant MRSA organisms are less sensitive to silver than some other organisms. In 1998, Wright et al28 showed that a nanocrystalline silver dressing killed MRSA in 30 minutes with a 7-log reduction; the other silver-releasers had virtually no effect. These tests were done in a complex media with organic matter and chloride. Similar results were seen with VRE.
Biofilms, a collection of organisms on a surface, occur in wounds. Generally, biofilms are resistant to antibiotics and antimicrobials. For example, Chan, Burrell, et al (unpublished data) showed that gentamicin was not effective against S. aureus, MRSA, or P. aeruginosa expressing either the planktonic or biofilm phenotype. That is, minimal differences were found between the minimum biofilm eradication concentration (MBEC, the concentration required to kill the biofilm form) and the minimum bactericidal concentration (MBC, the concentration required to kill the planktonic form). With silver sulfadiazine, large differences were seen between the susceptibilities of the biofilm form and the planktonic form, with the biofilm form being very resistant. The nanocrystalline silver dressing was very effective against both phenotypes, as is shown by the small differences between the susceptibilities of the biofilm and the planktonic forms. This suggests that the efficacy of nanocrystalline silver in a wound is, in part, due to its control of the biofilm phenotypes.
An in vivo test compared the use of silver nitrate-saturated dressings with nanocrystalline silver dressings on rats with 18% to 20% total body surface area full-thickness burns.32 Burns on the various groups of rats were dressed with one of the experimental silver dressings (inoculated and silver dressing) or were placed in a control group. Controls included infection control (inoculated, gauze dressing, no silver) and burn control (not inoculated, gauze dressing, no silver). The end point was when a rat become moribund and had to be terminated. The rats in the nanocrystalline silver dressing group had an 85% survival rate; approximately 1% to 2% of the rats that were treated with silver nitrate survived. This clearly shows the advantages of nanocrystalline silver's quick kill.
ANTI-INFLAMMATORY PROPERTIES
Not all silver is anti-inflammatory. The anti-inflammatory properties depend on the delivery vehicle, the available concentration and species of silver, and the duration of release. This is true for other drugs as well. For example, gold chloride has been used as an intra-articular injectable for rheumatoid arthritis because of its potent anti-inflammatory effect. Cisplatin is also used as an anti-tumor drug because it shuts down production of tumor necrosis factor-[alpha] (TNF-[alpha]) and matrix metalloproteinases (MMPs) in tumors, which could be considered an anti-inflammatory response. The nanocrystalline silver dressing modulates the inflammatory process at or above the level of TNF-[alpha] expression, thus generating an anti-inflammatory effect.17 It also induces apoptosis, which is an anti-inflammatory process in the sense that it prevents cells from undergoing necrosis, which is a highly inflammatory process.15
Practitioners using nanocrystalline silver to control bacteria in wounds had reported that these wounds were healing faster than expected, leading them to believe that the nanocrystalline silver was doing something other than controlling bacteria (RH Demling, personal communication). A porcine model was developed to explore these observations. Full-thickness wounds that were 2 cm in diameter were created on the dorsum of triplicate pigs. These wounds were inoculated with P. aeruginosa, coagulase-negative S. aureus, and a Fusobacterium sp. The wounds were treated either with a nanocrystalline silver dressing, silver nitrate, or a saline soak. On day 1, the control wounds, which received either silver nitrate or a saline soak, showed raised edges, some edema, and a small amount of inflammation. Wounds treated with a nanocrystalline silver dressing did not exhibit an inflammatory response; histology showed active fibroblasts in these wound after 24 hours. On day 2, the inflamed control wounds were not improving. On day 4, the nanocrystalline silver dressing-treated wounds had a fully developed granulation bed that was the thickness of the original dermis. They showed no signs of contracture; wounds in the control group had started to contract, driven in part by the inflammatory process.15
The wounds treated with a nanocrystalline silver dressing were 95% healed at day 13. It took the wounds in the control group another 10 days (23 days total) to reach the same level of closure. These significant changes were apparently achieved by controlling the inflammatory response.
The nanocrystalline silver-treated wounds filled in and reepithelialized without contracting. The MMP levels in the wounds treated with silver nitrate and saline skyrocketed, indicating an inflammatory response. Matrix metalloproteinases are needed to heal a wound, but excess levels degrade fibronectin and peptide growth factors. This is exacerbated by diminished levels of tissue inhibitors of metalloproteinase (TIMPs) (Table 7).
Although the study was conducted to examine the level of MMPs as an indication of inflammation, examining total proteases provided some insight into what was happening. For the wounds treated with silver nitrate or saline soak, the protease levels mirrored the MMPs, but at higher levels. The wounds treated with nanocrystalline silver had low levels of proteases, indicating antimicrobial control.
The investigators also examined apoptosis, or programmed cell death, which prevents necrosis and inflammation. If cells are going to die, it is better to have them go through apoptosis than necrosis, which kills microorganisms. Apoptosis results in fragmentation of cells into membrane-bound particles that are then eliminated through phagocytosis. This process avoids the dumping of cell constituents that occurs in necrosis.
The effects of controlling the inflammatory response were also examined in a small clinical study (10 patients) conducted in Calgary, Alberta, Canada.17 One of the subjects was an 86-year-old female with a 10-year history of venous ulcers; for the past 2 years, she had been assessed daily and treated with pressure dressings and 1% silver sulfadiazine cream. She was enrolled in the clinical study to evaluate MMP inhibition by a nanocrystalline silver dressing. Figure 2 shows the subject's feet at the start of the study and after 60 (right foot) and 70 days (left foot) of treatment with the nanocrystalline silver dressing. Initially, only the right foot was enrolled in the study. However, after the investigators realized that wounds that had persisted for 10 years were going to heal, they treated the left foot as well. Figure 3 shows the baseline data that were collected from the last dressing change with silver sulfadiazine through the end of the treatment period with the nanocrystalline silver dressing. High levels of MMP-9 and MMP-2 were reduced but did not drop to zero; had they dropped to zero, the wounds would not have healed.
This effect was examined in more detail by Paddock et al17,who conducted a 10-patient study to determine if a nanocrystalline silver dressing would alter the concentration of cytokines and MMPs in chronic wounds over time. In the wounds in the control group, the level of MMP-9 stayed high, while it dropped significantly in wounds treated with the nanocrystalline silver dressing.
The investigators also looked at levels of interleukin-1 (IL-1) and TNF-[alpha]. Although the result was not statistically significant, a power analysis suggested that had there been 15 patients instead of 5 in each arm, the result may have reached statistical significance.
The study results suggest that nanocrystalline silver affects the inflammatory process of wounds, not only by inhibiting MMPs, but also by controlling the process at least at the TNF-[alpha] level. This may be responsible for improved wound healing.17
SUMMARY
With technology in hand, practitioners have options to intervene when a patient develops a chronic wound infection. Some wounds require surgical exploration to understand the extent of tissue involvement and the likelihood of an infectious process. Practitioners should keep in mind that every wound that looks infected probably is, and some that do not look infected may be. Necrosis needs to be properly debrided because it provides a fertile breeding ground for microorganisms, which may convert a contaminated and colonized wound to an infected wound. Once a wound progresses to invasive infection, optimum treatment is culture-specific systemic antibiotics. No topical agents will be sufficient.
Studies support the concept of eradicating infection to help wound healing. From a pathophysiologic standpoint, treating an infection reduces the wound's bacterial burden, which has favorable effects on the dynamics of oxygen delivery and utilization within the wounds. This favorably impacts cellular metabolism. Treating infection also helps diminish the chronic inflammatory response, which is primarily degradative. Finally, treating infection adjusts the tissue's capacity to respond to cell signaling and to develop sustained growth.
Bacterial contamination is variable and has an uncertain impact on wound healing. Infection impairs wound healing, but it is often unrecognized. Practitioners may underappreciate a clinical finding that actually points toward infection.
When assessing a wound infection, the practitioner should identify the infecting organism whenever possible. This is not always easy; anaerobic organisms are particularly difficult to identify. Host-immune competency should also be assessed and improved to the extent possible. Basic principles of wound care always apply: remove necrotic tissue, remove/drain an abscess, treat local edema, and maintain proper wound bed hydration without overhydrating it. A moist wound can mount a more appropriate inflammatory response to infection and support wound healing. Avoid using fully occlusive dressings on wounds in compromised hosts. These dressings may create an excessively moist environment that favors oxygen-reduction potential and supports anaerobic bacterial growth.
Watch the surrounding skin and assess for osteomyelitis. Remain cognizant of the guidelines about exposed bone and plantar neuropathic diabetic foot ulcers. Treat infection that extends beyond the wound margins and treat osteomyelitis with culture-specific systemic antibiotics. Use topical antiseptics and topical antimicrobials carefully, and take advantage of the new dressing technologies available.
In particular, silver-based technologies may provide added benefits, such as down-regulation of MMPs to levels that facilitate wound healing. Silver is a powerful, broad-spectrum antimicrobial with anti-inflammatory and healing properties, but only if it is delivered at the right concentration and species and over an appropriate amount of time. Nanocrystalline-derived silver meets these requirements. When the heavy bacterial burden in a wound is treated, the wound is moved back to an appropriate healing pathway.
The only constant with wounds is that they are constantly changing. Practitioners should stay vigilant about the adverse effects of bacteria in wounds and keep in mind the role of infection in producing wound healing failure. The interaction between organisms in an environment that is more favorable to them than to the host should be a concern for practitioners. A significant change in a previously healing wound should prompt reassessment for infection.
Remember to care for the whole patient; otherwise, infection will inevitably further impair wound healing. Most importantly, maintain a high index of suspicion and always look for the presence of infection in a chronic wound.
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