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Antimicrobial antiseptics for wound care

Valerie Edwards-Jones
22 May, 2013  
In the light of antibiotic overuse, other available topical antiseptics for wound care may be just as effective, including the simple application of honey
Valerie Edwards-Jones
Professor of Medical Microbiology,
School of Research and Innovation,
Faculty of Science and Engineering,
Manchester Metropolitan University,
Manchester, UK
Skin is the largest organ of the body and is covered with a diverse range of mixed microorganisms that form the normal flora. The skin contains immune mechanisms that maintain this balanced relationship but when there is a breach in the skin, infection can result. Wound healing can be affected by a variety of physiological conditions but generally will complete to full coverage within two weeks unless compromised. Wound dressings are used to protect the wound during the healing process and also to provide favourable physiological conditions. Some wound dressings containing antimicrobial agents can be used to prevent infection, to treat a superficial infection or reduce colonisation by microorganisms, but these antimicrobial agents can actually delay healing if used inappropriately. Antimicrobial agents used in wound care are various but are predominantly silver, chlorhexidine, iodine, polyhexamethylene biguanide (PHMB) and honey. 
The biology of wound healing
Closure of the skin is of prime importance to a wound-care specialist because with an open wound, the patient continues to be at risk of developing a life-threatening infection. Closure usually occurs within two weeks but can take longer depending on the clinical status of the patient. Wound healing occurs in a coordinated fashion and employs a wide variety of growth factors, enzymes and cells to result in wound closure. Many of these biomolecules have been identified and research continues to fully understand their role. 
Following an injury to the skin, if possible the edges of the wound are brought together using sutures or adhesive strips but if there is loss of tissue they are left open to form new granulation tissue and allow free drainage of exudate.(1) 
Unfortunately, wounds that do not heal can become chronic, which can cause increased pain, loss of function and odour and exudate problems – ultimately affecting quality of life. In order to make a difference, the wound-care specialist needs to prepare the wound bed to allow re-epithelialisation to occur. If there is evidence of high microbial bioburden (large amounts of exudate, unpleasant odour or laboratory data) the preparation may involve debridement (autolytic or interventional) and application of topical antimicrobial agents to interrupt the formation of biofilm following debridement.(2) Due to the increased carriage of antibiotic-resistant bacteria, prophylactic topical treatment for high microbe activity is imperative. Use of a topical antibiotic (for example, mupirocin, gentamicin or chloramphenicol) should be avoided as this may promote further development of antimicrobial resistance. The choice of topical antimicrobial agent should provide sustained antimicrobial activity, not impede healing, with low toxicity and no accumulation of the antimicrobial agent in tissues. These agents should remain active in the presence of secreted exudate, body fluids, necrotic tissue and any associated blood and pus.
Common microorganisms in wound infection
The common microorganisms that are isolated from wounds as causative agents of infection are Staphylococcus aureus (including methicillin-resistant Staphylococcus aureus; MRSA), Pseudomonas aeruginosa, Streptococcus pyogenes (group A strep) and other haemolytic streptococci (group C,D and G strep), Gram-negative bacteria including Escherichia coli (E. coli), Klebsiella spp and Acinetobacter baumanii, anaerobic bacteria and yeasts including Candida albicans. All these microorganisms are opportunistic pathogens but they do have pathogenicity (virulence) factors that allow wound infection in advantageous environmental conditions. These virulence factors are not expressed at all times and indeed some strains of the microorganism do not always carry the pathogenicity genes.
It is becoming more accepted that acute wound infection is frequently caused by a single organism (there are exceptions depending on the wound). However in a chronic wound, a heavy bacterial bioburden often colonises the area, which is often due to a diverse mixed culture of microorganisms living a synergistic existence as a biofilm. Currently, there is some debate by wound-care practitioners about what precisely constitutes a biofilm. Described as a complex aggregate of microorganisms and extracelluar material firmly attached to the surface of the wound bed, a biofilm should not be confused with a membrane or pseudomembrane that can be physically removed intact, which occasionally forms. Removal of a biofilm from any surface is extremely difficult and often involves reduction of biomass through debridement followed by intervention with antiseptics to prevention reformation.(2) 
Choice of appropriate topical antiseptics and dressings by the wound-care practitioner is based on availability, functionality and the nature of the individual wound. Dressings are chosen based on absorbency, comfort and wear time. The choice of topical antiseptic should be based on whether the patient is at high risk of developing an infection or has an infection control problem or from accurate laboratory diagnosis of the wound. Some topical antiseptics fulfil a number of these criteria and are available for use in wound care. There is also a developing market of preparations, such as honey, making an amazing return to this arena.
Antiseptic categories
Topical antiseptics can be subdivided into subgroups based on their chemical structure and mode of action. The subgroups used in wound care can be broadly categorised as biguanides (chlorhexidine and PHMB), halogens (iodine), silver salts and the natural antimicrobial product, honey.
Biguanides 
Chlorhexidine: Chlorhexidine gluconate is a water soluble, cationic (positively charged) bisbiguanide and is commonly used in skin preparations and increasingly in percutaneous endoscopic gastrostomy (PEG) sites. It binds strongly to the negatively charged bacterial cell wall, ultimately altering the osmotic equilibrium of the bacterial cell. The uptake of chlorhexidine by E. coli and S. aureus is very rapid and concentration dependent; pH also plays an important role in its activity. Its activity is bacteriocidal and at low concentrations it affects membrane integrity; but at high concentrations, the cytoplasmic contents coagulate, resulting in cell death.(3) At 0.05% it is effective against both Gram-positive and Gram-negative bacteria, aerobic and anaerobic bacteria, yeasts and some lipid-enveloped viruses. It is inactive against spores.(4) Chlorhexidine gluconate impregnated in catheter exit site foam dressing is effective at reducing the rate of catheter colonisation(5) and also generates significant reductions in the numbers of IV line-related infections and associated bloodstream infections.(6) In skin preparations, chlorhexidine is used with isopropyl alcohol prior to surgery and central venous line (CVC) insertion(7) to rapidly disinfect the skin. 
Tulle impregnated with 0.5% chlorhexidine acetate has been used for many years in wound care but its popularity is falling compared with some of the more modern dressings owing to poor release of chlorhexidine from the paraffin base, the need for daily dressing changes, potential maceration of the wound bed and the tendency to stick to the wound. 
PHMB: PHMB is a cationic polymeric biguanide with a molecular weight of 3000Da. It is active against Gram-positive and Gram-negative bacteria but has limited virocidal activity. PHMB interacts with negatively charged acidic phospholipids inducing aggregation, which changes membrane permeability resulting in disruption to the integrity and function of the membrane.(8) The neutral phospholipids in human cell membranes are only affected marginally.(9,10) PHMB is rapidly bacteriocidal at high concentrations and disrupts bacterial cytoplasmic membranes eventually causing leakage and precipitation of cell contents.(8) PHMB is tissue compatible and does not have any apparent negative effects on wound healing when used in solution. 
PHMB is currently incorporated into solutions, gels (for irrigation), foams, gauze and bio-cellulose wound dressings. It is also used as a universal antiseptic and is found in contact lens solutions and baby wipes.
Halogen-releasing agents
Iodine: Iodine has a broad spectrum of antimicrobial activity, rapidly inhibiting bacteria, yeasts, moulds, protozoa and viruses. It rapidly penetrates into microorganisms, attacking thiol and sulphydryl groups in proteins and enzymes, nucleotides, phospholipids and membrane structures in the cell. This results in changes to the structure and function of proteins in cell walls, membranes and cytoplasm causing rapid cell death. Aqueous solutions of iodine are unstable so newer formulations combine iodine with carriers: polyoxymer iodophores, cationic surfactant iodophores, non-ionic surfactant iodophores and polyvinyl-pyrrolidone iodophores (PVP-I).(11) These complexes create stability and give sustained delivery of iodine without marked tissue damage, and with reduced toxicity, pain and irritation. 
Iodine is available in two forms: povidone iodine (10%) and cadexomer iodine. Povidine iodine is available as a solution, aerosol spray, ointment, cream and wound dressing at varying concentrations, and cadexomer iodine is aggregated onto starch beads, which are incorporated into ointment, powders and wound dressings. On application, the starch beads swell with wound exudate and slowly release the incorporated active iodine.(12)
 
A novel approach to releasing iodine into the wound bed through a two-component, occlusive, hydrogel layer dressing has recently been made available for wound-care practitioners. When this absorbs wound fluid, glucose oxidase within the dressing produces a low level of hydrogen peroxide, which in turn generates iodine from the iodide salt held within the dressing. The amount of oxygen and iodine generated are related to the thicknesses of the gel sheets. 
Silver compounds
A number of silver compounds are used as antiseptics and many modern silver dressings use complex chemistries to release silver slowly into the wound bed or the dressing, reducing toxicity and sustaining the antimicrobial activity over a number of days. Not all products are the same but the release of the Ag+ ion from the silver salt is necessary to maintain antimicrobial activity and this has to occur under moist conditions to ensure the silver ion is active. 
Traditionally, silver nitrate (AgNO3) soaks were used to prevent infection in burn wounds.(13) Unfortunately, as AgNO3 dissociates quickly, the silver ions are rapidly inactivated by protein material in the wound, requiring up to four dressing changes daily. Silver sulphadiazine (SSD), introduced in 1968, allowed a slower release of Ag+ ions but still required daily dressing changes. Four different ionic silver states are known to exist: Ag+, Ag++, Ag+++ and Ag0. Single-charged silver, Ag+, is the most biologically active and is dependent upon solubility. Ag++ and Ag+++ are more likely to form insoluble complexes that are readily inactivated.
Silver ions attack multiple sites within the bacterial cell, including the cell membrane, respiratory enzymes, intracellular enzymes and DNA.(14) Silver is incorporated into alginates, hydrogels, hydrocolloids and foams. The efficacy depends on the nature of the silver used and the effective levels released. Nanocrystalline silver has also been shown to have some immunological properties.(15–17) The Ag+/Ag0 equilibrium allows a continual sustained release of Ag+ when it is exposed to water with antimicrobial activity still being observed over three to seven days. 
Honey
Honey is used as a topical treatment for heavily colonised chronic wounds. It contains a number of antimicrobial factors and the exact mode of action on bacterial cells is under research. It is known that the high sugar content creates an osmotic effect and sequesters fluid and bacteria from the wound bed. Hydrogen peroxide, as well as non-peroxide antibacterials including propalis, can also be found in honey and have an effect on the bacteria.(18) Additionally, in medical-grade Manuka honey, there are trace amounts of terpenoids of Leptospermum scoparium (Manuka oil) and these are also known to have antimicrobial properties. 
Conclusions
Current concerns that there may be limited treatment options available for patients with life-threatening infections has made microbiologists re-appraise the routine administration of antibiotics for superficial infections and revisit the potential of antiseptics. In addition, current opinion on the reduction of wound bioburden through debridement and application of an antiseptic to inhibit reformation of a biofilm and facilitate healing is an interesting new approach to wound healing. Other novel compounds are being explored and a new product based on an antibacterial enzyme system is available for wound care. This relies on two naturally occurring enzymes: glucose oxidase and lactoperoxidase. Glucose oxidase converts water and oxygen absorbed from the wound exudate into peroxide ions, which are then used by the lactoperoxidase to form free radicals. These free radicals attack the cell walls of absorbed bacteria. 
There are concerns that resistance may develop in antiseptics if they are used in the same unstructured way as antibiotics. However, unlike antibiotics, topical antiseptics usually have a non-specific mode of action and there is a reduced chance of resistance developing through natural selection or mutation. It is possible that transfer of resistance genes may occur and this must be carefully monitored in the future. 
References
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  15. Bhol KC, Alroy J, Schechter PJ. Anti-inflammatory effect of topical nanocrystalline silver cream on allergic contact dermatitis in a guinea pig model. Clin Exp Dermatol 2004;3:282–7.
  16. Bhol KC, Schechter PT. Topical nanocrystalline cream suppresses proinflammatory cytokines and induces apoptosis of inflammatory cells in murine model of allergic contact dermatitis. Br J Dermatol 2005;152:1235–42.
  17. Wright JB et al. Early healing events in a porcine model of contaminated wounds: effects of nanocrystalline silver on matrix metalloproteinases, cell apoptosis, and healing. Wound Repair Regen 2002;10:141–51.
  18. Cooper RA, Molan PC, Harding KG. The sensitivity to honey of gram positive cocci of clinical significance isolated from wounds. J Appl Microbiol 2002;93: 857–63.