Rose Cooper | |
| Published: Feb 2004 Last updated: Feb 2004 Revision: 1.0 |
Keywords:Antimicrobial agents; wound healing; antibiotic resistance; silver; iodine.
The development of antibiotics during the 20th century markedthe decline of many former remedies, but the emergence of antibioticresistant strains of pathogens has led to the need to findalternative treatments.
The judicious, prophylactic use of antiseptics may prevent thedevelopment of infections that will minimise antibiotic use, as wellas promoting healing.
The evidence concerning the efficacy of topical antimicrobialagents in the management of wounds is confused.
Larger betterdesigned trials to assess clinical efficacy and cost implications arenecessary.
Although reports of resistance are limited, misuse and abuse ofantiseptics must be avoided.
Antibiotics are potent antimicrobial agents with highspecificity. However the relentless emergence of antibiotic-resistantstrains of pathogens, together with the retarded discovery of novelantibiotics has led to the need to find alternative treatments. Themost frequently used topical antimicrobials in modern wound carepractice include iodine and silver containing products. In the pastacetic acid, chlorhexidine, honey, hydrogen peroxide, sodiumhypochlorite, potassium permanganate and proflavine have been used.Some of these products seem to be making a return, and otheralternatives are being investigated. This review attempts to provideinsight into the controversy that surrounds the use of topicalantimicrobials by describing their respective mechanisms of action,reviewing supporting evidence and outlining perceivedlimitations.
Throughout history man has had to contend with dermal wounds.In primitive societies substances derived from animals, plants andminerals formed the basis of crude remedies[1]needed to staunch bleeding, reduce swelling,minimise pain, remove damaged tissue, treat infections, mask foulsmells and promote healing. The earliest documented records oftopical wound treatments were found in Mesopotamia; theseinscriptions on clay tablets have been dated to approximately 2500BCE. The development and dissemination of later wound treatments canbe traced from the ancient Egyptians, via the Greeks to Romanmedicine[1], but the history of progress inwound care during the Middle Ages to the present time isincomplete[2].
Although topical antimicrobial agents were utilised in woundcare for thousands of years[3], during the19th century the discovery of chemical preservatives anddisinfectants[4], as well as a betterunderstanding of the nature of infection and inflammation, allowedincreased control of wound infection. In particular the use ofcarbolic acid by Joseph Lister in operating theatres from 1865significantly reduced mortality rates associated with surgicalprocedures. Later, when it was accepted that micro-organisms were thecausative agents of infections, it became possible to consider morespecific targeting. Paul Ehrlich began the search for chemicals withselective toxicity for infectious agents, rather than non-specificinhibitors, such as antiseptics and disinfectants.
The discovery and development of antibiotics during the 20thcentury provided potent antimicrobial agents with high specificity,which revolutionised clinical therapy and marked the decline of manyformer remedies. However, the relentless emergence of antibioticresistant strains of pathogens, often with multiple antibioticresistance[5], together with the retardeddiscovery of novel antibiotics[3] has led tothe need to find alternative treatments. Faced with the prospect ofincreased prevalence of antibiotic-resistant pathogens, and thediminished effectiveness of current therapies, careful considerationof treatment options is now important.
Antibiotics are indicated in cases of overt wound infectionwhere the classical signs are evident. Yet even in the treatment ofthe diabetic foot, where infection may precede amputation, assessmentof the whole patient and the rational use of antibiotics as part ofan integrated treatment plan is recommended[6].Antibiotics are not indicated simply to limit microbial numbers inuninfected wounds. Many wounds support relatively stable mixedcommunities of micro-organisms[7], oftenwithout signs of infection[8]. In chronicwounds reduction of certain microbial species, such as anaerobicbacteria in order to limit undesirable odours[9], or perhaps mixed communities of four ormore bacterial species that impede healing[10]may be justified. The eradication of beta-haemolytic streptococci[11], or staphylococci and pseudomonads[12] before grafting is essential, andintervention to prevent the development of systemic infection incritically colonised or locally infected wounds is reasonable. Heresystemic antibiotics are not always appropriate and topicalantimicrobial treatments may be more suitable.
Controversy has long surrounded the use of topicalantimicrobial agents because of reports of cytotoxicity. In thisreview generic products, rather than named examples will beconsidered, and topical antibiotics have been excluded. Thisre-evaluation of accumulated evidence is intended as a basis to helppractitioners make informed decisions.
Chlorhexidine was discovered in 1946 and introduced intoclinical practice in 1954[13]. It is widelyused as an antiseptic in handwashing, and as a surgical scrub, but inwounds its application has been limited largely to irrigation. Themode of action has been studied extensively.
Chlorhexidine is available as diacetate, digluconate anddihydrochloride; the digluconate is most frequently used in woundmanagement. It has rapid, bactericidal activity against a widespectrum of non-sporing bacteria by damaging outer cell layers andthe semi-permeable cytoplasmic membrane to allow leakage of cellularcomponents. It also causes coagulation of intracellular constituents,depending on concentration[14]. Antibacterialactivity againstStaphylococcus aureus,Pseudomonas aeruginosa and arange of clinical isolates has beendocumented[15], however in MRSA, resistance hasbeen observed[16].
Although the efficacy of chlorhexidine as a topical agent intreating wounds is generally not well characterised, a recentevaluation of seven animal studies and three human studies hasdemonstrated that it is associated with few adverse effects onhealing[17]. Despite reports of decreasedbacterial counts, increased healing rates, and lack of toxicity, itwas concluded that at present there is insufficient data to assesssafety and efficacy, and that further clinical trials are requiredbefore the use of chlorhexidine on open wounds is either recommendedor condemned[17].
Honey is an ancient remedy[18] whichhas been re-discovered for the treatment ofwounds[19]. Many therapeutic properties havebeen attributed to honey including antibacterial activity and theability to promote healing[20]. Evidence ofantibacterial activity is extensive, with more than 70 microbialspecies reported to be susceptible[21].Laterin vitro studies have shown that activemanuka honey is bactericidal against strains of antibiotic resistantbacteria isolated from infected wounds[22],[23],[24], so adding MRSA, vancomycin-resistant enterococci (VRE)andBurkholderia cepacia to the list of susceptiblebacteria. Osmolarity, acidity, the generation of hydrogen peroxide ondilution and the presence of unidentified phytochemicals have beensuggested to contribute to the antimicrobial potency of honey[21], but geographical location, floral origin,and post-harvesting treatment conditions may also beimportant.
Mechanisms of microbial inhibition and cellular target siteshave not yet been fully investigated, but multiple, non-specificsites are predicted. Similarly,in vitro studieswith cell lines exposed to honey solutions have demonstratedmodulation of monocytic cell activity. It is thought that this islikely to influence the wound healingprocess[25],[26],although this is not yet fully explained. Studies with animal modelshave provided evidence of the stimulation of healing byhoney[27],[28],[29], and there areextensive reports of the clinical efficacy of honey in treating awide range of wounds[20]. Much of this researchis, however, with uncharacterised honeys and has therefore beendescribed as anecdotal evidence. In a small number of case reports,sterilised active manuka honey was used[30],[31],[32]. In a review of the clinical evidence, thedesign of the clinical trials was criticised[33]; there is therefore a paucity ofdouble-blinded, randomised, controlled trials to date.
The development of wound care products containing honey hasbeen limited by the availability of standardised, quality assuredpreparations, although the situation is changing. Honey can beevaluatedin vitro for antibacterialpotency[34], and the production of registered,sterile dressings impregnated with honey, as well as sterilised honeyin tubes and innovative dressings (honey and alginate) will improvereliability and accessibility in the near future.
Hydrogen peroxide has been widely used as an antiseptic anddisinfectant. A 3% (10 volumes) solution has most often been used toclean wounds. It is a clear, colourless liquid that decomposes incontact with organic matter. It has a broad spectrum of activityagainst bacteria, with greater effect on Gram positive species thanGram negatives.
Hydrogen peroxide functions as an oxidising agent by producingfree radicals that react with lipids, proteins and nucleic acids toaffect cellular constituents non-specifically. Its use in cleaningsuperficial trauma wounds has declined since the formation of airemboli was reported[35], yet analysis ofstudies in animals and humans[17] failed tofind any negative effects on wound healing. At present there seems tobe insufficient evidence to base definitive judgements about themerits of hydrogen peroxide on wound healing.
Iodine is an element that was discovered in 1811. It is a darkviolet solid that dissolves in alcohol and potassium iodide. Itsfirst reported use in treating wounds was by Davies in1839[36], and later it was used in the AmericanCivil War. Early products caused pain, irritation and skindiscolouration, but the development of iodophores (povidone iodineand cadexomer iodine) since 1949 yielded safer, less painfulformulations.
Povidone iodine is a polyvinylpyrrolidone surfactant/iodinecomplex (PVP-I); cadexomer iodine is composed of beads of dextrin andepichlorhydrin that carry iodine. Both release sustained lowconcentrations of free iodine whose exact mode of action is notknown, but involves multiple cellular effects by binding to proteins,nucleotides and fatty acids. Iodine is thought to affect proteinstructure by oxidizing S-H bonds of cysteine and methionine, reactingwith the phenolic groups of tyrosine and reacting with N-H groups inamino acids (such as arginine, histidine and lysine) to blockhydrogen bonding. It reacts with bases of nucleotides (such asadenine, cytosine and guanine) to prevent hydrogen bonding, and italters membrane structure by reacting with C=C bonds in fattyacids[37]. It has a broad spectrum of activityagainst bacteria, mycobacteria, fungi, protozoa and viruses.
Despite prolonged use of iodine, reports of resistance arelimited to one[38]. Many authors havecommented that resistance to iodine has not become a problem, and themethodology in the case of iodine resistance cited above has beencriticised[39].
Povidone iodine is availablecommercially in several formulations (solution, cream, ointment, dryspray or dressings). There is extensiveinvitro evidence of the efficacy of PVP-I as a cidal agent,from varying methodology[15],[40],[41],[42]. In one study it was shown that PVP-Ilethally damaged >99% cells within 10 seconds of exposure, and aslittle as 2.36 �105 atoms of iodine were required to kill onebacterial cell[39]. Activity at lowconcentration is affected by the presence of organic matter, but notallin vitro tests incorporate this factor intotheir design.
Clinically, PVP-I has application not only in the managementof wounds, but as a skin antiseptic prior to surgery, and in thedisinfection of inert surfaces[43]. Whereasits efficacy as a skin disinfectant is undisputed, numerouspublications describe the use of iodine in cleansing wounds, and as atopical agent to prevent or treat localised wound infections, butcontroversy surrounds its safety andefficacy[44]. Since 1994 PVP-I has beenapproved by the US Food and Drugs Administration for the 'first aid'treatment of small, acute wounds, but it was not recommended for usewith pressure ulcers by the US Department of Health & Human Services.
A report that absorption of PVP-I gave rise to severemetabolic acidosis, which complicated the management of two burnspatients who died of renal failure[45],supported opinion that PVP-I should be restricted to brief topicalapplication on superficial wounds rather than long-term use on largewounds. Two comprehensive reviews of the accumulated evidence forPVP-I derived fromin vitro studies, animalmodels and human clinical use have attempted to analyse the confusedpicture[46],[17].
Overall observations from animal models have indicatedcytotoxicity against leukocytes, fibroblasts and keratinocytes, butconversely human studies on balance suggest that PVP-I reducesbacterial load, decreases infection rates and promoteshealing[17]. In one study healing rates ofchronic venous leg ulcers, each treated with one of three topicalagents were compared to untreated control ulcers in each respectivepatient. All agents were seen to reduce bacterial load; silversulphadiazine and chlorhexidine digluconate caused slightimprovements in healing rates and times, but PVP-I yieldedstatistically significant increases. Furthermore, histologicalassessment indicated lack of cytotoxicity because PVP-I induced lesschanges in microvessels and dendrocytes[47].Additionally, a report of the ability of iodine released from adressing to modulate the secretion of cytokines by human macrophagesin vitro has provided another justification ofits role in promoting healing[48].
Cadexomer iodine is availableas an ointment, as well as a dressing. Analysis of animal and humanstudies has shown the emergence of a similar picture to PVP-I, thatis reduction of MRSA[49]andPseudomonasaeruginosa[50] respectively, withevidence from clinical reports of efficacy in stimulatinghealing[17]. Its lack of toxicity for humanfibroblastsin vitro suggests lack of toxicityfor chronic woundsinvivo[51].
Proflavine is a brightly coloured acridine derivative that wasextensively used during the Second World War in the treatment ofwounds[52]. Modern use is as a prophylacticagent in surgical wounds packed with gauze soaked in proflavinehemisulphate solution, even though calcium alginate has been reportedto promote better results[53]. It is anintercalating agent that inhibits bacteria by binding to DNA andprevents unwinding prior to DNA synthesis. Although it is effectiveagainst sulphonamide-resistant bacteria[52],strains of MRSA that are resistant to proflavine by possessing effluxpumps (mechanisms associated with bacterial membranes that exportmaterials from cells) have been isolated[54].Acridines are photosensitive; it has therefore been proposed that newderivatives should be sought for topical therapy promoted bylight[55]. However, the ability to inducemutations in bacterial[56] and cellcultures[57] raises suspicion about the safetyof proflavine.
Silver has a long history as an antimicrobialagent[58],[59],especially in the treatment of burns. An awareness of its role ininhibiting micro-organisms has developed since the late 19thcentury[60]. Metallic silver is relativelyunreactive, but in aqueous environments silver ions are released andantimicrobial activity depends on the intracellular accumulation oflow concentrations of silver ions. These avidly bind to negativelycharged components in proteins and nucleic acids, thereby effectingstructural changes in bacterial cell walls, membranes and nucleicacids that affect viability. In particular silver ions are thought tointeract with thiol groups, carboxylates, phosphates, hydroxyls,imidazoles, indoles and amines either singly or in combination, sothat multiple deleterious events rather than specific lesionssimultaneously interfere with microbialprocesses[61]. Hence silver ions that bind toDNA block transcription, and those that bind to cell surfacecomponents interrupt bacterial respiration and ATP (adenosinetriphosphate) synthesis[62]. InCandida albicans, but not inEscherichia coli, irreversible binding of silverions to cysteine residues in phosphomannose isomerase interrupts cellwall synthesis, which in turn leads to loss of essentialnutrients[63]. The role of 'other' silverradicals in antimicrobial activity remains less clearlyunderstood[60].
The complex issues concerning the toxicity of silver tomammalian systems, and its effects on the healing process, have beenconsidered by Lansdown[64], who concluded thatfurther research into this area is required. Skin discolouration andirritation associated with the use of silver nitrate is welldocumented; absorption of silver, systemic distribution and excretionin urine has also been reported[64].
In wound care silver has been utilised in severalformulations. Silver nitrate is no longer widely used, but silversulphadiazine (SSD) and silver releasing dressings remain popular.When introduced in 1968[65], SSD wasrecommended as a topical treatment for the prevention of pseudomonadinfections in burns, but it has since been demonstrated to possessbroad-spectrum antibacterial[66],antifungal[67],[68] andantiviral activity[69].
SSD is an established treatment for burns patients, butconcern about its efficacy arose when the emergence ofsulphadiazine-resistant bacteria was reported in a burns unit in aBirmingham hospital following SSD treatment of patients withextensive burns[70]. Reports of thedevelopment of silver resistant strains are rare, but SSD-resistantbacteria have been recovered[71],[72],[73]. Silver resistance has been linked toplasmids[62], and on occasions these plasmidsconfer multiple antibiotic resistance[74],[75]. Resistance to silver inSalmonella has been located in a cluster ofseven genes that were organised into three discretely transcribedunits. The gene products were deduced to be a periplasmic proteinthat binds silver ions and two efflux pumps that export silver ionsfrom the bacterial cell[76].
In clean wounds in pigs, SSD increased the rate ofepithelialisation by 28%, indicating a beneficial effect in woundsadditional to antimicrobial activity[77]. Thismodel also showed that PVP-I did not affect the rate ofhealing.
A number of wound dressings containing silver have recentlybeen developed. These function by the sustained release of lowconcentrations of silver ions over time, and generally appear tostimulate healing, as well as inhibiting micro-organisms. Theevaluation of silver impregnated dressings, as with other topicaltherapies, includes in vitro antibacterial studies, animal models,and clinical testing. A number of laboratory studies have madecomparisons between different products[78],[79],[80],[81],[82], but varying silver concentrations anddiffering modes of delivery of silver ions makes direct comparisoninappropriate.
At present human studies with silver containing dressings arerather limited, yet trials conducted inGermany[83],[84],France[85] and Italy[86]provide encouraging results. In Canada anuncontrolled, prospective study of a series of chronic wounds treatedwith an ionised nanocrystalline silver dressing demonstrated improvedclinical parameters together with decreased surface wound bioburden,but unchanged deep tissue loads[87]. Theimplication was that surface flora contributed more significantly todelayed healing than deeper flora.
It has been argued that antimicrobial efficacy alone isinsufficient benefit in modern wounddressings[88], and that additional propertiespromoting wound healing are required. Based on this, the ability toremove any undesirable bacterial products in the wound environmentthat impinge on healing would be a bonus, for example bindingbacterial endotoxin (toxins released on cell death) to a silverdressing would be of benefit[89].
Overall the evidence concerning the efficacy of topicalantimicrobial agents in the management of wounds is confusing. Itmust be remembered that it originates from multiple sources, whichare not directly comparable. In laboratory studies the evaluation ofan antimicrobial agent often begins with the determination of theMinimum Inhibitory Concentration (MIC) to determine potency,continues with suspension tests (both qualitative and quantitative)to assess rates of inhibition, and may include capacity tests toevaluate persistence. Numerous factors influence activity, such asthe concentration tested, temperature, the extent of the contacttime, the type of species tested, the number of organisms present andthe presence of organic matter. Specifications forinvitro tests are not consistent in all countries, althougha standardised European suspension test has been proposed for testingantiseptics against clinically significantorganisms[15]. Innovative, non-invasive andnon-destructive assays of inhibitory activity that give results inreal time are becoming available[90], but sofar these have limited application.
Animal models may also yield inconsistent evidence as theyutilise different species, different types of wound and differentchallenge organisms. Two landmark studies that demonstratedcytotoxicity of topical agentsin vivo haveprobably influenced practitioners to limit the use of antiseptics onwounds since the late 1980s. One study used a rabbit earchamber[91], and the other cultured humanfibroblasts, followed by acute wounds in adult rats[92]. It is debateable whether such models arerelevant to the situation in chronic wounds. The development of afibroblast model using cells from non-healing equine wounds may haveadvantages over models utilising cell lines or cells from acutewounds[93], but in vitro models will neverfaithfully mimicin vivo conditions.
Whilein vitro testing is required toscreen and eliminate agents that are likely to be unsuitableforin vivo testing, only those agents withgreater potential are selected for clinical trials. Ultimately suchstudies provide the best evidence of efficacy, providing thatappropriate control groups and statistically empowered studies aredesigned. Of the agents discussed here, there is insufficientevidence to make conclusive judgements about efficacy in thetreatment of chronic wounds, except perhaps for silversulphadiazine[94]. A systematic review of theeffectiveness of antimicrobial agents used for chronic wounds debatedthe difficulties in making comparisons between studies and called forlarger, better designed trials to assess clinical efficacy and costimplications[94].
The future threat of ineffectual control of wound infectionscaused by antibiotic-resistant strains of pathogens is sufficientreason to consider modifying our present reliance on antibiotics. Theimpact of micro-organisms on the healing process is not fullyunderstood, but infection is known to interrupt healing and at worstcan lead to death. In chronic wounds, it has been suggested thatbacteria delay healing. If this is true, then reducing the woundbioburden should reduce adverse influences that impede healing. Incases of infections, antibiotics may be indicated, but holisticassessment of the patient is required. The judicious, prophylacticuse of antiseptics may prevent the development of infections thatwill minimise antibiotic use, as well as promoting quicker healing.Topical antimicrobial therapy may be especially helpful to overcomethe deleterious effects of bacteria in specific circumstances, forexample, the eradication of bacteria[12] priorto grafting. Another application, as yet hardly considered, might bein the removal of biofilms, where bacteria encased in slime layersare less susceptible to antibiotics and have been implicated inpersistent infections[95]. Alreadyinvitro tests on biofilms withiodine[96] show inhibition, and hydrogenperoxide (and hence peroxide generating honeys) also offer potentialfor their disruption.
Topical antimicrobial agents usually have a non-specific modeof action and therefore the opportunity for unwanted patient effectsexist, but there is a lesser chance of the development of resistancein microbial species. The development of antiseptic resistance,however, has already been noted with chlorhexidine, and it has beenlinked to antibiotic resistance[14]. Misuseand abuse of antiseptics must, therefore, be avoided, and additionalantimicrobial therapies will always be needed. Tea tree oil hasalready been assessed forinvitro[97] andinvivo[98] activity, and antimicrobialpeptides isolated from amphibian skin offer promise in treatinginfections[99].
The removal of infectious agents provides another approach tocontrolling infection, and two therapies in particular are suitablecandidates for wound care. In biosurgery the ingestion and subsequentdigestion of bacteria in the gut of greenbottle larvae (maggots) notonly reduces the bioburden in difficult to heal wounds, but alsoreduces the risks of hospital acquiredinfection[100]. In addition, maggot therapy isbeneficial in the debridement of necrotic tissue and the productionof substances that stimulate the healingprocess[100]. Bacteriophage therapy began soonafter the discovery of bacterial viruses in the early 20th century,but it was overlooked, except in Georgia and Poland, when antibioticsbecame widely available for clinical use. Because bacteria, such asstaphylococci, pseudomonads and enterococci, are killed by infectionwith their respective viruses, topical application of viralsuspensions has the potential to limit bacterial colonisation andinfection of wounds[101]. This therapy has yetto be fully evaluated, but may be appropriate for the management ofburns.
This review extends the information included in a previouspublication[102], and hopes to give insightinto past, present and future topical treatment strategies for woundinfection. Future wound care products are likely to be sophisticatedformulations that incorporate not only antimicrobial components, butwill be designed to maintain a moist wound environment and optimisethe wound environment to promote healing. Since evolution is fasterin microbial species than in other species, there will be a continualneed to search for novel treatments.
This article is supported by an educational grant from Johnson & Johnson.The views expressed in this article are those of the author and do notnecessarily reflect those of Johnson & Johnson.
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