As far as I know there is still no evidence that bacteria can develop resistance to iodine... :)
Although it has been speculated that iodine delays healing and is cytotoxic, there is substantial evidence to suggest that the commonly-used low concentration, slow release iodophors improve healing rates and are effective as highly potent antimicrobials with a broad spectrum of activity, including antibiotic-resistant strains such as MRSA.
Authors: Sibbald RG, Leaper DJ, Queen D.
Full author details can be found on page 4.
What is iodine?
Iodine is a natural dark violet, non-metallic element that plays a key role in human metabolism. It is essential for the production of thyroid hormones and an iodine deficiency can result in hypothyroidism. Iodine occurs naturally in the form of iodide ions in sea water, fish, oysters and certain seaweeds. It can also be found in vegetables grown in iodine-rich soil and dairy products. It has been described as 'the most potent antiseptic available'.
What is the history of iodine in wound healing?
In the 4th century BC, before iodine had been discovered, Theophrastus, a pupil of Aristotle, recorded that iodine-rich seaweeds could be used to reduce the pain of sunburn . One of the first antiseptic iodine preparations to be used in wound care was Lugol's solution containing elemental iodine and potassium in water, which was developed in 1829. This solution was also used to treat wounds in the American Civil War.
The antimicrobial properties of iodine were first demonstrated in 1882 by Davaine6. In the First World War, iodine was found by Alexander Fleming to reduce the incidence of gas gangrene in the wounds of soldiers when compared to carbolic acid . Since the mid-19th century, iodine-based preparations have also had an important role in the prevention of surgical site infections. Povidone iodine preparations are popularly used as an antiseptic to prepare the patient's skin before surgery and are also used by surgeons and theatre staff as a skin cleanser and antiseptic in preoperative hand scrubs.
Early uses of iodine involved aqueous and alcoholic iodine preparations, which were associated with unpleasant side effects including pain, irritation and skin staining.
Why is iodine safer today?
Iodophors were developed in the 1950s to overcome the side effects associated with elemental iodine. These were found to be safer and less painful, but just as effective as elemental iodine, allowing widespread use.
Bonding iodine with another molecule makes it less toxic and instead of high concentrations of iodine being released in a single application, the iodine is slowly released from the reservoir carrier molecule over a sustained period of time.
Iodophors are preparations that bind iodine to a solubilising agent or carrier. The water-soluble complex allows the slow release of a low concentration of free iodine when the carrier comes into contact with wound exudate. This controlled release of low concentrations of iodine helps to minimise the negative side effects of using free elemental iodine.
Modern iodine preparations
The two most commonly used iodophors in modern wound dressings (Table 1) are:
Povidone iodine preparations were introduced in the 1960s and it is now the most common iodophor in clinical use. It is available in different formulations, including solution, cream, ointment, spray and wound dressings.
What is the evidence to support iodine use?
There is extensive evidence to support the use of povidone iodine in wound healing  (Table 2), but its use is not without controversy due to perceived issues with toxicity, systemic absorption and delayed healing. It has been suggested that iodine has a negative impact on cells involved in the wound healing process and because of this its safety and efficacy have been questioned.
Some reviews have analysed the conflicting evidence and have found that studies based on animal models tend to support the argument for iodine's cytotoxicity, whereas human studies suggest that PVP-I can help the wound healing process by reducing bacterial load and decreasing infection rates [9,10]. One study demonstrated that not only does PVP-1 significantly improve the healing rates of chronic venous leg ulcers, but also that it lacks cytotoxicity in vivo .
The efficacy of cadexomer iodine has been demonstrated using both animal models and clinical studies. Cadexomer iodine was found to significantly reduce symptoms associated with infection (eg exudate, erythema, oedema and pain) in patients with pressure ulcers  and venous leg ulcers .
In addition to providing an antimicrobial effect, in vitro studies have reported a lack of toxicity for human fibroblast activity  and that cadexomer iodine may increase epithelialisation of chronic wounds [15,16]. However, its mode of action is not understood and further research is needed to determine whether wound aetiology has a contributory role .
How does iodine work as an antimicrobial?
Iodine's exact antimicrobial mode of action is not fully understood, but it is believed to be associated with its ability to rapidly penetrate the cell wall of micro-organisms.
Schreier et al also investigated the effects of PVP-1 on microbial cells and found that it affects the structure and functions of enzymes and cell proteins and damages bacterial cell function by blocking hydrogen bonding and altering the membrane structure. These multiple modes of action ensure the rapid death of microbes and help to prevent the development of bacterial resistance. Because the microbicidal action of iodine is related to several directly toxic effects on the cell wall, rather than through specific molecular pathways (as used by antibiotics), resistance is highly unlikely and reports of iodine-resistant strains are exceptionally rare (Figure 1).
Is iodine effective against MRSA?
There is substantial in vitro evidence demonstrating that PVP-I is a highly effective and broad spectrum antimicrobial. Activity has been demonstrated against both common bacterial wound isolates[18,19[ and antibiotic-resistant species20. Lacey and Catto determined that more than 99% of meticillin-resistant Staphylococcus aureus (MRSA) cells were killed within 10 seconds of exposure to PVP-I. Mertz et al found that cadexomer iodine significantly reduced MRSA and total bacteria in partial thickness porcine wounds compared with a no-treatment control and a vehicle group.
Is iodine effective against biofilms?
At the most basic level, a biofilm can be described as being bacteria embedded in a slimy, protective mucopolysaccharide glycocalyx. The effectiveness of iodine in the management of biofilm is currently unclear, although it is known that low dose, slow release iodine is effective in killing free-floating planktonic micro-organisms and is therefore likely to be a good choice of antiseptic dressing when the intention is to suppress biofilm formation or prevent recontamination.
Recent evidence suggests that sustained release iodine may penetrate biofilms more effectively than silver or polyhexamethylene biguanide (PHMB).
Can patients build up bacterial resistance to iodine?
Despite 170 years of prolonged and extensive use of iodine in medicine and wound care, iodine-resistant microbial strains are exceptionally rare.
The validity of the one documented case of resistance to iodine products26 has been questioned and the methodology of the study criticised.
When is iodine indicated?
An international consensus document on managing wound infection, recommends the use of antiseptic dressings as being part of an overall management plan in the following circumstances:
Slow release iodine dressings have been used to treat a range of wound types where infection is present or suspected.
These include pressure ulcers, venous leg ulcers, diabetic foot ulcers, minor burns and superficial skin-loss injuries[24,28].
When are iodine dressings contraindicated?
Iodine dressings must be used under medical supervision in patients with thyroid diseases, known or suspected iodine sensitivity, in pregnant or breastfeeding women or in newborn babies and up to the age of six months8.
Long-term use of PVP-I has been loosely associated with mild hyperthyroidism and long-term use is not recommended for patients with impaired thyroid function. However, a number of studies have monitored thyroid function during PVP-I clinical trials and have reported that it remains unchanged[30-32].
To avoid toxicity or the hypothetical risk of thyroid-related complications, iodine products should be used with caution in children, in those with large burn areas, and where prolonged treatment of large open wounds is required. The use of iodine dressings should also be avoided before and after the use of radio-iodine diagnostic tests (until permanent healing).
Reports of systemic effects following short-term PVP-I treatment are extremely rare. Iodine absorption has been found to be dependent on the size of the wound and the duration of treatment. Hunt et al also discovered a relationship between wound area and iodine levels in serum and urine following the treatment of burn wounds with PVP-I, but it was proposed that renal function was a factor in the determination of this. Iodine should, therefore be avoided in patients with significant renal disease.
How to apply iodine dressings
The method of application depends on the mode of delivery. Dressings should be applied directly to the surface of the wound and covered using a secondary dressing as appropriate. All dressings should be applied according to the manufacturer's instructions for use.
Why does iodine stain the skin brown?
The skin is sometimes stained brown after treatment with iodine products. This is because of the effects of the tri-iodide ion and, to a lesser extent, free molecular iodine.
However, any staining that may occur is harmless and will quickly fade.
Why does the application of iodine sometimes sting?
Iodine-based products can be associated with a transient burning or stinging sensation immediately after they are applied to open wounds. However, this is not harmful. The stinging probably relates to the osmotic loads delivered by higher concentrations of iodine in some preparations.
The prevalence of allergic reactions to the topical application of iodine varies considerably (between 0.7% and 41%), depending on which study is reviewed.
For example, in a dermatological study by Juhász35 involving 50 patients, no cases of sensitisation to PVP-I after patch-testing were recorded.
How frequently should dressings be changed
The slow release of the iodine in these preparations allows the wound to remain in continuous contact with the antiseptic, whereas with a single exposure to a product such as PVP-I tulle the iodine is soon broken down[36,37]. It is important to remember that even with modern iodine dressings (eg Inadine, Iodosorb and Iodoflex), which provide a slow release of iodine, this is only for a relatively short time and frequent dressing changes are required to constantly replenish the supply of antiseptic to the wound. It is assumed that once the dressing has lost its 'colour' the antiseptic effect has been lost and the dressing should be changed.
In heavily exudating wounds, dressings may need to be changed daily. With appropriate moisture balance iodine dressings can be applied 1-3 times per week.
When should treatment be discontinued?
Medical supervision should be sought if using iodine for more than one week. Treatment should be re-evaluated regularly and discontinued when the signs of infection resolve and the wound is healing.
If the wound does not improve after 10-14 days, the patient and the wound should be re-evaluated and an alternative antiseptic dressing regimen or systemic treatment with an antibiotic considered.
What are the economic arguments for iodine?
Povidone iodine dressings and irrigating solutions are relatively inexpensive compared to other antimicrobial therapies. Dressings that lose their colour (eg Inadine) may be more cost-effective in that they provide an indicator of how frequently dressings should be changed.
Iodine and the future
Iodine-containing dressings may be developed that combine the antimicrobial effect of iodine with autolytic debridement, moisture balance and active therapies to optimise wound healing.
Although it has been speculated that iodine delays healing and is cytotoxic, there is substantial evidence to suggest that the commonly-used low concentration, slow release iodophors improve healing rates and are effective as highly potent antimicrobials with a broad spectrum of activity, including antibiotic-resistant strains such as MRSA. It is unfortunate that the concerns about iodine are based on studies that are so varied in method and design that it is difficult to draw reliable comparisons and conclusions. The reputation of iodine wound products, used as antimicrobials, suffered as a result of these studies but it is now widely accepted that slow-releasing iodophor antimicrobials are safe and have minimal detrimental impact on wound healing.
To cite this publication
Sibbald RG, Leaoer DJ, Queen D. Iodine Made Easy. Wounds International 2011; 2(2): Available from http://www.woundsinternational.com
Sibbald RG, Leaper DJ, Queen D.
1. Professor of Public Health Sciences and Medicine, University of Toronto, Toronto, Ontario, Canada
2. Visiting Professor, Department of Dermatology and Wound Healing, Cardiff University, Cardiff, UK
3. Honorary Research Fellow, Department of Dermatology and Wound Healing, Cardiff University, Cardiff, UK
Healthcare practitioners are advised to consult the package inserts for any iodine-based products before applying the dressing to a wound.
This article is supported by an educational grant from Systagenix. The views expressed in this 'Made Easy' section do not necessarily reflect those of Systagenix.
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