WO2008149104A1

WO2008149104A1 - An antimicrobial composition comprising an aqueous solution of silver and copper

Info

Publication number: WO2008149104A1
Application number: PCT/GB2008/001945
Authority: WO
Inventors: John Newcomb Bostock; Duarte Maria Botelho Moniz Novoes Tito
Original assignee: Aguacure Limited
Priority date: 2007-06-07
Filing date: 2008-06-06
Publication date: 2008-12-11
Also published as: GB0710888D0; GB2449893A

Abstract

The invention provides an antimicrobial composition comprising an aqueous solution of silver and copper, devices/formulations comprising such a composition and a method of preparing such a composition. The invention also provides a use of a composition of the invention to destroy colonies of microorgansims and a method of treating a human or animal suffering from a microbial infection, the method comprising administering an effective amount of a composition of the invention.

Description

AN ANTIMICROBIAL COMPOSITION COMPRISING AN AQUEOUS SOLUTION OF SILVER AND COPPER

The present invention relates to antimicrobial compositions, methods for making antimicrobial compositions and the use of such compositions.

The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

Microorganisms are known to present health hazards due to infection or contamination. Antimicrobials are commonly used to prevent and/or retard infection or contamination. A problem with antimicrobials, for example antibiotics, is toxicity. This is a particular problem for antibiotics which are administered systemically. There are a number of possible side-effects that may be experienced when using antibiotics and these are more likely to occur if antibiotics are administered systemically rather than topically. A further disadvantage to using systemic antibiotics is that they are distributed through the body, therefore, only a small amount of the total dose reaches the local infection site.

Antibiotics (and other antimicrobial agents) that are administered topically have an advantage over systemic antibiotics, in that they significantly increase the concentration of the antibiotic locally compared to the concentration when administered systemically. See, for example, Mulligan, A. M.; Wilson, M.; Knowles, J. C. Biomaterials 2003, 24, 1797.

There exist further problems associated with antimicrobials (e.g. antibiotics), regardless of how they are delivered. Such problems include the development of resistance. For example, antibiotic-resistant strains are becoming more common and tools to fight them are scarce.

Silver is known as an antibacterial, but its biocidal mechanism remains unclear. See,* for example, Spacciapoli, P.; Buxton, D.; Rothstein, D.; Friden, P. Journal of Periodontal Research 2001, 36, 108; Klasen, H. J. Burns 2000, 26, 131; Klasen, H. J. Burns 2000, 26, 117; Silver, S.; Phung, L. T.; Silver, G. Journal of Industrial Microbiology & Biotechnology 2006, 33, 627; Gallant-Behm, C. L.; Yin, H. Q.; Liu, S. J.; Heggers, J. P.; Langford, R. E.; Olson, M. E.; Hart, D. A.; Burrell, R. E. Wound Repair and Regeneration 2005, 13, 412; Fox, C. L.; Modak, S. M. Antimicrobial Agents and Chemotherapy 1974, 5, 582; Dibrov, P.; Dzioba, J.; Gosink, K. K.; Hase, C. C. Antimicrobial Agents and Chemotherapy 2002, 46, 2668; Holt, K. B.; Bard, A. J. Biochemistry 2005, 44, 13214; Bragg, P. D.; Rainnie, D. J. Canadian Journal of Microbiology 1974, 20, 883; Feng, Q. L.; Wu, J.; Chen, G. Q.; Cui, F. Z.; Kim, T. N.; Kim, J. O. Journal of Biomedical Materials Research 2000, 52, 662; Ugur, A.; Ceylan, O. Archives of Medical Research 2003, 34, 130.

The silver cation (Ag⁺) is believed to be the active antimicrobial agent (see, for example Spacciapoli, P.; Buxton, D.; Rothstein, D.; Friden, P. Journal of Periodontal Research 2001, 36, 108; Silver, S.; Phung, L. T.; Silver, G. Journal of Industrial Microbiology & Biotechnology 2006, 33, 627; and Ugur, A.; Ceylan, O. Archives of Medical Research 2003, 34, 130). The chemical environment relevant to biomedical applications is typically complex. For example, life-supporting environments are generally nutrient-rich and contain a variety of salts (e.g. halogenates, phosphates and sulphates) that induce the precipitation of silver. This may retard and/or prevent delivery of Ag⁺ to the desired location. Consequently, it would be desirable to develop an efficient strategy for delivering silver (e.g. Ag⁺) at the point of use.

Living organisms require copper at low concentrations as cofactors for metalloproteins and enzymes. For instance, copper is needed for the function of several important enzymes in Escherichia coli including the cytochrome bo ubiquinole oxidase, copper-zinc superoxide dismutase, and amine oxidase (see, for example, Franke, S.; Grass, G.; Rensing, C; Nies, D. H. Journal of Bacteriology 2003, 185, 3804). However, copper can also be extremely toxic. At high concentrations, Cu^2τ induces an inhibition of growth in bacteria, and has a toxic effect on most microorganisms. For example, metallic copper and copper alloys have antibacterial activity over the bacterium E. coli and also inhibit the adhesion of bacteria on biofilm development (see Faundez, G.; Troncoso, M.; Navarrete, P.; Figueroa, G. Bmc Microbiology 2004, 4). However, resistance to copper is a problem. Microbial tolerance to high levels of copper is thought to be environmentally inducible, as metal exposure may exert a selective pressure for acquisition of resistance. Moreover, some bacterial species accumulate and resist copper without any prior exposure (Sawaidis, L; Hughes, M. N.; Poole, R. K. World Journal of Microbiology & Biotechnology 2003, 19, 117).

The present invention addresses the foregoing deficiencies by providing an antimicrobial composition comprising an aqueous solution of silver and copper. Unless otherwise stated herein, such compositions will be known as the compositions of the invention.

By the term "antimicrobial composition" we mean any chemical substance that can destroy microorganisms. By the term "microorganisms" we include all microscopic organisms. Preferably, the antimicrobial compositions of the invention are capable of destroying bacteria, fungi (e.g. yeast), algae, and molds, and mixtures thereof. The compositions are particularly suited as bactericidal and fungicidal agents.

The compositions of the invention may be chemically or electrochemically generated. By chemically generated we include adding water soluble silver and copper salts to water. Preferably, the compositions of the invention are electrochemically generated. Preferably, the compositions of the invention also comprise at least one metal (in addition to silver and copper) which has antimicrobial properties. For example, the at least one additional metal may be selected from potassium, zinc, selenium, titanium, gold, palladium, platinum and other metals suitable for electrochemical delivery and mixtures thereof. Preferred compositions of the invention comprise zinc. Without being bound by theory, it is believed that the presence of zinc may help to retard and/or prevent the precipitation of silver and/or copper from solution.

The silver, copper and any additional metal(s) may be in the form of ions and/or particles (e.g. nanoparticles) and/or one or more compounds of the metals. Preferably, the silver, copper and any additional metal(s) are in the form of ions and/or particles. For example, the particles may comprise ions of the silver, copper and any additional metals present. The invention also provides methods of preparing the compositions of the invention. One method of the invention comprises providing an electrochemical cell comprising silver (Ag) and copper (Cu) electrodes in water and applying a potential difference across the electrodes to drive a current, thereby delivering silver and copper into the water. Unless otherwise stated herein, such a method will be known as the method of the invention. If one or more additional metals (e.g. zinc) are present in the compositions of the invention, the electrochemical cell described above may also comprise an electrode of the or each metal and a potential difference is applied across the or each electrode, thereby delivering the additional metal(s) into the water.

Description of the Figures (which are for illustrative purposes only and not limiting to the scope of the invention):

Fig 1 illustrates the relationship between current and concentration of silver and copper .

Fig 2 illustrates the relationship between flow rate and concentration of silver and copper .

Fig 3 illustrates the results of biological assays of solutions containing copper only, silver only and both silver and copper against Staphylococcus Aureus.

Fig 4 illustrates the results of biological assays of a solution containing Ag and Cu

(Ag 6788 ppb Cu 2457 ppb) against Staphylococcus Aureus.

Fig 5 illustrates the results of biological assays of a solution containing Ag and Cu

(Ag 6788 ppb Cu 2457 ppb) against Pseudomonas Aeruginosa.

Fig 6 illustrates the results of biological assays of three solutions containing Ag, Cu and Zn (various concentrations) against Staphylococcus Aureus.

Fig 7 illustrates the results of biological assays of an electrochemically prepared solution and a chemically prepared solution both containing Ag and Cu against

Staphylococcus Aureus.

Any suitable silver and copper electrodes may be used. The invention envisages using electrodes comprising eutectic alloys or single element electrodes of the metals. Suitable electrodes are commercially available from Goodfellow (http://www.goodfellow.com/csp/active/gfHome.csp^') or Cooksons

(http://www.cooksongold.com/uk/home.isp). If separate electrodes of Ag, Cu and any additional metal(s) are used, it is preferred if they are coupled to separate power supplies. This enables the concentration of silver and copper delivered into solution to be separately controlled. Specific ions of, for example, Ag, Cu or any additional metal, may be added chemically if antimicrobial efficiency is not significantly impaired by doing so.

Any suitable water may be used according to the invention, including tap, distilled and/or deionised water. Preferably, distilled or deionized water is used.

The equations below describe the electrochemically driven phase change of the elements from the metallic state to the aqueous medium taking place at the positive electrodes (anodes).

Ag(_S) ► Ag⁺ _(aq)+ e-

0.8 V (V₅. NHE)

CU(_S) ► Cu²⁺ _(aq) + e- E⁰ C_u ²⁺Z_Cu - 0.34 V (vs. NHE)

On the cathodes (negative electrodes), the complementary reaction is the hydrolysis of water, resulting in the evolution of hydrogen and production of hydroxide ions:

H₂O₀) ► K H₂ (_g) + OH-_(aq) E° H⁺ZH₂ = 0 V (vs. NHE)

A conducting agent may be added to the water in order to increase the conductivity. However, it may be desirable to avoid the use of a conducting agent as this may increase the complexity and decrease the purity of the composition thus formed. If used, preferred conducting agents are highly soluble salts. A preferred group of conducting agents are nitrate salts, particularly alkali metal (lithium, sodium, potassium, rubidium and caesium) and alkaline earth metal (beryllium, magnesium, calcium, strontium and barium) nitrate salts, or mixtures thereof. Currently preferred nitrate salts are sodium nitrate (NaNO₃) and potassium nitrate (KNO₃), especially KNO₃.

Typically, the concentration of conducting agent, when used, is less than about 6.2 x 10^~3 mol/L, for example in the range of from about 4.9 x 10^"5 to about 4.9 x 10^~3 mol/L. Preferably, the conducting agent concentration is less than about 2.6 x 10^~3 mol/L, for example in the range of from about 4.95 x 10^~5 to about 1.24 x 10^"3 mol/L.

The concentration of conducting agent is proportional to the conductivity of the solution in the electrochemical cell. Typically, the conductivity of the aqueous solution in the electrochemical cell will be in the range of from about 0 to about 1000 μS (e.g. 0.001 to 1000 μS). If a conducting agent is used, the conductivity preferably is in the range of from about 8 to about 800 μS. If no conducting agent is used, the conductivity preferably is in the range of from about 0.05 to about 4 μS. As a general rule, as the conductivity in the electrochemical cell increases, so does the rate at which silver and copper and any additional metal(s) present are delivered into solution. Therefore, increasing the conductivity of the electrochemical cell generally results in compositions of the invention with greater concentrations of silver and copper and any additional metal(s). However, the concentration of silver and copper and any additional metal(s) delivered into solution also depends on other factors, as explained in more detail below.

For example, if a potential difference is applied across the silver and copper electrodes in order to drive a current of set magnitude, the amount of Cu and Ag delivered to solution will depend at least in part on the magnitude of the current to the electrodes. This is because the greater the current, the greater the rate at which the reactions below proceed:

Ag_(S) → Ag⁺ _(aq)+ e^" Cu{_S) — ► Cu²⁺ _(aq) + e^"

Consequently, the concentration of Cu and Ag produced typically is directly proportional to the current flow. Of course, the amount of time for which current is driven through the electrodes will also be important.

As the above reactions progress, it is believed that several surface phenomena occur such as development of concentration gradients, (uneven) etching of electrode surface, competing reactions and polarisation of the electrode surface. These phenomena are thought to have a variable impact on the reaction rate, and thus on the concentration of Cu and Ag in the resulting solutions. These effects can be minimised by polarisation switching, which is cyclically altering the polarity of the electrodes. It is believed that such polarity switching prevents and/or retards one electrode being sacrificial and the other accumulating. Fig 1 illustrates the results of a series of experiments in which the current was varied and implementing polarisation switching. The results show that the concentration of Cu and Ag in the solutions produced was directly proportional to the current flow.

Typically, the current at the electrodes will be in the range of from about 0 to about 1000 mA/cm² (e.g. 0.001 to 1000 rnA/cm²), preferably in the range of from about 0.5 to about 600 mA/cm². The skilled person will appreciate that the current at the silver electrode and the current at the copper electrode or at the electrode of any other metal(s) present may be the same or different. This may be dictated by the desired concentration of Cu and Ag and any additional metal(s) in the resulting solution.

Preferably, water is flowed through the electrochemical cell at a given rate. Since the flow rate determines the amount of time a given volume of water spends in contact with the electrode surface as it flows through the cell, it is preferable to employ a means (e.g. a pump) to control the flow of water (e.g. a calibrated peristaltic pump).

A slower flow rate results in a longer contact time between water and electrode surface, hence the greater the concentration of Cu and Ag and any additional metal(s) in the resulting solution will be for a given current. Typically, the flow rate will be in the range of from about 0.1 to about 50 L/min, preferably from about 1 to about 10 L/min. Fig 2 illustrates the results of experiments in which the flow rate was varied. The concentration of silver and copper ions decreased linearly as the flow rate was increased from 2 to 4 L/min.

The current and flow rate may be defined in relation to each other (i.e. current per flow rate). Typically, the current per flow rate will be in the range of from about 0.002 to 2000 mA.cm^'VL.min^"1, preferably from about 1 to about 1200 mA.cm^' ^.min^"1.

In summary, the current determines the rate at which Ag, Cu and any additional metal(s) are delivered to solution and the flow rate of water through the cell determines the residence time (i.e. the contact time between solution and electrodes). In other words, the combination of current and flow rate determines the amount of Ag and Cu and any additional metal(s) delivered per unit volume of water and, therefore, the concentration of Ag and Cu in the compositions of the invention.

The intensity of the current flowing between electrodes is a result of the applied potential whose magnitude, in turn, is related to the resistance of the system according to the equation:

Potential (V) = Resistance (R) x Current (I)

The resistance is related to the conductivity (C) (R = 1/C) and thus the amount of power required for producing the compositions of the invention is directly related to the conductivity of the water used. Typically, the more conductive the solution, the less power is necessary to drive reactions taking place at the electrode surface. The conductivity may be varied by the provision of a conducting agent. Typically, the greater the concentration of conducting agent, the more conductive the solution will be.

Overall, therefore, the combination of current, flow rate and conductivity governs the concentration of silver and copper and any additional metal(s) in the solutions produced. Typically, the greater the concentration of silver and copper and any additional metal(s), the greater the antimicrobial strength of the compositions of the invention.

The compositions of the invention may typically comprise from about 10 to about 1,000,000 ppb of each of copper, silver and any additional metal(s) present. Preferably, the compositions comprise from about 100 to about 15000 ppb or about 300 to about 12000 ppb of each of copper and silver. For example, the compositions of the invention may comprise from about 500 to about 12000 ppb or about 1000 to about 10000 ppb, such as from about 3000 to about 7000 ppb of silver (e.g. about 5000 ppb) and from about 500 to about 12000 ppb or about 1000 to about 8000 ppb, such as from about 2000 to about 4000 ppb of copper (e.g. about 3000 ppb). If any additional metals (e.g. zinc) are present in the compositions of the invention, they are each typically present in an amount of from about 100 to about 10000 ppb, such as from about 200 to about 5000 ppb, for example from about 300 to about 3000 ppb, preferably from about 500 to about 2000 ppb (e.g. about 1000 ppb).

The compositions of the invention may contain other components in addition to the Ag, Cu and any additional metal(s). For example, if KNO₃ is used as a conducting agent in the method of the invention, the resulting composition of the invention typically will comprise Cu, Ag, K, NO₃ ^", OH^" and H⁺. Such compositions may also comprise an additional metal (e.g. zinc). The concentration of the elements in such compositions may be monitored by any suitable means known in the art, for example by Inductively Coupled Plasma (ICP) against standard solutions (see, for example, A. Montaser and D. W. Golightly (eds.), Inductively Coupled Plasmas in Analytical Atomic Spectrometry, VCH Publishers, Inc., New York, 1992).

The pH of the compositions of the invention is typically in the range of from about 4 to about 9, preferably from about 5 to about 8, such as from about 6 to about 7 (e.g. about 6.5).

Without being bound by theory, it is believed that the compositions of the invention may contain silver and copper in the form of particles comprising silver and particles comprising copper. Compositions of the invention which also contain an additional metal (e.g. zinc) may contain particles comprising the additional metal. Particles comprising two or more of silver, copper and any additional metal(s) may also be present. The silver, copper and any additional metal(s) contained in the particles described above may be in the form of ions.

The particles described above may have a diameter of less than 450nm, preferably less than 200nm, such as less than lOOnm, e.g. less than 20 nm. An indication of the particle size in the compositions of the invention may be achieved by filtering the compositions using inert filters of various pore sizes (e.g. 20nm, 200nm and 450nm filters, available from Whatman). Of course, any other means to measure particle size known in the art may be used.

The compositions of the invention may be stored for long periods of time, for example for greater than about 1 day, greater than about 1 week, greater than about 1 month or even greater than about 1 year, without substantially reducing their antimicrobial efficacy.

The compositions of the invention are capable of destroying a wide range of microorganisms. For example, the compositions of the invention are believed to have potent antibacterial properties against all bacteria (e.g. Staphyloccus and Pseudomonas). The antimicrobial properties of the compositions of the invention are thought to be superior to the antimicrobial properties of the corresponding compositions comprising only copper or only silver. In other words, it is believed that the compositions of the invention unexpectedly exhibit a synergistic effect between the silver and copper and any other metals present. Additionally, it is believed that the combination of silver, copper and any other metal(s) present prevents and/or retards the bacteria from developing resistance to the compositions of the invention compared to the corresponding compositions comprising only copper or only silver.

Accordingly, the invention provides a use of a composition of the invention to destroy colonies of microorganisms. The invention also provides a method of treating a human or animal body suffering from a microbial infection, the method comprising administering an effective amount of a composition of the invention to the human or animal body.

The compositions of the invention may be used in any suitable form, depending on factors such as the microorganisms which it is desirable to destroy (e.g. bacteria) and the site at which the microorganisms are populated. For example, the compositions of the invention may be used in the treatment of microbial infections (see above), but also for use in sterilisation of human or animal body surfaces (e.g. human hands or cow udders) or inanimate surfaces (e.g. work surfaces or farrowing crates).

The invention also provides a delivery device loaded with a composition of the invention. Such devices include those which deliver the composition in any suitable form such as a spray, a gel, a cream or a liquid. Alternatively or additionally, such a device may be in the form of a transdermal patch, a dressing or an impregnated material (e.g. a wipe impregnated with the composition of the invention). The invention further provides a kit of parts comprising an aqueous solution of silver and an aqueous solution of copper. Such kits may be arranged so that in use, the aqueous solution of silver is combined with the aqueous solution of copper to provide a composition of the invention as herein described. Accordingly, the kits of the invention may exhibit any appropriate feature of the compositions of the invention as herein described, such as one or more additional metals.

The compositions of the invention may be formulated as antibiotics or antifungals for administration (e.g. to the eye, orally, nasally, intravenously, transdermally and topically) to a human or animal. By way of example, compositions for topical administration may be formulated in a device as a spray, a cream, a gel (e.g. for use in wound dressing) or a transdermal patch, a dressing or an impregnated wipe for application to the skin of a human or animal body. Other components may be added to the compositions of the invention to provide such formulations.

The invention will now be illustrated, but not limited, by the following Examples.

Example 1

The biocidal effectiveness of the compositions of the invention was assessed using screening tests with Pseudomonas Aeruginosa (gram negative bacteria) and Staphylococcus Aureus (gram positive bacteria).

Screening was done by comparing the number of colony forming units (CFU' s) grown on nutrient agar plates (37⁰C overnight) inoculated with equivalent amounts of control solution (bacteria in water; control plate) or test solution (bacteria in Ag and Cu (and optionally Zn) solution; test plate).

Control and test solutions were prepared by mixing aliquots of bacteria grown overnight (500 μl) with 5.5 ml of water (control sample) or 5.5 ml of antibacterial fluid (test sample). Solutions were left under mild agitation and exposed to sunlight during testing whilst samples were collected at set intervals and spread over plates for screening. Example Ia

A chemically prepared solution containing Ag⁺ and Cu²⁺ (1783 ppb and 1512 ppb, respectively) was compared with corresponding solutions containing Ag⁺ (1800 ppb) or Cu²⁺ (1429 ppb) (using AgNO₃ and Cu(NO₃ )₂) for activity against Staphylococcus Aureus. The results are illustrated in Fig 3.

Example Ib

The shelf life of the compositions of the invention was investigated by testing the biocidal activity of an electrochemically generated solution containing Ag = 6788 ppb and Cu = 2457 ppb against Pseudomonas Aeruginosa and Staphylococcus Aureus (see Figs 4 and 5). Population reductions in the order of 99.9999% were consistently achieved within 60 minutes of inoculation (populations in the order of 10⁸ reduced to 10²) despite 2 weeks of storage before use.

Example Ic

Aqueous solutions containing the following amounts of silver, copper and zinc were prepared electrochemically.

Solution Concentration / ppb

#6 Ag 5402; Cu 6226; Zn 6232

#7 Ag 1935; Cu 2716; Zn 3153

#8 Ag 903; Cu 701 ; Zn 626

The bactericidal activity of these solutions against Staphylococcus Aureus was measured and is illustrated in Fig 6.

Example Id

The bactericidal activity against Staphylococcus Aureus of an electrochemically prepared solution and a chemically prepared solution were compared. The solutions contained the amounts of Ag and Cu set out below. The results are illustrated in Fig

7.

Electrochemically prepared solution: 6787 ppb Ag and 2456 ppb Cu Chemically prepared solution: 7652 ppb Ag and 3038 ppb Cu

Example Ie

The shelf life of the compositions of the invention was investigated by testing the biocidal activity of an electrochemically generated solution containing Ag = 844 ppb and Cu = 13657 ppb against Staphylococcus Aureus (see Fig 8). The biocidal activity test was run over 8 months after the composition was originally prepared, thus demonstrating the long shelf life of the compositions of the invention.

Example 2

Bacterial cells from a single colony were grown up overnight at 37⁰C. Overnight cultures were centrifuged to pellet bacteria and subsequently resuspended in phosphate buffer. Pellets were formed again and washed twice with distilled water before addition to either test solution or distilled water (control).

Bacteria solutions (test and control) were vortexed for 30 seconds before collection of an aliquot at T₀. Test and control solutions were then kept under agitation (225rpm) from which samples were subsequently collected, diluted and spread over nutrient broth media plates every 5 minutes.

Plates were then incubated overnight at 37⁰C and the average number of colonies counted three times. The bacterial population from each time point was worked out from the average number of colonies grown on test plates.

Example 2a

Aqueous solutions containing the following amounts of silver, copper and zinc were prepared electrochemically. Solution Concentration / ppb

#3 Ag 3495; Cu 1654; Zn 492

#4 Ag 2550; Cu 2446; Zn 688

The bactericidal activity of these solutions against Esherichia coli was measured and is illustrated in Fig 9. T₀ was measure at approximately T_3Osecs, resulting in the zero count for solution #3 at T₀.

Example 2b

An aqueous solution containing 3495 ppb silver, 1654 ppb copper and 492 ppb zinc was prepared electrochemically. The bactericidal activity of this solution against Methicillin-resistant Staphylococcus Aureus (MRSA) was measured and is illustrated in Fig 10.

Claims

1. An antimicrobial composition comprising an aqueous solution of silver and copper.

2. A composition according to claim 1 further comprising at least one additional metal selected from potassium, zinc, selenium, titanium, gold, palladium and platinum.

3. A composition according to claim 2 comprising zinc.

4. A composition according to any of the preceding claims wherein the metals are in the form of particles and/or ions and/or compounds thereof.

5. A composition according to any of the preceding claims obtainable by a process in which the solution is generated electrochemically.

6. A composition according to any of the preceding claims comprising from about 10 to about 1,000,000 ppb of silver.

7. A composition according to any of the preceding claims comprising from about 10 to about 1,000,000 ppb of copper.

8. A composition according to any of claims 2 to 7 comprising from about 10 to about 1,000,000 ppb of the at least one additional metal.

9. A composition according to any of the preceding claims having a pH of from about 4 to about 9.

10. A composition according to any of the preceding claims in the form of a sprayable composition, a cream or a gel.

11. A delivery device loaded with a composition as defined in any of the preceding claims.

12. A device according to claim 11 in the form of a transdermal patch, a dressing or an impregnated material.

13. A kit of parts comprising an aqueous solution of silver and an aqueous solution of copper.

14. A kit according to claim 13 further comprising an aqueous solution of at least one additional metal selected from potassium, zinc, selenium, titanium, gold and platinum.

15. A kit according to claim 13 or 14 obtainable by a process in which the metals are generated electrochemically.

16. A method of preparing an antimicrobial composition comprising an aqueous solution of silver and copper, the method comprising: providing an electrochemical cell comprising silver and copper electrodes in water; applying a potential difference across the electrodes to drive a current, thereby delivering silver and copper into the water.

17. A method according to claim 16 further comprising adding a conducting agent to the water.

18. A method according to claim 17 wherein the conducting agent is potassium nitrate.

19. A method according to any of claims 16 to 18 wherein the antimicrobial composition further comprises at and least one additional metal selected from potassium, zinc, selenium, titanium, gold and platinum, and the electrochemical cell comprises an electrode of the or each additional metal.

20. A method according to any of claims 16 to 119 wherein the electrochemical cell has a conductivity of from about 0 to about 1000 μS.

21. A method according to any of claims 16 to 20 wherein the water is flowed through the electrochemical cell at a rate of from about 0.1 to about 50 L/min.

22. A method according to any of claims 16 to 21 wherein the current is from about 0 to about 1000 mA/cm².

23. Use of a composition according to any of claims 1 to 10 or a kit according to any of claims 13 to 15 to destroy microorganisms.

24. A method of treating a human or animal suffering from a microbial infection, the method comprising administering an effective amount of a composition according to any of claims 1 to 10 to the human or animal.

25. A method according to claim 24 wherein the composition is administered topically.

26. An antimicrobial composition according to any of claims 1 to 10 for use in medicine.

27. Use of a composition according to any of claims 1 to 10 in the manufacture of a medicament for treating a human or animal suffering from a microbial infection.

28. A composition according to any of claims 1 to 10 for use in treating a human or animal suffering from a microbial infection.

29. A composition according to any of claims 1 to 10 for use in the sterilisation of human or animal body surfaces or inanimate surfaces.

30. Use of a composition according to any of claims 1 to 10 for sterilising human or animal body surfaces or inanimate surfaces.

31. Any novel composition of the invention generally as herein described.

32. Any novel composition generally as herein described with reference to the Examples.

33. Any novel method of the invention generally as herein described.

34. Any novel method generally as herein described with reference to the Examples.

35. Any novel kit of parts of the invention generally as herein described.

36. Any novel use of a composition generally as herein described.

37. Any novel use of a composition generally as herein described with reference to the Examples.