Due to the continuing progression of antibiotic resistant bacteria development in the medical community a number of individuals believe it is time to explore how to incorporate silver, a long standing antibacterial agent, into treatment strategies. On its face this appears to be an important new avenue for attacking the growing problem of resistant bacteria and the lack of development of new antibiotics to fight these bacteria. However, this excitement must be stemmed by appropriate knowledge of how silver helps fight bacterial infections and the potential pitfalls to its incorporation into actual antibiotics or other blood based treatment strategies versus its current medical uses as a sterilization and wound dressing tool.
The use of silver in medical and wound management dates back to the 18th century with the application of silver nitrate to treat ulcers.1,2 Anti-microbial activity for silver ions was first scientifically suspected in the 19th century, although anecdotal reasoning can be traced back much earlier.3 However, in the early 20th century the overall effectiveness of penicillin and later discovery of other antibiotics displaced silver use for over two decades. Silver use increased in the 1960s in response to burn treatments through the use of a silver sulfadizine (SSD) cream. In modern times silver is incorporated into wound dressings, antibiotic cream and first-aid plasters to reduce bacterial growth and limit infection.4,5
Both bulk silver and isolated silver nano-particles are effective at killing, not only standard Gram-negative and Gram-positive bacteria, but also antibiotic-resistant strains.6-8 This action also catalyzes antibacterial activity of current antibiotics including penicillin G, amoxicillin, erythromycin, clindamycin, and vancomycin.9 Various mechanisms have been suggested to explain the anti-microbial action of silver: 1) particle penetration of bacterial cell walls and membranes leading to DNA damage and apoptosis possibly due to free radical incursion; 2) alteration of bacterial cell wall and membrane, most likely targeting lipopolysaccharides, forming pits increasing membrane permeability for antibiotics or disrupting respiration for aerobes; 3) lowering probability of biofilm formation increasing the probability of antibiotic effectiveness.10-13
There is no clear evidence regarding whether the anti-microbial effects of silver are found in colloidal silver or ionic silver or both, but some evidence suggests that colloidal silver is much less effective as an anti-microbial agent than ionic silver, especially because only oxidized silver nanoparticles exert a significant antibacterial effect.14,15 If colloidal silver has anti-microbial action its magnitude is largely dictated by the dimensions of the particles where smaller particles have a greater effect due to higher surface area to volume ratios.16-18 The shape of the nano-particle also seems important with truncated triangles having superior activity.19,20
Ionic silver is thought to activate its anti-microbial effects through the formation of complexes with negatively charged sulfur, nitrogen or oxygen functional groups in bacterial enzymes destabilizing the bacteria and preventing normal functions like metabolism and reproduction.15,21 So silver nano-particles may simply be an inert element that eventually degrades creating the actual silver anti-microbial agent with degradation occurring faster at smaller sizes and certain shapes. Therefore, when studying anti-microbial activity and possible silver toxicity it stands to reason that one should focus on oxidized silver versus neutral silver.
While the anti-bacterial properties of silver were suggested in the 19th century, silver resistant bacteria and the basis for that bacterial resistance was first reported much later in 1975, first in Salmonella typhimurium.22,23 This resistance is chiefly born from plasmid development or transfer.24,25 The chief silver resistant plasmid has been labeled pMG101, is 180-182 kb with nine Open Reading frames (seven known and two unknown) in three transcriptional units and like most plasmids is conjugally transferable between bacteria.24,26 In addition to silver, pMG101 grants additional resistance to mercury, tellurite, ampicillin, chloramphenicol, tetracycline, streptomycin, and sulphonamide as well.23,24,27 Note the wide variety and importance of this resistance against not only silver, but other frequently used antibiotics. pMG101 presence can resist up to six times minimum silver lethality for E. coli and more than likely other bacteria species.24,27
Overall silver resistance is driven by encoding two silver efflux pumps (one an ATPase and the other chemiosmotic) and two periplasmic Ag+-binding proteins.26 Basically silver ion uptake is decreased and silver efflux is increased. There is also some belief that an ATP-dependent copper efflux protein can mediate removal of silver ions.1 However, there is no direct evidence that this silver resistance mechanism induces cross-resistance to various antibiotics, but the rate of cross-resistances to other antibiotics and genetic linkage of silver resistance and antibiotic resistance genes provides positive circumstantial evidence.
Various bacteria species that have been identified as having silver resistance are E. cloacae, Enterobacteriaceae, Salmonella typhimurium, Klebsiella pneumoniae, Pseudomonas stutzeri, P. aeruginosa and E. coli.27-35 Resistance is principally generated through sil genes (one of the genes contained in the pMG101 plasmid) with its functions having been assigned due to homologous genes that encode resistance to other genes.22 However, there is question regarding the total level of silver resistance provided by sil genes because some research has demonstrated only partial phenotypic silver resistance, thus providing only partial resistance to silver.22,28,29 The reason for this partial resistance could be driven by lack of complete transcription due to a lack of consistent silver forcing selection pressure. Finally silver resistance has developed from acinetobacter baumannii based on seemingly separate 54 kb plasmid (pUPI199).31
Despite the variety of bacteria that can acquire silver resistance, the high-level and multi-faceted mode of action from silver (cell wall destruction and reduced antibiotic efflux) lead most to believe that the development of silver resistant is unlikely. While silver resistant bacteria appear rare, one of the reasons for this rarity is thought to be that silver is not utilized at high levels for antibacterial purposes similar to current antibiotics and if this were to change so would the rates of silver resistance development.
One of the big problems with regards to determining an accurate accounting of silver resistance is the lack of standardized metrics for this resistance. There is disagreement on the MIC50 breakpoints for bacteria susceptibility.34 When most silver microbial susceptibility studies are performed the conducting researchers tend to assign their own breakpoints to delineate susceptible and resistant strains. For example results from studies that explored MIC values for Staphylococcus aureus ranged from 8 to 80 mg/L32,33 where studies for Pseudomonas aeruginosa produced a range between 8 and 70 mg/L.34,35
Another problem is a general lack of standardization for silver ion anti-microbial testing methods mostly driven by solubility issues that change the rate of available silver ions due to formation of various silver-halide anionic complexes.1 This influence is most noted in the sodium chloride content of the microbiological medium used for susceptibility testing.1 Also the lack of standardization in silver wound dressings creates numerous delivery systems to release silver ions, which include elements beyond simple silver concentration to influence the ability to kill microorganisms.36 These discrepancies create questions regarding how different tests can be compared for similarities or differences in method.
One of the principle concerns with incorporating silver into antibiotic treatments that directly enter the blood is the potential for serious negative side effects. There are three important factors that govern the ability of a metal to produce systemic toxic effect: 1) the rate of absorption due to the solubility of the metal; 2) the capacity to bind to biological targets in the body; 3) the residence time of the metal after absorption both at biological targets and in the blood stream before elimination through excretion. The form of silver that is introduced to the body influences these three elements. Currently silver is largely applied as either silver nitrate (inorganic silver salts) or as organic compounds like SSD.1
Normal (no medical or heavy industry work) silver concentration in the body is very low with <2.3 ug/L (or 3 nmol/L) in the blood, about 2 ug/day (or < 8 nmol/L) expelled in urine and 0.05 ug/g (wet tissue) in the liver and kidneys.37-39 Some evidence has demonstrated that some SSD treated burn patients have plasma silver concentrations as high as 50 ug/L after 6 hours of treatment and can reach a maximum of 310 ug/L.37 In one particular case a patient that died of renal failure after 8 days of treatment had silver concentrations of 970 ug/g, 14 ug/g and 0.2 ug/g in the cornea, liver and kidney respectively.37,38 Due to silver derived renal toxicity from topical application (from wound bandages) most physicians believe that it is improper to apply topical silver treatments over long periods of time even on burn patients.
However, despite the above information most believe that silver application is generally safe with accumulation occurring in superficial layers of the liver and kidneys with full clearance after 28 days.39,40 Such limited consistent binding and the rate of clearance implies superficial binding and low absorption from a single application. However, there is limited information regarding how organ absorption would change with direct silver application to the blood.
Overall there are still numerous safety and toxicity studies that must be performed before even considering applying silver directly to oral or intravenous antibiotic treatments. Also new policy limiting the application of ionic silver, in either direct or indirect form, to non-medical products for an anti-microbial agent must be created because mass application dramatically increases the probability for widespread resistance development. Such an outcome has already occurred for numerous existing antibiotics and based on the apparent resistance profile for sil genes and their corresponding plasmids the widespread development of silver resistant bacteria could create a pathogen that cannot be effectively managed through modern medicine.
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