As can be seen in Table 3 of Nanocrystal Silver, the aqueous concentration of silver ions released from the nanocrystalline film is approximately 3% of that released from a 0.5% silver nitrate or a 1% silver sulfadiazine cream.  However, the biological properties of the silver released from nanocrystals are much greater.  Silver resistance has been reported in the literature and is mediated through one of two pathways.  Either the silver is tied up in the cell wall and membranes, or it is actively transported out of the cell.  Bacterial organisms that have either one of these resistance mechanisms, which are effective up to 1000 g/mL Ag+ have been tested against the nanocrystalline silver coated dressing.  These tests showed that these organisms were susceptible to the silver produced by the nanocrystals but not to Ag+ from silver nitrate.  These findings, as will be described, strongly suggest that other species of silver besides Ag+ are released from the nanocrystals. (ACTICOAT™).

In another study nanocrystalline silver was extracted from the silver delivery system Acticoat by incubating the dressing in pure water at 37°C in a shaking incubator, and silver concentrations were measured using atomic absorption spectrophotometry.  The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined using five bacterial isolates of clinical interest, and results were compared for nanocrystal silver, silver nitrate and silver sulfadiazine, based upon total silver.  Nanocrystalline silver had similar MIC and MBC values when compared to three silver containing agents.  Kill kinetics were also studied, using 2.0 cm x 2.0 cm of pieces of silver dressing, and the same sized pieced of dressing impregnated with either silver nitrate or silver sulfadiazine.

Bacterial survival was measured using a plate counting technique.  Nanocrystal silver demonstrated the fastest kill times for the five bacteria used.  In most instances with Nanocrystal silver, bacterial survival was undetectable 30 minutes after inoculation, whereas at least 2-4 hours elapsed before no viable cells were detected with silver nitrate or silver sulfadiazine.

Nanocrystalline Silver

Silver Sulfadiazine

Silver Nitrate

MIC

MBC μg/ml

MIC - minimal inhibitory concentration

MBC - minimal bactericidal concentration

Similar results are seen with vancomycin-resistant enterococcus (VRE).  The Acticoat™ dressing has been shown to inhibit the growth of Pseudomonas aeruginosa and S.aureus for a minimum of nine days while a silver film dressing was only able to inhibit the growth of P. aeruginosa for four days of repetitive challenge, and S. aureus for one day.  This may be due to the presence of phosphate in the film dressing which is known to reduce the bactericidal properties of silver.

Sustained release of silver is important in reducing bacterial burden.  Silver nitrate has to be applied every two hours to be effective, and the cream base in silver sulfadiazine reacts with serous exudates to form a pseudo-eschar that must be removed before the cream can be re-applied.  Acticoat™ can be left in place for up to seven days, meaning that the wound does not have to be manipulated during this period, which may cause trauma to the new epithelial growth and may spread bacteria into the blood stream.

Resistance to silver is rare, but not unknown.  There are two forms of resistance: silver can be bound by cells in the form of an intracellular complex: and it can also be excreted from microbes using cellular efflux systems.

Resistance can be induced using low concentrations of silver.  Exposure of various E. coli strains to silver nitrate started at half the minimum inhibitory concentration (MIC) value (2-4 mg Ag+/L) produced resistance which increased with each generation.  Bactericidal levels of silver do not produce resistance as dead cells cannot mutate, but MIC and sub-MIC levels can result in the development of resistance.  Resistant cells appear to have reduced permeability of the outer membrane to silver combined with an ability to pump silver out of the cell – an active efflux mechanism.

This emphasizes the importance of using clinically relevant levels of silver particularly as a range of silver dressings are now in widespread use.  Non-controlled use of silver (sub-lethal levels) may result in bacteria developing resistance in the way that antibiotic and biocide-resistant bacteria have emerged.