Higher oxidative states of silver exhibit enhanced antimicrobial activity and effectively disrupt wound biofilms

Helen Thomason, PhD1, David Warde, PhD2, Christian Stephenson, BSc2, Andrew McBain, PhD3, Karen Cuvelier, P. Eng4 and Michelle Woodward, BSc4, (1)Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom, (2)Research and Development, Crawford Healthcare, Knutsford, United Kingdom, (3)School of Pharmacy and Pharmaceutical Sciences, University of Manchester, Manchester, United Kingdom, (4)Exciton Technologies, Edmonton, AL, Canada

Ionic silver (Ag1+) is an effective antimicrobial agent and long been used for treatment of infected wounds through incorporation into dressings. Dressings containing silver oxysalts which release higher oxidative states of silver (Ag1+,2+,3+) have recently been developed. Here we compared the effectiveness of dressings containing Ag1+ or silver oxysalts against a broad spectrum of bacteria and biofilms.


The efficacy of gelling fibre dressings containing Ag1+ or silver oxysalts were assessed in log reduction studies against 3 gram negative and gram positive bacteria over 7 days with daily re-inoculation. Efficacy of silver oxysalts over 14 days was also assessed in daily re-inoculated log reduction assays. To determine the effects of Ag1+ and silver oxysalts on biofilms, Pseudomonas aeruginosa biofilms were cultured for 3 days before applying to porcine skin cultured ex vivo for a further 24 hours. Dressings were applied to the mature biofilms for 24 hours, after which, biofilms were assessed using a viability dye and scanning electron microscopy (SEM).


Over 7 days, dressings containing silver oxysalts exhibited significantly greater log reductions (5-7 log) compared to Ag1+ (3-6 log) dressings. In addition, silver oxysalts dressings maintained >5 log reductions for 14 days with daily re-inoculation of Staphylococcus aureus or P. aeruginosa. Viability staining and SEM revealed a greater effect against mature P. aeruginosa biofilms with silver oxysalts dressings compared to Ag1+ dressings. The higher oxidative states of silver disrupted the biofilm to reveal underlying porcine collagen fibres which were not evident with Ag1+ treatment.


The potent nature of silver oxysalts results in rapid and sustained log reductions against a broad spectrum of bacteria. Furthermore, these high oxidative states of silver effectively disrupt mature biofilms. The strong attraction for electrons of higher oxidative states of silver therefore makes Ag oxysalts more effective against microbes.