Combinatorial activities of ionic silver and sodium hexametaphosphate against microorganisms associated with chronic wounds

Objectives: There is growing evidence to suggest that biofilms may be involved in the aetiology of chronic wounds. The development of formulations with enhanced anti-biofilm activity could therefore represent a potential therapeutic strategy by reducing bioburdens. Here, the antimicrobial properties of ionic silver and sodium hexametaphosphate (polyphosphate) against bacteria in planktonic form and as biofilms were investigated. Methods: The MICs, MBCs and minimum biofilm eradication concentrations (MBECs) of the two test compounds against Candida albicans, Pseudomonas aeruginosa and Staphylococcus aureus were determined. The chequer-board microdilution method was used to determine the fractional inhibitory concentration index (FICI) and fractional biofilm eradication concentration index (FBCI) for the compounds against planktonic bacteria and monospecies biofilms. In addition, biofilms in continuous culture were exposed to hydrogel formulations containing ionic silver and polyphosphate, singly or in combination. Results: Ionic silver and polyphosphate, when combined, exhibited putative anti-biofilm synergy against P. aeruginosa (FBCI 0.08), C. albicans (FBCI 0.06) and S. aureus (FBCI 0.44). While silver hydrogels reduced the viability of continuous culture biofilms, the incorporation of polyphosphate resulted in significantly greater and more rapid bacterial inactivation (P<0.05). Conclusions: The antimicrobial and anti-biofilm potency of silver hydrogels was markedly enhanced by the incorporation of polyphosphate.