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Keywords:

  • antimicrobials;
  • biocides;
  • infection;
  • silver ions;
  • tea tree oil

Abstract

The activity of alternative antimicrobial agents such as tea tree oil (TTO) and silver ions (Ag+) with multiple target sites impedes the development of antibacterial resistance and might be useful in improving the current treatment strategies for various chronic wound infections. In this study, liposome-encapsulated TTO, Ag+ and TTO plus Ag+ were added to suspension cultures of Pseudomonas aeruginosa (Ps. aeruginosa), Staphylococcus aureus (Staph. aureus) and Candida albicans (C. albicans). Treatment of these cultures using the agents in combination at subminimal lethal concentrations resulted in an enhanced loss of viability compared to treatment with individual agents. The effective concentration, elimination time (to the limit of detection, LOD) and fractional lethal concentration index (FLCI) of liposomal agents in combination were as follows:

  • Candida albicans: 0·05% v/v TTO:PVA30-70 kDa: 8·9 × 10−5% w/v Ag+:PVA30–70 kDa: 2·0 h, FLCI = 0·73 (indifferent),

  • Staphylococcus aureus: 0·05% v/v TTO:PVA30–70 kDa: 6·0 × 10−4% w/v Ag+:PVA30–70 kDa: 1·5 h, FLCI = 0·38 (synergistic),

  • Pseudomonas aeruginosa: 0·25% v/v TTO:PVA30–70 kDa: 3·2 × 10−4% w/v Ag+:PVA30–70 kDa: 30 min, FLCI = 0·33 (synergistic).

These results show the potential for improving antimicrobial efficacy by delivering lower effective concentrations of alternative agents, via controlled release systems. NB All values denoted as %w/vAg+ refer to the concentration of silver ions.

Significance and Impact of the Study

In this study, we have shown that encapsulating silver (as the ion Ag+) and tea tree oil (singly and in combination) in a controlled release liposomal carrier system can improve their antimicrobial efficacy as well as reduce the effective concentration required. These findings may impact on the problems of agent toxicity caused by the need for high effective doses or microbial resistance where long term application is required.