Water-in-oil microemulsions exhibit antimicrobial activity

Objectives: Previous research from this group has identified significant antimicrobial activity associated with oil-in-water (O/W) microemulsions. This activity has been exhibited against both bacteria and fungi (including yeasts) and bacterial biofilms and is dependent upon the position of the microemulsion within its stability zone. This novel work aims to identify antimicrobial activity of water-in-oil (W/O) microemulsions. Materials & Methods: A simple, thermodynamically stable water-in-oil microemulsion was tested for its time-related antimicrobial activity against a selected panel of test microorganisms (i.e.: Pseudomonas aeruginosa ATCC 9027, Escherichia coli ATCC 8739, Candida albicans ATCC 10231 and Staphylococcus aureus ATCC 6538P) and its effectiveness as a self-preserving system against a similar panel (Pseudomonas aeruginosa ATCC 9027, Candida albicans ATCC 10231, Staphylococcus aureus ATCC 6538P and Aspergillus niger ATCC 16404). Results: The microemulsion exhibited significant antimicrobial activity against all the selected microorganisms. Decreases in the viability of cultures (P. aeruginosa, C. albicans, E. coli and S. aureus) were observed over a short period of time after exposure to a known concentration of the first microemulsion. The results for the four samplings in the preservative effectiveness test according to the European Pharmacopeia requires a significant reduction in bacterial count, and this requirement was achieved in all samplings. Conclusions: Thermodynamically stable water-in-oil microemulsions are antimicrobially active, self-preserving systems, as are their oil-in-water counterparts.