Encapsulation of Antimicrobial Peptides in Bicontinuous Microemulsions for Topical Delivery to Surgical Site Infections and Chronic Wounds

Surgical site infections and chronic wounds, especially those caused by antibiotic-resistant microorganisms, result in hospitalization and fatalities each year. Methods to prevent these infections, such as cleaning and preparing medical tools, have had minimal success in preventing infections. Further, antibiotic treatments have become less successful in treating infections and wounds because of antibiotic-resistant bacteria. Antimicrobial peptides (AMP) are a possible treatment solution for wound infections. AMPs are oligopeptides that occur in nature or can be synthesized in vitro which possess a broad spectrum of antimicrobial activity against bacteria and other harmful microorganisms. AMPs operate by disrupting the packing arrangements of biomembranes in prokaryotes through their insertion into negatively charged phospholipid bilayers. However, many AMP products fail clinical trials because of their difficulty to be encapsulated and delivered at high concentrations in an active form. This project proposed the use of bicontinuous microemulsions (BMEs) as a possible system to encapsulate and deliver AMPs. BMEs are thermodynamically stable mixtures consisting of a surfactant, oil, aqueous mixture, and sometimes a cosurfactant. They are optically clear and the surfactant(s) in BMEs solubilize nearly equal amounts of oil and water creating elongated nanodomains. AMPs are typically cationic, and the following hypothesis was tested: BMEs created with anionic surfactants would induce a more highly folded, hence more biologically active, conformation for melittin. Several different BME systems composed of biocompatible oils were identified and evaluated for their ability to encapsulate melittin, a model AMP, and to test the system’s antimicrobial activity. Small-angle x-ray scattering showed melittin effected the BMEs quasi-periodic repeat distance and correlation lengths. Circular dichroism data showed a higher percentage of melittin was in its active form when encapsulated in a BME compared to an aqueous solution. Fluorescence measurements showed melittin resided within the surfactant monolayers of the BMEs. Antimicrobial diffusion assays proved that there was a larger zone of inhibition against bacteria commonly found in surgical site infections and chronic wounds than the BMEs without melittin. This research was successful in adding an AMP into BMEs created with biocompatible materials and may be a viable option in combating the rise in antibiotic-resistant organisms.