Nanoencapsulation of Thymol and Eugenol with Chitosan Nanoparticles and the Effect against <i>Ralstonia solanacearum</i>

Abstract

Essential oils (EOs) are natural bioactive compounds with antibacterial activity against a variety of microorganisms including phytopathogens. The use of EOs and their components as viable therapeutic antibacterials is however greatly compromised by their volatile nature, hydrophobicity and instability when exposed to environmental and physiological factors. Encapsulation of these compounds in an appropriate carrier system can alleviate these challenges. This study therefore aimed at developing, characterizing and evaluating the efficacy of antibacterial potential of thymol and eugenol loaded chitosan nanoparticles (TCNPs and ECNPs) against Ralstonia solanacearum , the bacterial wilt-causing pathogen in potatoes. Synthesis of TCNP and ECNP was achieved via ionic gelation method and the prepared nanoparticles characterized by their particle size distributions, encapsulation efficiency, loading capacity (LC) and in-vitro release characteristics. Antibacterial activities of the nanoparticles were investigated using agar dilution and colony counting methods and their minimum inhibitory concentration (MIC) determined by 96-well broth micro-dilution method. Scanning electron microscope images of TCNPs and ECNPs showed that the nanoparticles were spherical in shape and were well separated with an average particle size of 590 nm and 555 nm respectively. The average size of chitosan nanoparticles alone was however 375 nm. The encapsulation efficiency was 72.9% for TCNP (with 48.3% LC) and 71.7% for ECNP (with 49.5% LC). The release of thymol and eugenol from the microcapsules was found to be pH dependent with the highest release at pH 1.5. The growth inhibition of R. solanacearum was 92% and 94% for TCNP and ECNPs respectively. The MIC of thymol and eugenol before encapsulation was 175 μg/ml and 275 μg/ml respectively, but this reduced significantly to 22.5 μg/ml and 45 μg/ml after encapsulation. Thus, encapsulation of thymol and eugenol in chitosan nanoparticles has shown promising potential as a bactericide alternative for R. solanacearum and could be useful in managing the soil borne phytopathogen.