Evaluation of the in vitro antimicrobial activity of a Pheroid®- entrapped plant extract against human pathogens

Abstract

This plant extract (PE) has proven activity against fungi affecting crops in South Africa. However, there is currently insufficient data on the activity of the PE against human pathogens. The general aim of this study is to determine whether the PE has in vitro antimicrobial properties against 11 human pathogens. Identification of compounds or treatments, which could have possible antimicrobial activity, will facilitate the development of novel treatments, such as a Pheroid® PE delivery system, and contribute to the indigenous knowledge of SA's plant. The PE was formulated and entrapped into the Pheroid® drug delivery system, containing different concentrations of the oil phase, i.e. 4%, 8%, 10%, 13% and 50%. The in vitro antimicrobial activity of these test formulations were compared to the activity of the PE and control formulation, i.e. Pheroid® without the PE, to determine if Pheroid® technology influenced the PE's activity. The formulations were subjected to accelerated stability testing (AST), after which the minimum bactericidal/fungicidal concentration (MBC/MFC) was determined at month 0, 1, 2, and 3. Improvements were made to this method by adding resazurin to determine the minimum inhibitory concentration (MIC) and MFC [1]. The formulations were subjected to characterization by Malvern Mastersizer, Malvern Nanosizer and confocal laser scanning microscopy (CLSM). The particle size, obtained from Malvern Mastersizer, for test formulations (4% and 8%) stayed relatively constant, while test formulations 10%, 13% and Pro-Pheroid® increased in size. There was statistically no difference in mean particle sizes for the control formulations. The zeta potential, obtained from Malvern Nanosizer, for both test- and control formulations generally decreased during the AST, resulting in more stable formulations. In general, after AST, test formulation (8%, 10%, and 13%) and PE tested against C. albicans had a 50% decrease in MFC, while the 4% test formulation's MFC stayed constant. All the control formulations had a N 90% decrease in MFC. The PE was found to have moderate activity against C. albicans, S. cerevisiae, and T. dermatis with a MIC of 1725 μg/mL, 54 μg/mL, and 108 μg/mL respectively. The test formulations had a general MIC of 108 μg/mL, 54 μg/mL, and 27 μg/mL against C. albicans S. cerevisiae, and T. dermatis respectively. Discussion Characterization showed that optimisation of the formulations was possible and that most of the formulations were stable i.e. no aggregation, flocculation or creaming was observed during AST. In conclusion, the addition of Pheroid® increased the MIC in all cases, however, did not seem to have an effect on the MFCs in general. This excludes S. cerevisiae where the PE had no MFC on its own, but by adding Pheroid® there were MFCs. Generally, adding the PE to Pheroid® resulted in more stable vesicles