Evaluation of the in vitro antimicrobial activity of the Pheroid®-entrapped plant extract of Agapanthus africanus against human pathogens
Van Lingen, Bianca
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Antimicrobial resistance is a major concern as existing antimicrobials are becoming less effective due to misuse and overuse (WHO, 2018b). Identifying novel compounds or treatments with a possible antimicrobial activity could facilitate the development of novel treatments, such as a Pheroid® plant extract delivery system. This Agapanthus africanus plant extract has proven in vitro and in vivo activity against fungi affecting crops in South Africa, however, activity against human pathogens have not been determined. In this study, the plant extract’s in vitro antimicrobial activity was tested against eleven human pathogens, namely Escherichia coli, Staphylococcus epidermidis, Staphylococcus aureus, Klebsiella pneumoniae, Salmonella enterica, Enterobacter aerogenes, Pseudomonas aeruginosa, Trichosporon dermatis, Cryptococcus neoformans, Saccharomyces cerevisiae, and Candida albicans. The plant extract 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 (Pheroid®-plant extract) was compared to the activity of the plant extract and control formulation (only Pheroid®), to determine if Pheroid® technology influenced the plant extract's activity. The formulations were subjected to accelerated stability testing, after which the minimum bactericidal/fungicidal concentration was determined at month 0, 1, 2, and 3. Improvements were made to this method by adding resazurin to also determine the minimum inhibitory concentration. The formulations were characterised by Malvern Mastersizer (particle size), Malvern Nanosizer (zeta potential) and confocal laser scanning microscopy (morphology) before and during accelerated stability testing. The characterisation tests showed that optimisation of the test formulations by adjusting the percentage oil phase was possible. Most of the formulations were stable i.e. no aggregation, flocculation or creaming was observed during accelerated stability testing. Test formulation PPE 4% proved to be the most promising formulation with the largest initial particle size and remained stable during accelerated stability testing. The A. africanus plant extract was identified to have moderate antimicrobial activity against three human pathogens, namely C. albicans, S. cerevisiae, and T. dermatis with a minimum inhibitory concentration of 1725 μg/ml, 54 μg/ml, and 108 μg/ml, respectively. In conclusion, the addition of Pheroid® drug delivery system increased the inhibitory activity in most cases (excluding S. cerevisiae), however, did not seem to have an effect on the fungicidal concentration. The formulation that showed the best initial antimicrobial activity against C. albicans was Pro-PPE 50%, however, after accelerated stability testing both Pro-PPE 50% and PPE 4% had the highest antimicrobial activity. Formulation PPE 13% had the highest initial fungicidal activity against both S. cerevisiae and S. epidermidis, while PPE 8% had the highest fungicidal activity after accelerated stability testing against S. cerevisiae.
- Health Sciences