Evaluation of antimicrobial, antidermatophytic, antiproliferative and cytotoxic activity of secondary metabolites isolated from calpurnia aurea subsp. aurea (aiton) benth (l’herit) and associated fungal endophytes

Abstract

Antimicrobial resistance has been recognized as a significant threat to healthcare systems globally, affecting both developing and developed countries, resulting in increased rates of skin infections caused by bacterial and dermatophytic pathogens. Cancer cases have increased over the years, and existing management options often entail undesirable side effects. Medicinal plants and endophytic fungi provide alternative solutions to these challenges. This research focused on the isolation of antimicrobial, antidermatophytic, antiproliferative, and cytotoxic secondary metabolites from Calpurnia aurea subsp. aurea and its associated endophytic fungi. The leaves and stem bark of this medicinal plant were used for the isolation of endophytic fungi, while the remaining portions were used for secondary metabolite extraction using methanol. Isolated fungal endophytes were identified through molecular techniques and fermented on rice media for the extraction of secondary metabolites. All extracts from leaves, stem bark, and endophytic fungi were purified using High Performance Liquid Chromatography (HPLC) to isolate pure compounds. The structures of the isolated compounds were elucidated using 1D and 2D NMR spectroscopy, as well as Liquid Chromatography Mass Spectrometry techniques. The bioactivities (antimicrobial, antidermatophytic, antiproliferative, and cytotoxicity) of the isolated compounds were also analyzed. Nineteen endophytes were isolated, classified into the classes Eurotiomycetes, Sordariomycetes, Coleomycetes, and Agariomycetes. Together with other known compounds, five (5) previously undescribed compounds were reported, namely; quinolizidine alkaloids (46-49) and a tirucallane triterpenoid (67), additionally ten (10) phenolic compounds were reported for the first time in this study. The phenolic and quinolizidine alkaloids demonstrated antidermatophytic activity, with minimum inhibitory concentration (MIC) values ranging from 6.6 μg/mL to 300 μg/mL against Trichophyton rubrum, Trichophyton interdigitalle, Trichophyton benhamiae, Microsporum canis, and Nannizia gypsea. The cinnamic acid (65) demonstrated antimicrobial activity against Bacillus subtilis (MIC of 16.6 μg/mL), comparable to oxytetracycline (MIC 16.6 μg/mL). The phenolic compounds also exhibited antiproliferative and cytotoxic effects on KB3.1 and L929 cell lines, with compound 65 demonstrating an IC50 value of 18 μg/mL. The results of this study demonstrate the significance of C. aurea subsp. aurea and endophytic fungi as sources of antimicrobial, antidermatophytic, antiproliferative, and cytotoxic secondary metabolites, which may serve as lead compounds in drug development. Additionally, this study scientifically validates the use of C. aurea subsp. aurea in alternative medicine.