Elsevier

Phytochemistry Letters

Volume 35, February 2020, Pages 152-155
Phytochemistry Letters

Antimicrobial metabolites from the endophytic fungus Aspergillus versicolor

https://doi.org/10.1016/j.phytol.2019.12.003Get rights and content

Highlights

Abstract

Two new diaryl ether derivatives [aspergillethers A (4) and B (5)] and five known metabolites [(22E,24R)-stigmasta-5,7,22-trien-3-β-ol (1), stigmasta-4,6,8(14),22-tetraen-3-one (2), orcinol (3), and butyrolactones I (6) and VI (7)] were separated from the endophytic fungus Aspergillus versicolor (Aspergillaceae) isolated from the roots of Pulicaria crispa Forssk (Asteraceae) growing in Saudi Arabia. The structures of these compounds were determined by various spectral techniques (e.g., UV, IR, NMR, and HRESIMS) and by comparison with the literature data. Compounds 4 and 5 were evaluated for their antimicrobial properties towards different microorganisms using a disc diffusion assay. Compound 5 exhibited significant antibacterial capacity towards Staphylococcus aureus, Bacillus cereus, and Escherichia coli with inhibition zone diameters (IZDs) of 14.3, 19.8, and 24.5 mm, respectively, and MICs values of 4.3, 3.7, and 3.9 μg/mL, respectively, compared to ciprofloxacin (MICs 3.1, 2.8, and 3.5 μg/mL, respectively). In addition, compound 5 exhibited prominent antifungal activity towards Candida albicans and Geotrichium candidum (IZDs 15.8 mm and 17.5 mm, respectively) in comparison to that of clotrimazole (IZDs 17.2 mm and 21.4 mm, respectively).

Introduction

Fungi have been proven to be a wealthy pool of bioactive and structurally unique secondary metabolites (Kellogg and Raja, 2016; Wang et al., 2015; Zhong and Xiao, 2009). Endophytic fungi are increasingly becoming a focal point of many studies due to an increase in the number of identified novel compounds (Gupta et al., 2019; Khayat et al., 2019; Ibrahim et al., 2019, 2018a, 2018b, 2018c; Liu et al., 2018a, 2018b; Yan et al., 2018). The Aspergillus genus (Aspergillaceae) includes more than 250 species of highly aerobic fungi that grow in oxygen-rich environments (Elkhayat et al., 2016; Khosravi et al., 2015; Samson and Varga, 2009). A. versicolor is a common soil mould that is a notorious producer of mycotoxins (Stein and Bulboacӑ, 2017). It has been previously isolated from Paris polyphylla var. yunnanensis rhizomes (Gao et al., 2018), leaves of Centella asiatica (Netala et al., 2016), and Eichhornia crassipes leaves (Ebada et al., 2018). In addition, it is known to produce a wide array of structurally diverse metabolites including isocoumarins, xanthones (sterigmatocystins), sesquiterpenoids, butyrolactones, anthraquinones, and peptides with varied bioactivities (El-Agamy et al., 2019; Ebada et al., 2018; Ibrahim et al., 2017a, b; Lee et al., 2010). As a part of our ongoing research for isolating bioactive fungal metabolites from endophytes, A. versicolor isolated from Pulicaria crispa Forssk (Asteraceae) roots growing in Saudi Arabia was investigated. In this study, we reported the separation and structural characterization of two new diaryl ether derivatives, aspergillethers A and B (4 and 5), and five known metabolites (1-3, 6, and 7) from the MeOH:CHCl3 extract of A. versicolor. Moreover, the antimicrobial activity of new metabolites (4 and 5) was evaluated.

Section snippets

Results and discussion

An extensive column chromatographic separation of the MeOH:CHCl3 extract of A. versicolor rice culture using Sephadex LH-20, silica gel, and RP-18 column chromatography (CC) yielded two new (4 and 5) and five known (1-3, 6, and 7) metabolites (Fig. 1).

The known metabolites were identified by comparing their physical and NMR spectral data with the literature as (22E,24R)-stigmasta-5,7,22-trien-3-β-ol (1) (Ibrahim et al., 2015; Ha et al., 1982), stigmast-4-ene-3-one (2) (Ibrahim et al., 2015;

General experimental procedures

Optical rotations were determined with a Perkin-Elmer Model 341 LC polarimeter (Perkin-Elmer, Waltham, MA, USA). The IR spectra were measured on a Shimadzu Infrared-400 spectrophotometer (Shimadzu, Kyoto, Japan). HRESIMS was recorded on an LTQ Orbitrap (ThermoFinnigan, Bremen, Germany). EIMS were recorded on a JEOL JMS-SX/SX 102A mass spectrometer (ThermoFinnigan, Bremen, Germany). NMR spectra were performed on a BRUKER Unity INOVA 400 instrument (Bruker BioSpin, Billerica, MA, USA).

Declaration of Competing Interest

The authors have no conflict of interest to declare

Acknowledgements

This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah under grant no. (G: 67-166-1440). Therefore, the authors acknowledge with thanks DSR for the technical and financial support.

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