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Titre : Chemical Profile and Antimicrobial and Antioxidant Activities of Endophytic Fungi Extracts Associated with Acacia nilotica (L.) Willd. Ex Delile Subsp. nilotica (Mimosaceae)

Auteur : Elham Abd El Rahman Mohamed Kheir

Université de soutenance : University of Khartoum

Grade : Ph.D. in Botany (Biochemistry) 2021

Résumé partiel
Endophytes are living organisms that colonize plant tissues and do not cause symptoms of disease. Endophytic fungi are considered as a source of novel biologically active compounds with wide range of applications such as antimicrobial and antioxidants agents or as alternative source of compounds originally isolated from higher plants. This study aimed to isolate endophytic fungi residing in the tissues of Acacia nilotica (L.) Willd. Ex Delile grown in the campus of University of Khartoum and the Nile Street. The antimicrobial and antioxidant activities of the isolated endophytic fungi extracts were evaluated and their chemical constituents were also determined. The endophytes were isolated after surface sterilization of the leaves, fruits and twigs of the host plant. The fungal isolates were cultured in potato dextrose agar medium. Extracts were prepared by maceration of cultures as well as the host plant parts in ethyl acetate and methanol respectively. Antimicrobial activity investigation was carried out against Gram positive (Bacillus subtilis and Staphylococcus aureus) and Gram negative (Escherichia coli and Pseudomonas aeruginosa) bacteria and two fungal species (Candida albicans and Aspergillus niger) using the disc diffusion method. Antioxidant activity was evaluated by their scavenging activity of the stable 1, 1-diphenyl-2-picryl hydrazyl (DPPH) radical. Chemical constituents of extracts were determined by colorimetric and chromatographic techniques including thin layer chromatography (TLC) and high-performance liquid chromatography (HPLC). The polyphenolic, flavonoid and tannin contents were also determined. Optimization of the growth conditions using the one-at-a time procedure was performed for the most active endophytic fungus. A total of thirteen endophytic fungi were isolated and morphologically identified from different A. nilotica tree parts ; four isolates from the leaves, three from the fruits and six from the twigs. Twelve isolates (92%) belonged to Ascomycota and only one (8%) belonged to Zygomycotina. Results of antimicrobial activity showed that 38.3 % of the endophytic fungal extracts were able to exhibit antimicrobial activity with inhibition zones diameters ranging from 11 to 30 mm. The highest activity was observed for the ethyl acetate and methanolic extracts of Alternaria sp. (30 mm) against A. niger and C. albicans. In addition, 31% (8/26) of endophytic fungal extracts displayed DPPH radical scavenging activity with inhibition values ˃ 50%. The highest scavenging activity was exerted by the methanolic extract of Aspergillus terreus 1 (IC50 0.132 mg/ml). A good positive correlation was observed between the DPPH scavenging activity and the polyphenolic (R2 = 0.704) and flavonoid (R2 = 0.627) contents of the endophytic fungal extracts.

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