Chemical constituents of three endophytic fungi: simplicillium subtropicum harboured in Duguetia staudtii (Annonaceae) and two new Diaporthe sp. Isolated from Trema guineensis (Ulmaceae); antibacterial activity and cytotoxicity of selected compounds

Abstract

The present work deals with the isolation, structural elucidation, and evaluation of antimicrobial and cytotoxicity activities of secondary metabolites isolated from potential endophytic harboured in two Cameroonian medicinal plants Duguetia staudtii and Trema guineensis (Ulmaceae). Notably, Simplicillium subtropicum isolated from the stem bark of D. staudtii showed veriaty of chemical compounds and therefore was cultured in a liquid medium. While two new Diaporthe sp. (1 and 2) isolated from the leaves and roots of Trema guineensis also produced remarkable chemical compositions and were separately cultured in a solid medium. Each extract was subjected to a variety of chromatographic techniques viz., liquid chromatography-mass spectrometry (LC-MS), column chromatography (CC), thin layer chromatography (TLC), and preparative high performance liquid chromatography (HPLC) to produce a total of twenty-two (22) compounds. The structures of all compounds were fully characterized by usual spectroscopic (UV, IR, 1D and 2D NMR), mass spectrometry (HRESIMS) techniques and by comparison with relevant data reported in the literature. However, four (04) compounds were isolated from S. subtropicum among which two (02) new polyketides, named simplicilones A and B whose the configuration of asymmetrics centers were completed using Mosher ester method, and one (01) known fatty acid (linoleic acid) and the ubiquitous fungal sterol (ergosterol). Eighteen (18) compounds were also isolated from the two new Diaporthe sp. and were sorted into several classes of secondary metabolites viz., four (04) benzopyrones (including one new derivative: 3,9- diacetylalternariol), one (01) new benzofuranone, diapobenzofuranone, together with three known (03) alternariol derivatives (including alternariol, 2-hydroxyalternariol and 4-hydroxyalternriol), seven (07) known cytochalasins (including cytochalasin H, cytochalasin N, epoxycytochalasin H, cytochalasin J, cytochalasin J2,cytochalasin J3 and cytochalasin RKS-1778), two (02) cytosporones (cytosporones C and E); one (01) macrolide (lasiodiplodin), (01) biphenyl (5′-methoxy-6-methyl-biphenyl-3,4,3′-triol), one (01) chromone (2,5-dimethyl-7-hydroxychromone), one (01) glycol (sorbitol). Extracts from Diaporthe sp. strains as well as some of the isolated compounds were screened in vitro for their antibacterial and antifungal activities on a plethora of microorganisms, including clinical strains. Extract from Diaporthe sp.1 displayed good antibacterial activity against E. coli with an MIC value of 15.6 μg/mL compared to the standard ciprofloxacin (MIC = 0.078 μg/mL) and moderate antibacterial activity against S. enterica muenchen and E. cloacae with MIC values Abstract xxxi of 125 and 250 μg/mL, respectively. Moreover, 2-hydroxyalternariol, lasiodiplodin showed moderate antibacterial activity against E. coli with MIC 62.5 – 125 μg/mL. Simplicilones A and B displayed moderate in vitro cytotoxicity with IC50 values of 25 and 29 μg/mL, respectively, against the cervix carcinoma cancer cell line KB3.1. They were inactive against the murine fibroblast cancer cell line. However, both compounds did not show any activity against Gram-positive and Gram-negative bacteria nor the used fungi strains.