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Background   Opuntia dillenii  is an invasive plant well established in the harsh South-Eastern arid zone of Sri Lanka. Evidence suggests it is likely that the endophytic fungal populations of  O. dillenii  assist the host in overcoming biotic and abiotic stress by producing biologically active metabolites. With this in mind there is potential to discover novel natural products with useful biological activities from this hitherto poorly investigated source. Consequently, an investigation of the antimicrobial activities of the endophytes of  O. dillenii,  that occupies a unique ecological niche, may well provide useful leads in the discovery of new pharmaceuticals.    Methods  Endophytic fungi were isolated from the surface sterilized cladodes and flowers of  O. dillenii  using several nutrient media and the antimicrobial activities were evaluated against three Gram-positive and two Gram-negative bacteria and  Candida albicans . The two most bioactive fungi were identified by colony morphology and DNA sequencing. The secondary metabolite of the endophyte  Fusarium  sp. exhibiting the best activity was isolated via bioassay guided chromatography. The chemical structure was elucidated from the ESIMS and NMR spectroscopic data obtained for the active metabolite. The minimum inhibitory concentrations (MICs) of the active compound were determined.    Results  Eight endophytic fungi were isolated from  O. dillenii  and all except one showed antibacterial activities against at least one of the test bacteria. All extracts were inactive against  C. albicans . The most bioactive fungus was identified as  Fusarium  sp. and the second most active as  Aspergillus  niger. The structure of the major antibacterial compound of the  Fusarium  sp. was shown to be the tetramic acid derivative, equisetin. The MIC’s for equisetin were 8 μg mL −1  against  Bacillus subtilis , 16 μg mL −1  against  Staphylococcus aureus  and Methicillin Resistant  Staphylococcus aureus  (MRSA).    Conclusions   O. dillenii,  harbors several endophytic fungi capable of producing antimicrobial substances with selective antibacterial properties. By producing biologically active secondary metabolites, such as equisetin isolated from the endophytic  Fusarium  sp., the endophytic fungal population may be assisting the host to successfully withstand stressful environmental conditions. Further investigations on the secondary metabolites produced by these endophytes may provide additional drug leads.

bmc complementary and alternative medicine (online)

Antimicrobial activities of endophytic fungi obtained from the arid zone invasive plant Opuntia dillenii and the isolation of equisetin, from endophytic Fusarium sp.