Isolation and purification of bioactive metabolites from an endophytic fungus Penicillium citrinum of Azadirachta indica

Endophytic fungi are the plant symbiont with highly diverse nature and poorly defined ecological importance in host fitness. Although there are the reports on the isolation and characterization of fungal endophytes from a variety of hosts, there is still no report of Penicillium citrinum from Azadirachta indica. In this study, an endophytic P. citrinum was isolated from A. indica. The purified fraction of secondary metabolites was characterized by combining TLC, GC-MS, ¹H NMR and ¹³C NMR analyses. The TLC purified fraction was identified as milbemycin. The pure fraction did not show any antioxidant activity while crude extract showed strong antioxidant activity (DPPH inhibition capacity; IC₅₀ = 52.13 μg ml⁻¹). The secondary metabolites displayed significant antimicrobial activity against human's pathogenic bacteria and fungi. The inhibition zones between 15 and 20 mm were recorded against Gram +ve Staphylococcus aureus, Enterococcus faecalis and Gram–ve Aeromonas hydrophila, while maximum inhibition of 29 mm was observed against Trichophyton mentagrophytes. P. citrinum can be a promising fungus that has broad spectrum antimicrobial activity and may provide future insight towards the production of bioactive compounds.