Isolation and characterization of antibacterial secondary metabolites from Polyscias fulva and its Endophytic fungi

Abstract

Infectious diseases remain to be a global health burden due to the development of antibiotic resistance by pathogenic microorganisms. Antibiotic resistance has led to increased number of deaths among children and adults. Medicinal plants have been used over the years in the treatment and remedy of various infections affecting human beings. The use of secondary metabolites from medicinal plants and fungal endophytes can be an alternative to disease management without the negative impact of synthetic antibiotics. Therefore, this study sought to isolate and characterize antibacterial secondary metabolites from the leaves of Polyscias fulva and its endophytic fungi. The leaves were collected from Kakamega rain forest; some of the leaves were dried under shade and ground into fine powder for extraction whereas the remaining fresh leaves were used for fungal endophyte isolation. The fungal endophytes were isolated from the internal tissues of P. fulva and cultured in Potato Dextrose Agar (PDA) media amended with streptomycin sulphate, followed by sub-culturing to obtain pure cultures. The pure cultures were then identified by DNA sequencing using ITS1F and ITS4 primers as Fusarium species (PF1 and PF2). The fungal endophytes were screened for bioactivity by carrying out antagonistic assay test against Staphylococcus aureus ATCC25922 and Klebsiella pneumoniae ATCC13883. Fusarium species (PF1 and PF2) were subjected to solid fermentation on rice media for 21 days; followed by ultra-sonication in methanol and subsequent liquid partitioning between hexane and ethyl acetate. The ethyl acetate extract was fractionated using column chromatography on sephadex LH 20. Extraction of the dried leaves’ powder was carried out using methanol. The bioactivity of the pure compounds was determined using disc diffusion method against K. pneumoniae and S. aureus. Structure elucidation of the compounds isolated was successfully identified through analysis of their 1D and 2D NMR spectra and mass spectrometry data as well as comparison with literature data. Compounds 15, 16 and 17 were isolated from the dried leaves of P. fulva while compound 18 was obtained from fungus Fusarium species (PF1). The methanol crude extract and isolated fractions showed antibacterial activities. Compound 18 was found to be the most active against K. pneumoniae (11.66±2.51 mm) and S. aureus (12.00±3.60 mm). Compound 17 was the least active against both K. pneumoniae (7.33±0.57 mm) and S. aureus (7.00±1.00 mm). These results suggest that these compounds could be lead compounds for drug development.