Biodiversity and antimicrobial activity of endophytic fungi isolated from native Sutherlandia frutescns (cancer bush)

Abstract

The current study focused on finding alternative bioactive compounds with the potentials to mitigate or control antimicrobial resistance. Natural products such as medicinal plants; are being studied for their therapeutic potentials in the race against antimicrobial resistance. The plant kingdom offers surfeit of biologically active compounds and most studied plants have been found to at least host one endophyte. In addition, studies on establishing the relationship between endophytes and plants and the products of their interaction have since gained traction. Endophytes are endosymbionts (bacterial or fungal) that produce bio-compounds that aid in plant protection and growth. The bioactive compounds produced by endophytes in symbiosis are used in the pharmaceutical industry for manufacturing secondary products. South Africa is well-known for its valuable and untapped information on medicinal plants. Since ancient times, people have been using plants for their essential wellbeing. This study centered around the medicinal plant, Sutherlandia frutescens, commonly known as cancer bush indigenous to Southern Africa, and produces bioactive compounds of medicinal value. The study aimed at evaluating the biodiversity and antimicrobial activity of possible endophytic fungi isolated from the plant. A total of fifty-one (51) fungal endophytes were isolated, identified and classified into various genera. The predominant genus found was Penicillium (25%), followed by the Mucor (12%), Alternaria (10%) and Coniochateta (10%) genera. The endophytes were further characterized to species-level based on known identities in the GenBank database, and their relatedness and evolutionary lines were determined by phylogenetic analysis with a total of four (4) clusters and six (6) sub-clusters constructed. The endophytes were evaluated for their

antibacterial activity against environmental Gram-negative and Gram-positive bacteria. The antibacterial properties of the secondary metabolites were investigated using the disc diffusion and agar plug assays. A disk diffusion assay showed Salmonella enterica (environmental strain) was the most sensitive and with its growth being inhibited by the activity of the secondary metabolites from the sixteen (16) out of the fifty-one (51) endophytes isolated. In overall, 51% of the endophytic fungi produced bioactive compounds that exhibited antimicrobial activity against at least one tested bacterial. However, the agar plug assay showed less activity compared to the latter with only eight (8) endophytic fungal plugs that managed to inhibit the growth of four (4) bacteria. The largest zone of inhibition with a diameter of 21.0 mm was exhibited by Coniochaeta hoffmannii (CB016) observed against E. coli 017. Some fungal isolates were further tested for their antifungal activity against plant pathogenic fungi. The dual culture test gave 84% antifungal effectiveness against fungal pathogens, with Mucor, Penicillium genera and Aspergillus brasiliensis exhibing broad-spectrum antifungal activities for most plant pathogenic fungi tested in this assay. During the culture filtrate test, 78% showed inhibition activity against one or more pathogens. The culture filtrate of the endophytic fungus CB011 (Purpureocillium lilacinum) exhibited a broad range of antifungal activity against all the pathogens. Under salt conditions, most isolates were able to grow at a 3% concentration, giving a total of 90% fungal growth rate with a maximum within the range of 75-80.5 mm growth in diameter. At pH of 5, an overall 44% of the total fungal growth was observed, with most growth

noted at a temperature of 25 °C. The antimicrobial activity showed 19% of growth activity. The results suggest endophytic fungi isolated from Sutherlandia frutescens may be good sources of bioactive molecules, including those capable of inhibiting or controlling both human and plant pathogens.