The antimicrobial activity of secondary metabolites produced by bacterial endophytes isolated from Pellaea calomelanos

M.Sc. (Biotechnology)Abstract: Medicinal plants have been used worldwide as traditional remedies for the treatment of various diseases such as asthma, skin disorders, cardiovascular diseases, gastrointestinal symptoms, respiratory and urinary problems. These plants have shown to synthesize biologically active compounds that can be applied in drug discovery and other industries. The use of medicinal plants has led to deforestation and extinction of plant species; therefore, this has compelled scientists in search for new alternates for drug discovery and development. One of these are endophytes, which are microorganisms associated symbiotically with medicinal plants. Endophytes produce bioactive compounds similar to their plant host; in addition, endophytes produce various bioactive secondary metabolites some of which protect the plant host from pathogens and enhance growth potential of the plant host. This research study aimed at isolating and identifying bacterial endophytes from Pellaea calomelanos, a medicinal plant used for the treatment of headaches, chest and head colds, asthma, nasal and mouth ulcers, and to further evaluate the antibacterial activity of crude extracts produced by the plant and its bacterial endophytes. Six putative bacterial endophytes were isolated from surface sterilized leaves of P. calomelanos and identified using morphological and phenotypic data, the identification was confirmed by the 16S rRNA gene sequence, viz Arthrobacter sp. MHSD1 (MF613647), Pantoea sp. MHSD2 (MF613648), Bacillus infantis strain MHSD3 (MF613649), Pantoea sp. strain MHSD4 (MF613650), Pantoea ananatis strain MHSD5 (MF613651) and Pantoea sp. MHSD6 (MF613652). Crude extracts of the plant and its bacterial endophytes were tested against Gram-negative strains: Escherichia coli (ATCC 25922), Pseudomonas aeruginosa (ATCC 27853) and Klebsiella pneumoniae (ATCC 13182); Gram-positive strains: Staphylococcus aureus (NCTC 6571) and Bacillus cereus (ATCC 10876) using the disk diffusion method with Streptomycin used as a positive control and Dimethylsufoxide (DMSO) as a negative control. Methanol extracts of P. calomelanos and ethyl acetate extracts of the six bacterial endophytes showed good antibacterial activity against Escherichia coli, Pseudomonas aeruginosa, Klebsiella oxytoca, Staphylococcus aureus and Bacillus cereus. Eighty percent methanol P. calomelanos crude extracts was subjected to liquid chromatography mass spectrometry-ion trap and time-of-flight (LCMS-IT-TOF) and data obtained revealed the presence of two chlorogenic acid compounds, one phenolic acid and one flavonoid compound. The crude extracts of both the plant (methanol crude extracts) and its bacterial endophytes (ethyl acetate extracts) were further analysed using gas chromatography-high resolution time-of-flight mass spectrometer (GC-HRT) to investigate their volatile secondary metabolites and the data generated showed the presence of various volatile secondary metabolites..