Investigating Plant-Derived Compounds with Microorganisms: Biodegradation Studies and Antimicrobial Screening of Nigerian Plants Via NMR-Based Analysis.

ABSTRACT The antimicrobial activities of selected Nigerian plants were examined against clinical bacterial and fungal pathogen namely Staphylococcus aureus 29213, S. aureus MRSA, S. epidermidis, Pseudomonas aeruginosa PA01, Klebsiella pneumoniae BAA1705, and Candida albicans 10231. Plant materials were extracted using different solvents such as methanol, dichloromethane, chloroform and ethyl acetate in several chromatographic steps. Crude extracts were tested for bioactivity and further purification and separation were carried out on active extracts. A better inhibitory activity of A. paniculata chloroform leaf crude extract was observed against C. albicans (51.2%) as well as the methanol crude extract (28.3%) at MIC of 128 µg/ml. The chloroform and methanol extracts of A. paniculata stem bark also showed promising activity against C. albicans (29.2% and 16.4% inhibition respectively) at 128 µg/ml. Further purification of the leaf crude extracts led to the isolation in pure form and characterization of Andrographolide, Andropanolide, 14-deoxy-12 hydroxyandrographolide, 14-deoxy, 11, 12-didehydroandrographolide, 14-deoxyandrographolide, Deoxy-andrographiside, and Andrographiside by NMR analysis. These pure compounds were tested for their antimicrobial activities against S. aureus 29213 and they exhibited antimicrobial activities ranging between 18.5% to 20.5% at MIC of 8µg/ml. On the other hand, the S. aureus strains, were more sensitive to both methanol extract of the leaf (18.7% for S. aureus 29213; 26.8% for S. aureus MRSA) and stem bark (22.4% for S. aureus 29213; 28.9% for S. aureus MRSA) of Nauclea latifolia at 128 µg/ml. The MIC of the leaf crude extract was 64µg/ml with 29.2% inhibition. The methanol extract of the stem bark also exhibited slight antifungal activity against C. albicans, the highest being 18.3% at 128 µg/ml. The methanol crude extract of N. latifolia stem bark was purified to afford one monoterpenoid indole alkaloid (Strictosamide) and a flavonol glycoside (Quercetin-3-O-ꞵ-D-Glucopyranoside) which were isolated in their pure form and characterized by 1D and 2D NMR analysis. Another round of plant collection done in December 2023 were also tested for their antimicrobial activity against S. aureus and S. epidermidis, seven of which exhibited significant activities against the two bacterial strains. These active plants include; Caesalpinia pulcherrima, Ricinus communis, Sesamum indicum, Senna alata, Datura metel, Euphorbia tirucalli, and Luffa cylindrica gourd. Further purification and antimicrobial activity studies was carried out on the active plants. Partially purified fractions of L. cylindrica gourd showed very significant antimicrobial activity against S. aureus up to 87.5% inhibition at 64µg/ml. The 1D and 2D NMR analysis of the bioactive fraction of L. cylindrica suggested the presence of hydroxyl polyunsaturated fatty acids. S. indicum was also purified and Acteoside, a phenylethanoid glycoside, was isolated from it in its pure form. In a second study, the antimicrobial efficacy of Juglone from Juglans regia was carried out against phytopathogenic bacteria which causes walnut blight, a disease affecting J. regia. These phytopathogens include: Xanthomonas arboricola pv. juglandis NCPPB1659, X. a. pv pruni NCPPB2583, X. a. pv fragariae PD2780, Pseudomonas avellanae CRA-PAV013, P. syringae pv actinidiae CRA-FRU 8.43, P. viridiflava NCPP-B1382, P. syringae pv. syringae CRA-FRU10.31. The antimicrobial efficacy was assessed by evaluating the MIC of juglone capable of inhibiting the growth of these phytopathogens. At 10mM, juglone exhibited growth inhibition against all bacterial strains except X. a. pv. juglandis. The mechanism behind the less susceptibility of this bacteria to juglone was discovered to be its ability to degrade juglone in a time dependent manner which was assessed by NMR analysis. These studies support the folk use of plants in the treatment of diseases and calls for the need for continued research on plants.