Green extraction routes for conversion of Nerium oleander L. biomass into bioactive agents: phytochemical insights and antimicrobial promise

Abstract Background Nerium oleander L. ( N. oleander) is a species of shrub or small tree in the Apocynaceae family that has high levels of digitalin linear activity and cardiac glycosides, especially nerine and oleander. The study was researched to assess the in vitro proximate compositions, phytochemical analysis, and antimicrobial activity of the dried stems, leaves, and flowers of N. oleander extracted using various solvents. Methods Petroleum ether, ethyl acetate, acetone, methanol, and water extracts of the various solvents were tested for total phenol, flavonoid, and tannin contents using various phytochemical assays, while the antimicrobial activity was evaluated using the agar well diffusion method against six bacterial and fungal strains. Results The flowers had a higher moisture content (9.40%), whereas the stems had a higher ash content (18.58%) than the leaves and flowers. The flowers also had higher lipid, carbohydrate, and crude protein contents (4.33, 66.16, and 15.29%, respectively). The leaves of the N. oleander produced a maximum yield followed by flowers and stems, respectively. The highest percentage extractive yield was demonstrated by water extracts, which were followed by methanol, acetone, ethyl acetate petroleum ether extracts. The findings of the study showed that alkaloids, flavonoids, tannins, glycosides, steroids, coumarins, quinones, phenols, cardiac glycosides, and terpenoids were found by phytochemical analysis of N. oleander in different parts, while saponins and anthraquinones were completely absent in all parts. On the other hand, anthocyanins are present only in flowers and completely absent in stems and leaves. The greatest levels of phenol content, flavonoids and tannins were found in acetone extract of the flowers (157.12 ± 17.62 mg GAE/g, 187.43 ± 15.91 mg QE/g, and 89.93 ± 18.77 mg TAE/g, respectively), while the petroleum ether extract of stems had the lowest amount (5.45 ± 2.65 mg GAE/g, 6.43 ± 0.29 mg QE/g and 3.45 ± 2.05 mg TAE/g, respectively). Antimicrobial tests revealed the extract’s ability to inhibit several Gram-positive bacteria ( Staphylococcus aureus ATCC 6538 and Bacillus subtilis ATCC 6633), Gram-negative bacteria ( Pseudomonas aeruginosa ATCC 9027, Salmonella Typhimurium ATCC 14028 and Escherichia coli ATCC 11229), and eukaryotic strains such as unicellular fungi ( Candida albicans ATCC 10231). Conclusion These results highlight the potential of N. oleander extracts as natural antimicrobials.