Antibacterial activity of methanolic and aqueous crude plant extracts of Acanthus polystachyus Delile and Gymnanthemum amygdalinum Delile against Staphylococcus aureus and Escherichia coli

Abstract

The escalating global health crisis of antimicrobial resistance (AMR) necessitates the exploration of alternative antimicrobial agents. This study investigated the antibacterial activity of methanolic and aqueous crude plant extracts from Acanthus polystachyus Delile and Gymnanthemum amygdalinum Delile against Staphylococcus aureus and Escherichia coli. Plant samples were collected from Nakyesasa farm, authenticated by confirming the identity of the plants by the botanist at the Makerere university Herbarium, and subjected to methanolic and aqueous crude extraction with 90% methanol and distilled water respectively. Antibacterial activity of the crude plant extracts was assessed using agar well diffusion, followed by serial broth dilution for Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) determination. Data was managed and analyzed using STATA software, employing one-way analysis of variance (ANOVA) comparing mean MIC and mean MBC values, and Bonferroni’s Post Hoc comparison tests, with P < 0.05 considered statistically significant at 95% level of confidence. The One-way ANOVA analysis indicated that there were statistically significant differences across the different plant extract groups (P < 0.05) in the mean MIC and MBC values against S. aureus and E. coli. Further analysis with Bonferroni Post Hoc test revealed that methanolic extracts of Acanthus polystachyus Delile exhibited the greatest antibacterial activity. The agar well diffusion results indicated varying zones of clearance of the different plant extracts with extracts exhibiting greater zones of clearance up to 16.5mm against S. aureus than E. coli with up to 9.0mm. A. polystachyus Delile exhibited a higher antibacterial activity with lower mean MIC and MBC values of 1.29mg/ml and 5.16mg/ml respectively against S. aureus and 20.63mg/ml and 31.25mg/ml respectively against E. coli, than G. amygdalinum Delile with MIC and MBC values of 2.58mg/ml and 5.16 mg/ml respectively against S. aureus and 82.5mg/ml and 165mg/ml respectively against E. coli. The methanolic extracts demonstrated a superior antibacterial activity than the aqueous extracts of their counterpart plants with an MIC value as low as 1.29 mg/ml against S. aureus and 20.63 mg/ml against E. coli. The general MBC/MIC ration of 1.725 of the plant extracts being <4 indicated the bactericidal effect of the plant extracts. These findings highlight the potential of Acanthus polystachyus Delile and Gymnanthemum amygdalinum Delile as sources of natural antimicrobial compounds, offering a viable strategy to mitigate AMR and contribute to the development of plant-based alternatives to conventional antibiotics. Therefore, further research to isolate and optimize these plants bioactive compounds as well as conducting clinical trials to validate their efficacy and safety in humans and animals is highly recommended.