Comparative Phytochemical Screening and Antimicrobial Activities of Acacia nilotica (L.) Delile and Morinda citrifolia (L.) Extracts against selected Bacterial strains

Plants have long been recognized as reservoir of bioactive compounds with therapeutic potentials, many of which possess antimicrobial properties. Among these, Acacia nilotica and Morinda citrifolia have attracted scientific interest due to their ethnomedicinal application and rich phytochemical profiles. This study investigated the phytochemical constituents and antibacterial efficacy of Acacia nilotica (L.) Delile and Morinda citrifolia L. extracts against five clinical bacterial strains: Escherichia coli, Staphylococcus aureus, Proteus mirabilis, Salmonella typhi, and Klebsiella pneumoniae. Ethanol extracts of stem bark and leaves of A. nilotica, as well as leaves and fruit of M. citrifolia, were evaluated using standard phytochemical screening, agar well diffusion assay, and MIC/MBC determinations. Phytochemical analysis revealed a rich presence of bioactive compounds. A. nilotica stem bark and leaves contained high levels of tannins (+), flavonoids (+), and saponins (+), while M. citrifolia fruit and leaves showed strong positivity for steroids (+), alkaloids (+), and flavonoids (+), though they lacked terpenoids and tannins. Antimicrobial results showed that all extracts exhibited concentration-dependent inhibition. At 100 mg/mL, A. nilotica leaf extract recorded the highest zone of inhibition (28.00 ± 0.70 mm) against P. mirabilis, followed closely by A. nilotica stem bark (27.65 ± 0.58 mm) and M. citrifolia fruit (23.93 ± 0.94 mm) against S. aureus. Streptomycin (100 µg/mL) served as control, producing inhibition zones ranging from 26.43 mm (S. aureus) to 27.82 mm (P. mirabilis). Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) tests confirmed the potency of A. nilotica extracts. The lowest MICs were recorded by A. nilotica stem bark against S. aureus (6.81 mg/mL) and P. mirabilis (6.97 mg/mL), while M. citrifolia leaf and fruit showed higher MICs ranging from 12.01–15.46 mg/mL, with the highest MIC observed against E. coli (15.46 mg/mL). Corresponding MBCs followed a similar trend, with A. nilotica generally achieving bactericidal effects at half the concentration required by M. citrifolia. Overall, A. nilotica extracts demonstrated superior antibacterial activity compared to M. citrifolia, likely due to higher concentrations of polyphenolic compounds and tannins. These results support the traditional use of both plants in ethnomedicine and highlight the therapeutic potential of A. nilotica in combating bacterial infections, particularly those involving P. mirabilis and S. aureus.