Combined Antibacterial Effect of Azadirachta indica (Neem) Leaf Extract and Clindamycin Against Staphylococcus aureus

Antimicrobial resistance in Staphylococcus aureus, including resistance to commonly used agents such as clindamycin, continues to challenge infection management and has encouraged investigation into plant-derived antimicrobial adjuncts. This study evaluated the individual and combined antibacterial activity of 70% ethanolic Azadirachta indica (neem) leaf extract and clindamycin against S. aureus ATCC 25923. A laboratory-based experimental design was employed using qualitative phytochemical screening, broth microdilution, and checkerboard assay across nine independent trials conducted over three experimental days. Phytochemical screening confirmed the presence of saponins, flavonoids, tannins, and phenols in the neem leaf extract. The individual minimum inhibitory concentration of neem extract showed a modal value of 25 mg/mL, with observed values ranging from 12.5 to 25 mg/mL. The individual MIC of clindamycin was not determined within the tested range of 0.125-2 µg/mL, as bacterial growth persisted across all trials. In the checkerboard assay, the neem extract-clindamycin combination inhibited bacterial growth at sub-MIC concentrations of both agents. The fractional inhibitory concentration index ranged from 0.50 to 0.75, with a mean of 0.64 ± 0.14, indicating an additive antibacterial interaction, with some boundary-level evidence of potential synergy. These findings suggest that neem leaf extract may enhance clindamycin activity against S. aureus under in vitro conditions. Further studies using confirmed resistant and susceptible strains, expanded concentration ranges, and molecular validation are recommended.