Synergistic Antimicrobial Activity of Azadirachta indica (Neem) and Psidium guajava (Guava) Leaf Extracts against Multidrug-Resistant Bacteria

Multidrug-resistant (MDR) bacteria are eroding the usefulness of conventional antibiotics, and plant-derived combinations are being explored as alternatives. Azadirachta indica (neem) and Psidium guajava (guava) are widely used medicinal plants, but their combined activity against MDR pathogens is not well characterised. Leaves of both plants were extracted in ethanol by Soxhlet extraction, screened for phytochemical classes by standard qualitative tests, and profiled by GC-MS. Each extract and their combination was tested against MDR clinical isolates of Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa by agar well diffusion. Minimum inhibitory concentration (MIC) was measured by broth microdilution, interaction was assessed by checkerboard assay (fractional inhibitory concentration index, FICI), and IC50 values were estimated. Both extracts contained alkaloids, flavonoids, tannins, saponins, terpenoids, glycosides and phenols, with tannins more abundant in guava. The combination gave larger inhibition zones than either extract alone against all three isolates (S. aureus 19.6 mm, E. coli 18.1 mm, P. aeruginosa 15.2 mm). Combination MICs were 0.25, 0.5 and 0.75 mg/mL, respectively, against 1.0 to 2.5 mg/mL for the single extracts. The FICI was 0.42 for S. aureus (synergy), 0.58 for E. coli and 0.62 for P. aeruginosa (both additive). IC50 fell from 0.55 mg/mL (neem) and 0.45 mg/mL (guava) to 0.22 mg/mL for the combination.