In vitro evaluation of the effect of Azadirachta indica (Neem) leaf extract on Escherichia coli from gut source

Abstract

The human gut microbiota plays a crucial role in maintaining host health, and disruption of its balance can promote the growth of opportunistic bacteria such as Escherichia coli. Medicinal plants such as Azadirachta indica (Neem) have gained attention as potential sources of alternative antimicrobial agents. This study evaluated the in vitro effects of Neem leaf extract on a gut-derived E. coli isolate. Phytochemical characterization using UV–Vis spectrophotometry indicated the presence of diverse bioactive compounds. Antibacterial activity was assessed using optical density (OD₆₀₀), colony-forming unit (CFU) enumeration, biofilm formation, growth kinetics, and haemolysis assays. Neem extract exhibited concentration-dependent inhibitory effects, with 250 μg/mL of plant extract, bacterial growth reduced by approximately 23% and viable counts by ~52.8%, while 125 μg/mL reduced CFU (~45.6%). Growth kinetics analysis demonstrated delayed but sustained inhibition after 12 h of exposure. No significant effect on biofilm formation was observed, and haemolytic activity remained unchanged across treatments. The discrepancy between OD₆₀₀ and CFU highlights limitations of turbidity-based measurements when evaluating crude extracts. Overall, Neem extract demonstrated moderate inhibitory effects on bacterial viability, supporting its potential as a source of antimicrobial compounds targeting gut-associated bacteria. Jahangirnagar University J. Biol. Sci. 14(1 & 2): 175-189, 2025 (June & December)