Objective: Ornithine decarboxylase (ODC) is one of the key enzymes involved in polyamine synthesis that has a prominent regulatory role in cell growth in prokaryotes and eukaryotes. Its regulation/inhibition is a potential means of controlling cell growth. The purpose of the study was to check the effect of the methanol extracts of leaves and seeds of the mangrove plant Excoecaria agallocha in controlling/inhibiting ODC enzyme produced by Escherichia coli.Methods: The methanol extracts of leaves and seeds of the mangrove plant E. agallocha were made. The inhibitory effect of the extracts on ODC enzyme produced by E. coli was tested by a modified spectrophotometric assay (measured in terms of µM of putrescine produced), and the antibacterial effect of the extract was determined by well diffusion assay.Results: It was found that both the leaf and seed extracts inhibited ODC activity thereby preventing the conversion of ornithine to putrescine (the prominent by-product of ODC activity) in a concentration-dependent manner. The leaf extract exhibited 50% inhibition of ODC activity at a concentration of 17.5 µg/ml whereas seed extract inhibited 50% of the activity at a concentration of 16.1 µg/ml. In the growth inhibition assay by well diffusion method, the effect of the extracts on the growth of E. coli was studied and a concentration-dependent growth inhibition was found - 100 µl of leaf extract showed a zone of inhibition of 12.31 mm (diameter) and seed extract showed 12.64 mm, confirming the antibacterial activity of the extracts.Conclusion: The ODC inhibition-mediated growth inhibition of E. coli is a novel finding as far as the extracts of E. agallocha are concerned. The extract was potent enough to be suggested as an antibacterial agent.Keywords: Ornithine decarboxylase, Excoecaria agallocha, Escherichia coli, Ornithine decarboxylase inhibition, Putrescine, Ornithine, Antibacterial effect.